ISRO-PAX-300 Issue 5, November 2012

Workmanship Standards for the Fabrication of Electronic Packages

ISRO Reliability Standards

Directorate of Systems Reliability and Quality, ISRO Headquarters, Bangalore

Indian Space Research OrganisationDepartment of SpaceGovernment of IndiaAntariksh Bhavan New BEL Road, Bangalore - 560 231, IndiaTelephone : +91-80-2341 5241/2217 2333Fax : +91-80-23415328e-mail : chairman@isro.gov.in

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Dr. K. RadhakrishnanChairman

MESSAGE

ISRO Reliability Standards, addressing the various disciplines of Engineering, have been in vogue for

almost three decades now. These standards are followed across ISRO centres as well as external work

centers for design, fabrication, testing, analysis and other processes involved in the realization of Launch

Vehicles, Spacecraft, Space Applications, Ground support systems and other launch infrastructure. The

need for standardization of processes towards achieving high reliability systems can never be over

emphasized, and ISRO Reliability Standards are just an attempt towards explicitly stating this.

With the advent of newer techniques and with the evolution of technology itself, over the last 30 years,

it has become essential to revisit the existing ISRO Reliability Standards and revise and update the

standards wherever essential. Towards this, the Directorate of Systems Reliability and Quality (DSRQ) at ISRO Headquarters

has taken an initiative to re-invigorate the reach and visibility of ISRO Reliability standards across all the Centres of ISRO. Specific

Inter-centre teams were formed to revise each of these documents and I would like to place on record their commendable

efforts in bringing out these documents.

There is a pressing need for ensuring uniformity of practices, across various functions of design, fabrication, testing, review

mechanisms etc., across the centres and units of ISRO. Towards this goal, the mandatory adoption of ISRO Reliability Standards

will ensure standardization in quality processes and products. I am certain that this will go a long way towards ensuring overall

system level Quality and Reliability and in achieving the goal of zero defects in the delivery of space systems of ISRO.

K Radhakrishnan

Chairman, ISRO

PREFACE

ISRO Reliability standards are a result of the need for standardization of processes towards achieving high reliability systems.

The transfer of knowledge and techniques from the seniors to their successors is best done with proper documentation and

checklists translating the entire know-how into black and white.

This document on ‘Workmanship standards for the fabrication of electronic packages’ addresses the complete

assembly of launch vehicle, spacecraft, and critical check-out systems for all projects of ISRO, from the point of view of quality

and workmanship requirements to be met during fabrication of electronic and electromechanical packages. This document has

undergone a large scale revision, compared to its previous issue, considering the advancement of technology. The details regarding

facility, tools, materials, soldering and cleaning of Printed Circuit Board assemblies are discussed at length. Particulars related

to crimping, interconnecting cables, harnesses and wiring are also given specific attention. The role of Quality professionals and

aspects of Quality assurance are also elucidated. Additional details regarding polymeric applications, conformal coating, electro

static discharge, repair and rework and bonded stores are also made clear.

It is deemed essential that these standards be strictly adhered to, in order to ensure uniformity of practices across ISRO centers

and achieve zero defects in the delivery of space systems.

I am grateful to Chairman ISRO, for being the source of inspiration in the release of these documents. Thanks are also due to the

centre Directors for their encouragement. I am also thankful to the Heads of SR Entities/Groups of various ISRO centres for

their relentless support and guidance. I am also indebted to the members of the Integrated Product Assurance Board (IPAB) for

the meticulous review of these documents. I also owe gratitude to the task team members and other experts for putting efforts

in the realization of these documents. I am glad to carry forward this rich lineage of ISRO reliability standards, championed by

Shri R Aravamudan, a revered pioneer in the area of Quality & Reliability in ISRO.

S Selvaraju

Sr. Advisor (SRQ)

Directorate of Systems Reliability & QualityISRO HeadquartersAntariksh BhavanNew BEL Road, Bangalore -560231Ph :080 - 2341 5414 Fax :080 – 2341 2826Cell:09448397704Email: sselvaraju@isro.gov.in

S Selvaraju Senior Advisor, Systems Reliability and Quality

LIST OF CONTENTS

1 SCOPE 01

2 APPLICABLE DOCUMENTS 02

2.1 Other Related Documents 02

2.2 Definitions 02

3 FACILITY 03

3.1 Environmental Conditions for Work Area 03

3.2 Lighting Requirements 03

3.3 ESD Requirements 03

3.4 Wiring & Assembly Area 03

3.5 Cleaning Area 03

3.6 Tinning Area 03

3.7 Conformal Coating & Potting Area 04

3.8 Mechanical Assembly Area 04

3.9 Special Processes Area 04

4 TOOLS 05

4.1 Tools and Equipments 054.1.1 Brushes 054.1.2 Cutters and pliers 054.1.3 Bending tools 054.1.4 Clinching tools 064.1.5 Antiwicking Tools 064.1.6 Holding Devices 064.1.7 Insulation strippers 064.1.8 Thermal Shunts 07

4.2 In-Process Storage and Handling 07

4.3 Soldering, cleaning and Inspection Equipments 074.3.1 Contact Type (Soldering irons) 074.3.2 Non-contact Type Soldering machines 084.3.3 Solder Baths 094.3.4 Cleaning equipment and systems 094.3.5 Inspection Optics (Magnification Aids) 09

5 MATERIALS 11

5.1 General 11

5.2 Solder 115.2.1 Solder Preform 115.2.2 Solder Composition 11

5.2.3 Maintenance of paste purity 12

5.3 Flux 125.3.1 Rosin-based fluxes 12

5.4 Cleaning Solvents 135.4.1 Approved Cleaning Solvents 13

5.5 Flexible insulation materials 13

5.6 Terminals 135.6.1 Terminal Material 135.6.2 Type of terminal 145.6.3 Shape of terminals 14

5.7 Wires 14

5.8 PCBs 145.8.1 Boards 145.8.2 Gold finish on conductors 145.8.3 Classification of boards 14

5.9 Adhesives (potting compounds & heat sinking), Encapsulants & conformal coatings 15

6 COMPONENT MOUNTING 16

6.1 Principles of reliable soldered connections 16

6.2 Preparatory conditions 166.2.1 Facility cleanliness 166.2.2 Preparation of Components leads, conductors, terminals and solder cups 16

6.3 Surfaces to be soldered 176.3.1 Cleaning 176.3.2 De-golding of gold-plated leads and terminals 176.3.3 Methods for degolding 186.3.4 Pretinning of stranded wires 186.3.5 Pre-tinning of Component leads and solid-wire conductors 186.3.6 Preparation of the soldering bit 19

6.4 Storage 196.4.1 Components 196.4.2 PCBs 196.4.3 Storage of wired PCBs 19

6.5 Preparation of PCBs for soldering 19

6.6 Parts Mounting 206.6.1 General requirements 206.6.2 Stress Relief 206.6.3 Stress relief of components with bendable leads 206.6.4 Dual in-line package 216.6.5 Part Positioning 236.6.6 Visibility of Markings 236.6.7 Heavy components 236.6.8 Metal-case components 236.6.9 Glass Encased Parts 24

6.6.10 Hookup /Jumper Wire 246.6.11 Lead Bending and Cutting 246.6.12 Coated Parts 256.6.13 Splices 256.6.14 Location 25

6.7 Parts Mounted To PWB’s 256.7.1 Axial Lead Mounting 266.7.2 Boards Lead Terminations, Printed Wiring 286.7.3 Lead bending requirements 326.7.4 Mounting of terminals to PCBs 32

6.8 Mounting requirement for SMD 356.8.1 General 356.8.2 Registration of devices and pads 356.8.3 Lead forming 356.8.4 Mounting devices in solder paste 356.8.5 Leadless devices 366.8.6 Area array devices 366.8.7 Potting of heavy devices 36

7 SOLDERING 37

7.1 Securing conductors 377.1.1 Thermal shunts 37

7.2 Solder application to terminals 377.2.1 Soldering of swaged terminals onto PCBs 377.2.2 Soldering of conductors onto terminals (except cup terminals) 377.2.3 Soldering of conductors onto cup terminals 37

7.3 Solder application to PCBs 377.3.1 Application of flux 377.3.2 Solder application 387.3.3 Solder coverage 387.3.4 Solder fillets 387.3.5 Wicking 397.3.6 Solder rework 39

7.4 Soldering of SMDs 397.4.1 General requirements 397.4.2 End-capped and end-metallized devices 407.4.3 Hand soldering of chip capacitors and resistors 407.4.4 Bottom terminated chip devices 417.4.5 Cylindrical end-capped devices 417.4.6 Castellated chip carrier devices 417.4.7 Devices with round, flattened, ribbon, “L” and gull-wing leads 427.4.8 Devices with “J” leads 437.4.9 Tall profile devices 44

7.5 Ceramic Column Grid Array Devices 447.5.1 Handling Precautions for CCGA Devices 457.5.2 Bare CCGA Device Inspection 457.5.3 Bare PCB Inspection (For CCGA assembly point of View) 477.5.4 Post soldering CCGA Assembly Inspection 477.5.5 Visual Inspection 477.5.6 Radiographic Inspection (X-ray) 50

7.6 High-voltage connections 52

7.7 BGA devices 52

7.7.1 Handling Precautions for BGA Devices 52

7.7.2 Bare BGA Device Inspection 53

7.7.3 Bare PCB Inspection (For CCGA assembly point of View) 53

7.7.4 Post soldering BGA Assembly Inspection 54

8 Cleaning of PCB assemblies 56

8.1 Acceptable cleaning systems 568.1.1 Manual Cleaning 568.1.2 Vapour Degreasing – General Requirements 56

8.2 Monitoring for cleanliness 578.2.1 Cleanliness testing 578.2.2 Test limits 578.2.3 Test method 57

9 Quality assurance 58

9.1 Data 58

9.2 Nonconformance 58

9.3 Calibration/ Validation 58

9.4 Inspection 58

9.5 Acceptance criteria 58

9.6 Rejection criteria 59

9.7 Operator and inspector training and certification 59

9.8 Quality records 60

9.9 Typical accept / reject illustrations 609.9.1 Workmanship illustrations for SMDs 60

10 CRIMPING, INTERCONNECTING CABLES, HARNESSES, AND WIRING 73

10.1 Principles of Reliable Cabling and Wiring 73

10.2 General requirements 73

10.3 Tool and Equipment Control 74

10.4 Solvents and Cleaners 74

10.5 Mounting of Terminals 74

10.6 Attachment of conductors to terminals, solder cups and cables 7510.6.1 General 7510.6.2 Conductors 7510.6.3 Breakouts from cables 7510.6.4 Insulation clearance 7610.6.5 Solid hook-up wire 7610.6.6 Stress relief 7610.6.7 Insulation clearance 76

10.7 Stripping insulation from conductors and cable 7710.7.1 Stripping Round Conductors 7710.7.2 Stripping Jackets over Shields 78

10.8 Turret, Bifurcated, hook and cup terminals 7810.8.1 Turret and Straight Pin Terminals 7810.8.2 Bifurcated terminals 7810.8.3 Hook terminals 8110.8.4 Pierced terminals 8210.8.5 Solder cups (connector type) 8210.8.6 Insulation sleeving 82

10.9 Wire and cable interconnections 8310.9.1 General 8310.9.2 Preparation of shielded wires and cables 8310.9.3 Pre-assembly 8310.9.4 Soldering procedures 8410.9.5 Cleaning 8510.9.6 Workmanship 8510.9.7 Connection of stranded wires to PCBs 85

10.10 Interconnecting cable/harness fixturing 8610.10.1 General 8610.10.2 Mockup and Wiring Board Design Parameter 8610.10.3 Temporary Identification 8610.10.4 Interconnecting Cable and Harness Protection 86

10.11 Forming wires and cables into harnesses 8610.11.1 General 8610.11.2 Fabric Braid Sleeving (Pre-woven) 9010.11.3 Lacing 9110.11.4 Continuous Lacing 9210.11.5 Straps 9310.11.6 Insulation Sleeving/Tubing 93

10.12 Cable shielding and shield termination 9410.12.1 General RFI/EMI Practices 9410.12.2 Shield Termination 9510.12.3 Individual Shield Termination Using Heat Shrinkable Solder Sleeves 9510.12.4 Long Lengths of Shrinkable Sleeving 9510.12.5 Floating Shield Terminations 9610.12.6 Unshielded Wire Exposure and Total Length of Grounding Wires 97

10.13 Wire crimping 9710.13.1 Crimping Requirements: 9810.13.2 Crimping Operations 9810.13.3 Crimping Tools 9910.13.4 Calibration of Crimping Tools 10310.13.5 Insulation Clearance 10410.13.6 Insulation Support 10410.13.7 Integrity of Crimped Connections 10410.13.8 Examination of Test Samples 10510.13.9 Inspection 10510.13.10 Inspection Prior to Crimping 10510.13.11 Microsectioning of Crimped Pin: 106

10.14 Connector assembly 10710.14.1 Assembly of Crimp-Type Connectors (Including Terminal Junctions) 107

10.15 Interconnecting harness and cable cleaning 10810.15.1 General 10810.15.2 Cleaning the Harness Assembly 10810.15.3 Cleaning Harness Connectors 10810.15.4 Cleaning Coaxial Connectors (Assembled) 10910.15.5 Harness handling and protection 10910.15.6 Interconnecting Harness and Cable Storage Protection 10910.15.7 Connector mating 109

10.16 Testing and inspection 11010.16.1 General 11010.16.2 Wet Probe Testing 111

10.17 Quality assurance provisions 11110.17.1 Method of Inspection. 11110.17.2 Magnification Aids 11210.17.3 Documentation Verification 112

10.18 Wire visual aids and illustrations 11610.18.1 Wiring: connectors, cabling, and harnessing - wire dress to connectors 11610.18.2 Wiring: connectors, cabling, and harnessing - stress relief shrinkable sleeving on solder cups 11610.18.3 Wiring: connectors, cabling, and harnessing, wire preparation, thermal stripping 11710.18.4 Wire preparation: mechanical stripping 11810.18.5 Wiring: connectors, cabling, and harnessing, wire preparation, thermal stripping 11910.18.6 Wiring: connectors, cabling, and harnessing, wire preparation, tinning stranded conductors 11910.18.7 Wiring: connectors, cabling, and harnessing - installation of straps 12010.18.8 Crimps: insulation clearance 12110.18.9 Crimps: Acceptable and Unacceptable 121

10.19 Critical problems in coaxial cable assembly 122

11 SEMI-RIGID CABLE ASSEMBLY 125

11.1 Introduction 125

11.2 Principles of Reliable Soldered or Crimped Semi-Rigid Cable Connections 125

11.3 Material 125

11.4 Tools 12611.4.1 Fabrication tool kits from following manufacturer’s are available. 12611.4.2 Cutting Tools 12611.4.3 Cable Forming Tools 12611.4.3 Cable Forming Tools 12611.4.4 Cable Stripping and Dressing Tools 12711.4.5 Heat Treatment Chamber 12711.4.6 Soldering Equipment 12711.4.7 Crimping Equipment 127

11.5 Semi Rigid Cable Assembly Process 12711.5.1 General 12711.5.2 Cable Straightening 128

11.6 Cable Assembly Drawing 128

11.7 Cable Cutting 128

11.8 Preconditioning Heat Treatment 129

11.9 Cable Templates 129

11.10 Cable Bending 129

11.11 Cable Bending General Requirements 13011.11.1 Cable Bending Tools & Aids 13011.11.2 Cable Bending Procedure 131

11.12 Cable Assembly Support Requirements 132

11.13 Cable Outer Jacket Stripping 13211.13.1 Inspection of Stripped Cable Ends 132

11.14 Stripping the Dielectric 13311.14.1 Stripping the Dielectric Alone After Outer Jacket Stripping 13311.14.2 Stripping of Dielectric & Outer Jacket Simultaneously 133

11.15 Centre Conductor End Forming 133

11.16 Preparation for soldering of Cable Outer Jacket and Centre Conductor Tinning 134

11.17 Degolding of Gold Plated Connector Parts and Pre-tinning 13411.17.1 De-golding By Three Solder Pot Method 13411.17.2 Solder Preforms 13511.17.3 Assembly Plan 13711.17.4 General Requirements for Connector Assembly 137

11.18 Specific Requirements 13711.18.1 SMA Right Angle Connector 13711.18.2 SMA female connector 138

11.19 Solder Assembly of Semi Rigid Cables 13811.19.1 Straight cable end connector 138

11.20 Right Angle Cable End Connector 139

11.21 Teflon Bush Insertion In Connector 14011.21.1 In Case of Straight SMA Connector 14011.21.2 In case of TNC connector 140

11.22 Semi Rigid Cable Preconditioning 14111.22.1 Necessity 14111.22.2 Phase-I Preconditioning 14211.22.3 Phase-II Preconditioning 14211.22.4 Phase-III Preconditioning 143

11.23 Inspection & Acceptance/Rejection Criteria 14311.23.1 Inspection of Cable After Cutting To Required Length 14311.23.2 Inspection after Cable Bending 14411.23.3 Inspection After Cable Jacket Cutting, Dielectric Stripping Pin Forming and Tinning 14411.23.4 Inspection of De-Golded Connector Parts 14611.23.5 Inspection of Pin Soldering 14611.23.6 Inspection After Soldering of Connector Parts To Sem-irigid Cable Before Phase III Preconditioning 14611.23.7 Inspection of Finished Cable Assembly after Phase –III reconditioning 147

11.24 Specific 14711.24.1 Right angle connector cable assembly 14711.24.2 Straight connector cable assembly 14811.24.3 TNC connector cable assembly 148

11.25 Semi-rigid cable fabrication flow charts 149

11.26 Sample diagram of cable assembly 160

11.27 Typical stress relieving bends used in Semi rigid cable assembly 161

12 POLYMERIC APPLICATIONS 163

12.1 Preparation for polymeric applications 16312.1.1 Surface Preparation 16312.1.2 Masking 16312.1.3 Priming 16312.1.4 Local Potting 16312.1.5 Requirements 164

13 CONFORMAL COATING 171

13.1 Purpose 171

13.2 Safety Precautions 171

13.3 Poly Urethane Type Coating Applications 17113.3.1 Spraying 17213.3.2 Brush Method 17213.3.3 Dipping Method 17213.3.4 Pouring Method 172

13.4 Curing 172

13.5 Parylene Conformal Coating 17213.5.1 Preparation for Coating (For Polyurethane and Parylene) 172

13.6 Application Procedure 17313.6.1 Procedure for coating with Parylene: 173

13.7 Deposition Process 17313.7.1 Sublimation 17313.7.2 Precautions for Local Potting & Conformal Coating 178

13.8 Bonding 17813.8.1 General 178

14 REPAIR & REWORK 179

14.1 Repair/Rework 179

14.2 Repair criteria 179

14.3 Number of repairs 179

14.4 Modifications 17914.4.1 Modification criteria 179

14.5 Number of modifications 179

14.6 Rework 17914.6.1 Rework criteria 17914.6.2 Number of reworks 17914.6.3 Other requirements 179

14.7 Removal of conformal coating 17914.7.1 Requirements 17914.7.2 Procedure 18014.7.3 Acceptance criteria 180

14.8 Solder joint removal and unclinching 18014.8.1 Procedure 18014.8.2 Acceptance criteria 180

14.9 Repair of damaged conductor tracks 18014.9.1 Requirements 18014.9.2 Procedure 18014.9.3 Acceptance criteria 180

14.10 Repair of lifted conductors 18114.10.1 Requirements 18114.10.2 Procedure 18114.10.3 Acceptance criteria 181

14.11 Wire to wire joints 18114.11.1 Requirements 18114.11.2 Procedure 18114.11.3 Acceptance criteria 181

14.12 Removal and replacement of axial and multi lead components 18214.12.1 Requirements 18214.12.2 Procedure 182

14.12.3 Acceptance criteria 182

14.13 Removal and replacement of flat pack components 18214.13.1 Procedure 18314.13.2 Acceptance criteria 183

14.14 Modification of component connections 18314.14.1 Requirements 18314.14.2 Procedure 18314.14.3 Acceptance criteria 184

14.15 Quality assurance 184

14.16 Removal of conformal coating 18414.16.1 Introduction 18414.16.2 Tools and materials 184

14.17 Methods for the removal of conformal coating 18514.17.1 Method for the removal of polyurethane and silicone type coating 185

14.18 Solder joint removal and unclinching 18614.18.1 Introduction 18614.18.2 Tools and materials 18614.18.3 Methods for solder joint removal and unclinching 187

14.19 Repair of damaged conductor tracks 18914.19.1 Introduction 18914.19.2 Tools and materials 18914.19.3 Method for the repair of damaged conductor tracks 189

14.20 Repair of lifted conductors 190

14.21 Methods for repair of lifted conductors 19014.21.1 Method for the use of epoxy under conductor 19014.21.2 Method for the use of epoxy over conductor 191

14.22 Wire to wire joints 19114.22.1 Introduction 19114.22.2 Method for wire to wire joining 191

14.23 Addition of Components 19114.23.1 Method for additional component mounting on reverse (non component side) of board 19114.23.2 Method for additional components mounting on component side of board 192

14.24 Method for the addition of a wire link onto metallized cap of chips directly glued on PCB 192

14.25 Method for the addition of a wire link onto terminal pad of soldered chips 193

15 SPECIAL PROCESSES 194

15.1 SPLICING 19415.1.1 General 19415.1.2 General Information 19415.1.3 Design Considerations 19415.1.4 Splicing Methods 194

15.1.5 Soldered Splices 195

15.2 Lap Splice 19515.2.1 Preparation. 19515.2.2 Soldering. 19515.3 Lash Splice 19515.3.1 Preparation 19615.3.2 Soldering 196

15.4 Solder Sleeve 19615.4.1 Preparation 19615.4.2 Soldering. 197

15.5 Crimped Splices 197

15.6 Modified Crimp Contact 197

15.7 Butt Splice 19815.7.1 Preparation. 19815.7.2 Contact Sizing 19815.7.3 Assembly 19915.7.4 Inspection 199

16 ELECTRO STATIC DISCHARGE (ESD) 200

16.1 General 200

16.2 ESD Modeling 200

16.3 Triboelectrification 20016.3.1 Induction charging 200

16.4 Need of ESD Control 202

16.5 Classifications of ESD Devices 202

16.6 Type of ESD Failure 20216.6.1 Catastrophic failure 20216.6.2 Parametric failure 20216.6.3 Latent failure 203

16.7 ESD Control Program 20316.7.1 ESD Sensitivity Levels 20316.7.2 Methods of ESD Control 20416.7.3 Personnel safety 20616.7.4 ESD protected areas (EPA) 206

16.8 ESD Control Requirements For Facilities 20616.8.1 General 20616.8.2 Identification and access - ESD areas 20616.8.3 Prohibited Materials And Activities 20816.8.4 ESD Protective Work Surfaces 20816.8.5 ESD-Protective floor surfaces 20916.8.6 Personal grounding devices 21016.8.7 Integrity testing of personal grounding devices 21016.8.8 Equipment and facilities 21116.8.9 ESD safe protective packaging 215

16.8.10 Clothing requirements 215

16.19 ESDS Item Handling 21616.9.1 General 21616.9.2 Special Requirements for Highly Sensitive Items 21616.9.3 Equipment 21716.9.4 Identification and marking 218

17 BONDED STORES 220

17.1 Introduction 220

17.2 Environment of the bonded stores 220

17.3 Operation of the bonded stores 22017.3.1 Contents of the bonded stores 220

17.4 Storage 22017.4.1 General 22017.4.2 Electronic Component Storage Area 22117.4.3 Storage of Materials and Chemicals 22117.4.4 Operation 22117.4.5 Operator 22217.4.6 Documentation 222

18 TERMS AND DEFINITIONS 223

19 TECHNICAL STANDARD IMPROVEMENT PROPOSAL 236

19.1 Instructions 236

LIST OF FIGURES

Figure 4.1 : Profiles of correct and incorrect cutters for trimming leads 05

Figure 4.2 : Typical lead forming/bending tool 06

Figure 4.3 : Typical mechanical wire stripper 06

Figure 6.1 : Methods for incorporating stress relief with components having bendable leads 21

Figure 6.2 : Assembly of under filled TO-39 and TO-59, and adhesively staked CKR06 22

Figure 6.3 : Not acceptable body and seal conditions 23

Figure 6.4 : Minimum lead bend 25

Figure 6.5 : Horizontal Mount 26

Figure 6.6 : Radial Leaded Parts 26

Figure 6.7 : Obstruction of solder flow (Not acceptable) 27

Figure 6.8 : Stress Relief Part Termination 27

Figure 6.9 : Bend Angle 27

Figure 6.10 : Lapped Lead Height above Board 28

Figure 6.11 : Lapped Round Termination 29

Figure 6.12 : Lapped Ribbon Leads 30

Figure 6.13 : Clinched Termination 30

Figure 6.14 : Lead Bend 31

Figure 6.15 : Straight-Through Termination 31

Figure 6.16 : Straight-Through Lead Retention 31

Figure 6.17 : Leads with solder termination on both sides 32

Figure 6.18 : Types of terminal swaging 33

Figure 6.19 : Terminal swaging sequence 33

Figure 6.20 : Method of stress relieving parts attached to terminals 34

Figure 6.21 : Fuse mounted on bifurcated, where post is cut 35

Figure 6.22 : Exposed element 36

Figure 7.1 : Solder fillet for plated through holes 39

Figure 7.2 : Solder fillet for non through holes where leads are clinched 39

Figure 7.3 : Mounting of rectangular and square end-capped and end-metallized devices 40

Figure 7.4 : Mounting of bottom terminated chip devices 41

Figure 7.5 : Mounting of cylindrical end-capped devices 42

Figure 7.6 : Mounting of castellated chip carrier devices 42

Figure 7.7 : Mounting of devices with round, flattened, ribbon, “L” and gull-wing leads 43

Figure 7.8 : Mounting of devices with “J” leads 43

Figure 7.9 : Dimensions of tall profile components. 44

Figure 7.10 : Typical CCGA device build -up 44

Figure 7.11 : Typical assembled CCGA device 45

Figure 7.12 : Underside view showing missing column 46

Figure 7.13 : Solder fillet 360° coverage around the column circumference: Accept 46

Figure 7.14 : Side view showing column; column by more than 5°: Reject 46

Figure 7.15 : X-ray view showing voids in column solder joint more than 25%: Reject 47

Figure 7.16 : Example of acceptable solder fillet coverage around column, more than 50%: Accept 48

Figure 7.17 : Example of acceptable column tilt up to 10º 48

Figure 7.18 : CGA mounted on PCB showing columns tilted < 5°: Accept 49

Figure 7.19 : Micrograph of CGA mounted on PCB 49

Figure 7.20 : Micrograph of CGA mounted on PCB 49

Figure 7.21 : Radiograph of CGA mounted on PCB 50

Figure 7.22 : Radiograph of CGA mounted on PCB showing missing column: Reject 50

Figure 7.23 : Radiograph of CGA mounted on PCB showing insufficient solder: Reject 51

Figure 7.24 : Radiograph of CGA mounted on PCB showing solder bridge: Reject 51

Figure 7.25 : Radiograph of CGA showing excessive voiding in solder fillets at base of columns: Reject 51

Figure 7.26 : High voltage connection 52

Figure 7.27 : Missing of balls 53

Figure 7.28 : Sum of voids in some BGA balls exceeds 25 % of ball’s cross section diameter: Reject 53

Figure 7.29 : Non wetted ball in X-Ray (Absence of tear drop shape) 54

Figure 7.30 : Ball shall be centered on land 54

Figure 7.31 : Ball bridging is not accepted 55

Figure 7.32 : Insufficient wetting of left most ball 55

Figure 7.33 : Crack on the ball to PCB solder joint 55

Figure 9.1 : Preferred solder for chip devices 62

Figure 9.2 : Maximum acceptable solder 62

Figure 9.3 : Un acceptable solder due to poor wetting 62

Figure 9.4 : Acceptable, minimum solder: Terminal wetted along end, face and sides 64

Figure 9.5 : Preferred solder 64

Figure 9.6 : Unacceptable Excessive solder 64

Figure 9.7 : Unacceptable insufficient solder 65

Figure 9.8 : Ribbon/Gull wing leaded devices 67

Figure 9.9 : Unacceptable : Excessive solder (middle joint) 67

Figure 10.1 : Terminal Damage 74

Figure 10.2 : Roll Flange Terminal 74

Figure 10.3 : V-Funnel Type Swage Roll 75

Figure 10.4 : Flare and extension of funnel flanges 75

Figure 10.5 : Elliptical funnel swage 75

Figure 10.6 : Wrap Orientation 77

Figure 10.7 : Side- and bottom-route connections to turret terminals 79

Figure 10.8 : Bottom-route connections to bifurcated terminal 79

Figure 10.9 : Side-route connection to bifurcated terminal 80

Figure 10.10 : Top-route connection to bifurcated terminal 81

Figure 10.11 : Connections to hook terminals 81

Figure 10.12 : Connections to pierced terminals 82

Figure 10.13 : Connections to solder cups (connector type) 83

Figure 10.14 : Methods for securing shielded wires 84

Figure 10.15 : Connection of stranded wires to PCBs 85

Figure 10.16 : Line Drawing of Typical Harness Layout 87

Figure 10.17 : Starting Stitch 88

Figure 10.18 : Spot Tie (Typical) 88

Figure 10.19 : Closing Stitch and Single Thread—Illustration 89

Figure 10.20 : Alternate Closing Stitch and Single Thread—Illustration 89

Figure 10.21 : Running Lockstitch 89

Figure 10.22 : Flat Lacing Stitches 90

Figure 10.23 : Securing Fabric Braid Sleeving 90

Figure 10.24 : Spot Tie Principle 91

Figure 10.25 : Spot Tie 91

Figure 10.26 : Serve Method of Tying 92

Figure 10.27 : Serve at the Point of Origin 92

Figure 10.28 : Running Stitch 92

Figure 10.29 : Single Lock Stitch 93

Figure 10.30 : Double Lock Stitch 93

Figure 10.31 : Plastic Strap Orientation 93

Figure 10.32 : Individual Shield Termination Using a Heat shrinkable Solder sleeving 95

Figure 10.33 : Installation of Long Lengths of Sleeving to Achieve Controlled Dimensions 96

Figure 10.34 : Floating Shield Termination 96

Figure 10.35 : Conductor Exposure for Individual Shield Termination Types 97

Figure 10.36 : Folded back shield with splice termination to multi strand wire 97

Figure 10.37 : Specific Interconnection 98

Figure 10.38 : Crimp Joint Tensile Failure Categories 105

Figure 10.39 : Example of a typical connector barrel and single wire crimping 106

Figure 10.40 : Example of a typical connector barrel and multi-wire crimping 106

Figure 10.41 : Visual Examination Inside the Socket Contact for Flux Residue 109

Figure 10.42 : Illustration of Proper trim back of Jacket to Isolate it from the Clamping Sy stem 123

Figure 10.43 : Broken Solder Joint Caused by Insufficient Solder Fill 124

Figure 10.44 : Problem Point for Kynar Stress Relief Sleeving 124

Figure 11.1 : Typical cable cut off fixture 129

Figure 11.2 : Typical cable forming tool 131

Figure 11.3 : Dimensional inspection requirements 133

Figure 11.4 : Method of producing solder preforms 136

Figure 11.5 : Approved and non approved straight solder type cable end connectors 136

Figure 11.6 : Centre contact assembly 138

Figure 11.7 : Right angle cable-end connector assembly 141

Figure 12.1 : Default Potting for Horizontally-Mounted Sleeveless Cylindrical Part 165

Figure 12.2 : Single Wire Potting 165

Figure 12.3 : Potting for Radial Lead Components 166

Figure 12.4 : Potting for Radial Multi-lead Rectangular Components 166

Figure 12.5 : Default Potting of a Single Vertically-Mounted Rectangular Part 167

Figure 12.6 : Default Potting for an Array of Vertically-Mounted Rectangular Parts 167

Figure 12.7 : Wire Bundle Potting 168

Figure 12.8 : Typical Toroid Potting 168

Figure 12.9 : Vibration Dampening Potting 169

Figure 12.10 : Typical Vibration Isolation Potting 170

Figure 13.1 : Conformal Coating – Bubbles 175

Figure 13.2 : Conformal Coating – Scratches 176

Figure 13.3 : Conformal Coating - Lifting and Peeling 177

Figure 13.4 : Conformal Coating – Coverage Defects 178

Figure 14.1 : Removal of multi-lead components, clipping of component leads 182

Figure 14.2 : Removal of flat pack components 183

Figure 14.3 : Removal of coating by thermal parting device 186

Figure 14.4 : Continuous vacuum solder extraction on stud lead 187

Figure 14.5 : Pulse type solder sucker in use 188

Figure 14.6 : Hot Jet Blower Method 188

Figure 14.7 : Cross-sectional view of wicking method 198

Figure 14.8 : Hot unclinching with thermal parting device 189

Figure 14.9 : Lifted conductors 190

Figure 14.10 : Repair using epoxy under conductor 190

Figure 14.11 : Repair using epoxy over conductor 191

Figure 14.12 : Additional components mounted on reverse (no component) side of board 192

Figure 14.13 : Addition of a wire link onto metallized cap of chips directly glued on PCB 193

Figure 14.14 : Addition of a wire link onto terminal pad of soldered chips 193

Figure 15.1 : Pre-Tinned Conductors 195

Figure 15.2 : Soldered Conductors 195

Figure 15.3 : Sleeving over Soldered Connection 195

Figure 15.4 : Double Sleeving over Soldered Connection 195

Figure 15.5 : Pre-Tinned 196

Figure 15.6 : Lashing of Pre-Tinned Conductors 196

Figure 15.7 : Soldered Connection 196

Figure 15.8 : Pre-Lash End Type Splice 196

Figure 15.9 : Lash End Type Splice 196

Figure 15.10 : Soldered Lash Splice 196

Figure 15.11 : Sleeved Lash Splice 196

Figure 15.12 : Solder Sleeve Prior to Flow 197

Figure 15.13 : Fully Melted Solder Sleeve 197

Figure 15.14 : Stripped Wires Prior to Insertion 197

Figure 15.15 : Stripped Wire Bundle Prior 198

Figure 15.16 : Wires Crimped Within 198

Figure 15.17 : Contact Trimmed and Deburred 198

Figure 15.18 : Contact Covered With Shrink Sleeving 198

Figure 15.19 : Butt Splice 198

Figure 15.20 : Butt Splice Prior to Wire Insertion 198

Figure 15.21 : Butt Splice Prior to Crimp 199

Figure 15.22 : Properly Crimped Butt Splice 199

Figure 15.23 : Butt Splice with Shrink Sleeving. 199

Figure 16.1 : ESD Symbols 205

Figure 16.2 : Typical ESD Grounded Workstation 208

Figure 16.3 : Workstation Common Point Ground 209

Figure 16.4 : Main Service Box 212

Figure 16.5 : Sensitive Electronic Device Caution Symbol (With & without sensitivity class level) 218

Figure 16.6 : ESD Protective Item Symbol 218

Figure 16.7 : ESD Common Point Ground Symbol 219

Figure 17.1 : Segregation of electronic components 221

Figure 17.2 : Segregation of material 221

Figure 17.3 : Segregation of chemicals 221

LIST OF TABLES

Table 4-1 : Solder baths for degolding and pretinning 09

Table 5-1 : Guide to choice of solder types 11

Table 5-2 : Chemical composition of solders 12

Table 5-3 : Classification of printed circuit boards and substrates 14

Table 6-1 : Clearances for insulation 17

Table 6-2 : Baking conditions 20

Table 6-3 : List of material used for isolation 24

Table 7-1 : Dimensional and solder fillet requirements for rectangular and square end capped devices 40

Table 7-2 : Dimensional and solder fillet requirements for bottom terminated chip devices 41

Table 7-3 : Dimensional and solder fillet requirements for cylindrical end-capped devices 42

Table 7-4 : Dimensional and solder fillet requirements for castellated chip carrier devices 42

Table 7-5 : Dimensional and solder fillet requirements for devices with round, flattened, ribbon, “L” and gull-wing leads 43

Table 7-6 : Dimensional and solder fillet requirements for devices with “J” leads 43

Table 10-1 : Clearances for insulation. 76

Table 10-2 : Dimensions for Figure 10-16 86

Table 10-3 : Bend Radii for Completed Interconnecting Cable or Harness 87

Table 10-4 : Spot Tie, and Stitch Spacing Dimensions 88

Table 10-5 : Distances From Connectors or Connector Accessories to Beginning of Harness Ties 90

Table 10-6 : Selection Guide for Use of Polyolefin / Kynar sleeves 94

Table 10-7 : Shield Termination Control 97

Table 10-8 : Required ultimate axial strength for compactive and dispersive crimped joints 107

Table 11-1 : Cable diameter and bend radius 130

Table 11-2 : Cable pre-conditioning : Phase1 142

Table 11-3 : Cable pre-conditioning : Phase2 143

Table 11-4 : Cable pre-conditioning : Phase3 143

Table 13-1 : Conformal coating materials 171

Table 14-1 : Wire diameters for given conductor widths 181

Table 16-1 : Triboelectric Series 201

Table 16-2 : ESDS Component Sensitivity Classifications – HBM 203

Table 16-3 : ESDS Component Sensitivity Classifications – MM 204

Table 16-4 : ESDS Component Sensitivity Classifications – CDM 204

Table 16-5 : ESD Protective materials 205

Table 16-6 : ESD Control Program Verification Schedule and Measurements 207

Table 16-7 : ESD Sensitivity for Selection and Performance of Air Ionizers 214

Table 16-8 : Summary of Recommendations Applicable to HBM Class 0 and MM Class M1 217

Table 16-9 : Susceptibility of Devices to ESD 219

Table 16-10 : Typical Electrostatic Voltages 219

Table 16-11 : Effects of Electrical Current on Humans 219

1

1 SCOPE This specification states the quality and workmanship requirements to be met during fabrication of electronic and

electromechanical packages for complete assembly of spacecraft, launch vehicle systems and critical check out

systems of all projects of ISRO so as to maintain an acceptable uniform quality level. The fabrication requirements

specified herein are applicable for all onboard avionics elements and addtional requirements wherever necessary

are specified in relevant sections.

Adherence to the procedures specified herein shall be mandatory for all work centres of ISRO and their subcontractors

in order to realize reliable operation of the systems. The procedures are thus drawn to ensure that all modules

fabricated meet the performance and reliability requirements criteria. In general, greater importance shall be given

for preventive measures leading to defect free systems rather than allowing for possible rework at later stage,

although rework or repair cannot be totally dispensed with. ISRO reserves the right to undertake inspection at any

stage of fabrication at work centres including sub contractors.

2

2 APPLICABLE DOCUMENTS

Doc Number Title

ISRO-PAX-301 Design Requirements for Printed Circuit Board Layout Artwork

ISRO-PAX-304 Test specification for Printed Circuit Boards.

ISRO-PAS-207 Storage, Handling & Transportation Requirements for Electronic Hardware

ISRO-PAS-100 Non Conformance Control Requirements for ISRO Projects

2.1 Other Related Documents

ECSS-Q-ST-70-38C High-reliability soldering for surface-mount and mixed technology

ECSS-Q-ST-70-08C Manual soldering of high-reliability electrical connections

ECSS-Q-ST-70-28 Space product assurance - Repair and modification of printed circuit board assemblies for space use

ECSS-Q-ST-70-26C The Crimping of High Reliability Electrical Connections.

ANSI-J-STD-004 Flux Soldering Liquid (Rosin Base)

ANSI- J-STD-006 Tin Alloy, Tin Lead Alloy and lead Alloy Solder.

MIL-STD-1686Electrostatic discharge control program for protection of electrical and electronic parts, assemblies and equipment (excluding electrically initiated explosive devices)

MIL-HDBK-263Electrostatic discharge control handbook for protection of electrical and electronic parts, assemblies and equipment (excluding electrically initiated explosive devices) (metric)

In the event of any conflict, this specification along with the production details shall supersede the applicable

documents.

2.2 Definitions

Terms and definitions used in this document are given in Chapter 18.

3

3 FACILITY

3.1 EnvironmentalConditionsforWorkArea

Clean surroundings must be maintained in the area where electronic fabrication is carried out.

The soldering area shall have a controlled environment to limit the entry of contaminants.• Clean room area shall be class 100,000 or better.

• The clean room temperature shall be maintained at 22 °C ± 3 °C.

• The relative humidity (RH) at room temperature of the facility shall be maintained at 55 % ± 5 %.

• Clean room should have positive pressure difference to the outside area.

• Areas used for assembly or cleaning of parts and areas where toxic or volatile vapours are generated or released shall include a local air extraction system.

• Dirt, dust, solder particles, clipped wires etc., shall be removed at frequent intervals.

• The work area shall have good ventilation.

• The filter shall be changed every six months or earlier depending upon the use.

3.2 LightingRequirements• Lighting intensity shall be a minimum of 1100 lumens/sq. m on the work surface.

• The additional lighting near the operator coming from the sides with suitable shading on the eyes of the operator shall be provided, to be switched on by the operator whenever necessary.

3.3 ESDRequirements

A full fledged ESD proof work station shall be employed for fabrication of charge sensitive devices as listed in

Chapter 16

3.4 Wiring&AssemblyArea

Clean surroundings shall be maintained in the wiring and assembly area as listed in 3.1. Care shall be taken to remove

cut leads of parts, wires and wire braids. Care shall be taken to ensure cleanliness during pre-cleaning for flux

removal. Tissue papers and other materials used for pre-cleaning shall be disposed off away from the work table.

3.5 CleaningArea

Cleaning area shall have proper ventilation to avoid toxic fumes affecting personnel involved in cleaning operations.

Approved cleaning solvents shall be used for cleaning of PCBs, packages and subsystems. Precaution shall be taken

while handling these chemicals as they are susceptible to flammability.

3.6 TinningArea

Tinning area shall have proper ventilation to carry fumes away from the work area. Cleaning of part’s leads and wires

during pre tinning shall be done in a manner so that the loose particles removed shall not lie in the work area. They

shall be collected in cleaning solvent and shall be disposed off regularly. Tinning Pots shall be kept at locations with

fume hood to avoid contamination.

4

3.7 ConformalCoating&PottingArea

Conformal coating and potting area shall have proper ventilation to conduct away the toxic fumes from

materials used.

3.8 MechanicalAssemblyArea

Mechanical assembly area shall have clean surroundings with good ventilation and have provision for mechanical

operations such as minor fitting, cutting and filling operations for Semi-rigid Cables and correcting the hardware.

This area shall be isolated from the fabrication area to avoid contamination due to mechanical operations.

3.9 SpecialProcessesArea

Special Processes such as subsystems assembly, optical assembly such as VHRR assembly of packages having Microwave

Integrated Circuits (MIC), sensor elements like PRTs etc., shall be carried out in an area approved by the QA team

of the ISRO Centre. Integration of packages, subsystems on the panels etc., shall also be carried out in the special

process area. Preferably in class 100 laminar tables.

5

4 TOOLS

4.1 ToolsandEquipments

All equipments and tools shall be inspected to ensure that they are not defective prior to use.

4.1.1 Brushes• Medium-stiff natural or synthetic bristle, ESD-safe, brushes shall be used for cleaning provided that they do

not damage any surface to be cleaned or adjacent materials.

• Brushes shall be cleaned properly in a solvent.

• Brushes shall not be damaged by the solvents used for PCB cleaning.

• Wire brushes shall not be used.

4.1.2Cuttersandpliers• Cutting edge profiles and cutter usage shall be in accordance with Figure4‑1.• The cutter used for trimming conductor wire and component leads shall shear sharply, producing a clean, flat,

smooth-cut surface along the entire cutting edge.

• No twisting action shall occur during the cutting operation.

• Cutting edges shall be checked for damage and maintained in a sharp condition.

• Smooth, round long-nose pliers or tweezers can be used for attaching or removing conductor wires and component leads.

• Smooth round nose pliers are also used for making wire loops.

Lead cut correctly Lead cut incorrectly

Using correctly profiled cutters

Cutter

Incorrect lead cutting using incorrectly profiled cutters

Figure 4.1: Profiles of correct and incorrect cutters for trimming leads

4.1.3Bendingtools• Bare component leads shall be bent or shaped using bending tools, including automatic bending tools, which

do not cut, nick or damage the leads or insulation.

• Components shall not be damaged by the bending process. It is good practice to use bending tools with polished finish. The preferred surface finish for shaping tools is hard chromium plating.

• Bending tools shall have no sharp edges in contact with the component leads. Typical lead bending tool is shown in Figure4.2.

6

Figure 4.2: Typical lead forming/bending tool

4.1.4Clinchingtools

Clinching tools shall not damage the surfaces of printed-circuit conductors, components or component leads.

4.1.5AntiwickingTools

Antiwicking tools shall be of a design that fits only a specific conductor gauge size and shall be marked with that

conductor gauge size.

4.1.6HoldingDevices

Tools, fixtures, and materials used to hold or restrain conductors and parts shall be of a design that will not damage

or deform the conductors, conductor insulation, or parts.

4.1.7Insulationstrippers

4.1.7.1 Mechanical Strippers

Mechanical strippers shall be of the following types:

Mechanicalstrippers used to remove insulation from stranded or solid conductor wires may be of the hand

operated or automatic high volume machine type.

Automaticpower‑drivenstrippers shall be with precision, factory-set, cutting and stripping dies and wire guards, or

Precision‑typehandstrippers with accurately machined and factory-preset cutting heads.

The conductor shall not be twisted, ringed, nicked, cut or scored by the process.

Figure 4.3: Typical mechanical wire stripper

4.1.7.2 Thermal Strippers

• Thermal insulation strippers can be used for wire insulation types susceptible to damage by mechanical strippers.

• The temperature of the stripper shall not burn, blister or cause excessive melting of the insulation.

7

• Temperature controls shall be sufficient to prevent damage to the wire or unstripped insulation.

• It is a good practice to apply thermal strippers for use with AWG 22 and thinner wire sizes where there is a possibility of the wire stretching if a mechanical stripper is used.

4.1.7.3 Chemical Stripper

• Chemical solutions, pastes, and creams used to strip wires shall be suitable for removal of the insulation to be stripped and shall not cause degradation to the wire.

• The enamel shall be removed by chemical means.

• The enamel may be removed by mechanical means provided that visual inspection using a minimum magnification of 40x is carried out to ensure that the conductor is undamaged.

4.1.8 Thermal Shunts

Thermal shunts shall be used to absorb heat from part leads as necessary to protect parts, insulating materials, and/

or previously completed connections from damage during soldering operations.

4.2 In‑ProcessStorageandHandling

Each operator performing soldering operations shall develop and implement requirements and procedures that

control conditions to prevent damage to and degradation of, parts and deliverable items.

In particular, means shall be provided to prevent damage or contamination to printed wiring terminating areas,

terminals, connectors, wire ends, or part leads during handling and storage.

Contact with bare hands shall be avoided. When handling metal surfaces that are to be soldered is unavoidable,

clean, lint-free gloves or finger cots shall be used.

Gloves and finger cots used shall not generate electrostatic charges.

Electrostatic discharge sensitive (ESDS) parts or assemblies shall be stored, handled, or otherwise processed in

accordance with Para 16.

Controlled Environmental cabinets, Desiccators, dry nitrogen purged bags or Conductive bags shall be used for such

storage.

4.3 Soldering,CleaningandInspectionEquipments

4.3.1ContactType(Solderingirons)• The size and shape of the soldering iron and bit shall not damage adjacent areas or connections during

soldering operations.

• Temperature-controlled soldering irons shall be used. The idling temperature shall be controlled within ±5.5°C. It is good practice to verify periodically the soldering iron tip temperature.

• Files shall not be used for dressing plated copper soldering-iron tips.

• A selection of bit sizes, shapes & power appropriate to each soldering operation envisaged shall be available.

• The soldering iron shall maintain the set temperature at the joint throughout the soldering operation.

• Thermal shunts shall be used to protect thermally-sensitive components.

8

• For soldering conventional electronic components on PCBs (double sided and PTH), the soldering iron bit temperature shall be between 260 °C and 280 °C. For MLBs higher bit temperatures may be used if required, but limited to 320°C maximum.

• The bit temperature up to 320°C may be used for polyimide PCBs with heat sinks, wide tracks or ground planes.

4.3.2Non‑contactTypeSolderingmachines

4.3.2.1 General

• The soldering machine shall be grounded in order to avoid electrostatic discharge.

• Shall ensure that the soldering conditions do not exceed the values given by the individual component data sheets (e.g. maximum temperature to avoid internal melting, removal of marking ink, degradation of encapsulating plastic).

• Temperature and time profiles for assembly shall be identified and approved.

• When supplemental heat is applied by hot gases, radiant energy, or any other source for aiding the hand and wave soldering process, the equipment shall be set up, operated, and maintained by personnel using established and documented procedures.

4.3.2.2Hotgasreflowmachines

Hot gas reflow machines shall conform to the following requirements;

• There shall be no relative motion between the conductors, part leads, terminals and the printed wiring board termination areas during solidification.

• Preheats an assembly with solder paste to the temperature recommended by the solder paste manufacturer prior to soldering.

• Heats the area of the assembly to be soldered to a preselected temperature between 220 °C and 250 °C as measured on the substrate surface.

• Prevents the reflow of adjacent components.

• Maintains the preselected reflow temperature within 5 °C as measured at the substrate surface.

4.3.2.3Radiation(LASER&IR)reflowsystems

Radiation reflow machines shall be of design such that the system meets the following requirements;

• Provides a controlled temperature profile and does not transmit movement or vibration into the assembly being soldered.

• Preheats an assembly with solder paste to the temperature recommended by the solder paste manufacturer prior to soldering.

• Heats the area of the assembly to be soldered using focused or unfocussed energy, to a preselected temperature that is a minimum of 12 °C above the melting point of the solder being used as measured at laminate or substrate surface.

• Maintains the preselected temperature to within 6 °C in the reflow zone during soldering.

4.3.2.4 Solder Deposition equipment

• Equipment used to deposit solder pastes shall be of a screening, stenciling, dispensing, dotting type.

• Equipment shall apply pastes of a viscosity and quantity such that the positioned device is retained on the board before and during soldering operations, ensuring self-centering and solder fillet formation.

• Equipment used to apply solder preforms shall ensure alignment of the preform with the land or device lead and termination.

9

4.3.2.5 Automatic device placement equipment

• Automatic or computer controlled equipment used for device placement shall be of the coordinate-driven pick-and-place type or of the robotic type.

• Equipment shall not generate, induce or transmit electrostatic charges to devices being placed

• The placement equipment used shall be of a type that;

o Prevents device or board damages.

o Indexes devices with respect to the circuit.

o Aligns the device leads or castellation with the board terminal areas.

4.3.3 Solder Baths

Solder baths used for degolding and pretinning shall be in accordance withTable4‑1

• Surface impurities shall be removed from the bath surface before use.

• A controlled method shall be established and implemented for the replacement of solder baths, based on either:

o Contamination: Replace the solder bath alloy when the contamination limits exceeds as given in Table4‑1.

o Time: Establish a schedule of solder-bath replacement with justification of the replacement frequency.

• Solder pots shall be capable of maintaining the solder temperature at ±5°C of the preselected temperature. Solder pots shall be grounded.

Table 4‑1 : Solder baths for degolding and pretinning

Solderbath1 Solderbath2

Use Gold dissolution Pretinning

Temperature range (°C) 240 to 260 240 to 260

Contamination limits (weight %) Au < 1 Cu < 0.25; Au < 0.2; (Cu + Au) < 0.3;

Zn, Al and Fe: Trace.

4.3.4Cleaningequipmentandsystems

Cleaning tools shall be selected based on their ability to minimize the generation of static charge. Typical cleaning

tools include natural bristle brushes, lint-free tissue, cotton swabs, etc. Steel-wire brushes, knives, erasers, emery

cloth, sandpaper and other devices that produce an abrasive action or cause contamination shall not be used.

Vapour degreaser or manual cleaning (Three-tray method) shall be used for cleaning assembled PCBs. Refer

para 8.1.2 for vapour degreasing method.

4.3.4.1 Cleanliness testing equipment

Cleaning of the printed wiring assemblies shall be carried out using solvents listed in para 5.4.1. Cleaning method

followed shall be as per para 8.1. Also assemblies shall be tested for the cleaning as per para 8.2.

4.3.5InspectionOptics(MagnificationAids)

Visual inspection shall be performed using magnification aids conforming to the following:

• Magnification aids shall be capable of rendering true colors, proportional dimensions, and adequate resolution at the chosen magnification to perform the specified inspection.

10

• The light source shall provide shadow-less illumination on the area being viewed.

• Shall have anti-glare light source (preferably white light)

• Each soldered connection shall be visually inspected in accordance with the criteria specified in the clauses below.

• Inspection shall be aided by magnification appropriate to the size of the connections between 10X to 40X with stereo zoom microscopes or similar devices like AOI.

11

5 MATERIALS

5.1 General• Material selection shall be performed in accordance with approved material list of ISRO centres or ISRO

Declared Material List

5.2 Solder

5.2.1SolderPreform• For soldering, ribbon, wire, solder bar or preforms shall be used provided that the alloy meets the requirements

as given in Table5‑2.• For degolding and pretinning, solder alloys shall be supplied without flux.

5.2.2SolderComposition

The solder alloy with their composition and application are given in Table5‑1

Table5‑1:Guidetochoosesoldertypes

SoldertypeMeltingrange(°C)

UsesSolidus Liquidous

63 tin solder (eutectic) 183 183

Soldering printed circuit boards where temperature limitations are critical and in applications with an extremely short melting range. Preferred solder for surface mount devices.

62 tin silver loaded 179 190Soldering of terminations having silver and or silver palladium metallization. This solder composition decreases the scavenging of silver surfaces.

60 tin solder 183 188 Soldering electrical wire/cable harnesses or terminal connections and for coating or pretinning metals.

96 tin silver (eutectic) 221 221 Can be used for special applications, such as soldering terminal posts.

75 indium lead 145 162Special solder used for low temperature soldering process when soldering gold and gold-plated finishes. Can be used for cryogenic applications.

70 indium lead 165 175 For use when soldering gold and gold-plated finishes when impractical to degold.

10 tin lead 268 290For use in step-soldering operations, to avoid reflow of initial solder on making the second joint (limited to connections internal to devices).

12

Table 5‑2 : Chemical composition of soldersDesignation Sn Pb In Sb Ag Bi Cu Fe Zn Al As Cd Others

min% - max %

max % min % – max % max % min %– max %

max % max % max % max % max % max % max % max %

63 tin solder 62.5-63.5 remain - 0.05 - 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

62 tin silver loaded 61.5-62.5 remain - 0.05 1.8-2.2 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

60 tin solder 59.5-61.5 remain - 0.05 - 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

96 tin solder remain 0,10 - 0.05 3.5-4.0 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

75 indium lead max 0.25 remain 74.0-76.0 0.05 - 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

70 indium lead 0.00-0.10 remain 69.3-70.7 0.05 - 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

10 tin lead 9.0-10.5 remain - 0.05 - 0.10 0.05 0.02 0.001 0.001 0.03 0.002 0.08

5.2.3Maintenanceofpastepurity• When purchased premixed or mixed in house, the purity of solder paste shall be maintained.

• Manufacturers’ instructions shall be applied for the handling and storage of containers of solder paste purchased premixed.

• Refrigerated solder paste shall reach room temperature before opening the container.

• Neither paste purchased premixed nor paste mixed in-house shall be used if the use-by date or shelf life recommended by the manufacturer of the paste or paste constituents has expired.

• When the solder paste’s shelf life has expired, it shall not be used unless, relifing is performed.

• Tests that include visual inspection and viscosity measurements (according to the manufacturer’s recommendations) shall pass successfully.

• When relifing is performed, and the material passes the specified tests, the new shelf life shall be half the initial shelf life.

• Tools used for removing solder paste from the container shall not contaminate the paste dispensed or that remaining within.

5.3 Flux

5.3.1Rosin‑basedfluxes

The use of liquid rosin,mildly activated (RMA) flux is recommended for the soldering, wicking-off procedures, for

rework of soldered connections, tinning operations and reflow soldering. Liquid flux used with flux cored solder

shall be chemically compatible with the solder core flux and with the materials with which it will come into contact.

Flux shall conform to requirements of ANSI-J-STD-004.

Note: Flux residue shall be cleaned at the earliest as the residues may lead to performance deterioration of

the assembly.

13

5.3.1.1Applicationofflux

• The quantity of flux used shall be such that the solder joint is in accordance with acceptable criteria as pera. 9.5

• When flux-cored solder is used, it shall be positioned such that the flux flows and covers the components to be joined as the solder melts.

• When an external liquid flux is used in conjunction with flux-cored solders, the fluxes shall be compatible.

• When external flux is used, liquid flux shall be applied to the surfaces to be joined prior to the application of heat.

5.4 CleaningSolvents• Shall be electrically non-conductive and non-corrosive.

• Shall not dissolve or degrade the quality of parts or materials.

• Solvents shall not remove component identification markings.

• Solvents showing visual evidence of contamination or decomposition shall not be used.

• Solvents shall not be used such that dissolved flux residue contaminates electrical contact surfaces.

5.4.1ApprovedCleaningSolvents

The following solvents are acceptable for cleaning electronic assemblies during soldering operations.

• Isopropyl alcohol, electronic grade, 99.5% pure by volume.

• Trichloro-trifluoro-ethane, clear 99.8% pure. This shall not be used when assembly contains silicone rubber elastomer.

• Aziotropic mixture of the above two solvents as below shall be used.

o 50% weight of Isopropyl alcohol and 50% by weight of trichloro-trifluoro-ethane.

• De-ionised water with resistivity greater than 1.0 M ohms.

• Water-based solvents containing saponifiers shall not be used.

5.5 Flexibleinsulationmaterials• Materials shall have low outgassing properties and shall meet the requirements of Declared Material List of

respective centre.

• The following flexible insulation materials may be used in a space environment:

o ETFE, FEP and PTFE.

o Polyolefin and Kynar® sleeving for heat-shrinkable wire terminations.

o Irradiated polyethylene, fluorinated resin and polyimide.

• PTFE materials shall not be heated above 250 °C.

5.6 Terminals

5.6.1TerminalMaterial• Terminals shall be made from one of the following materials:

o Bronze (copper/tin) alloys. It is good practice to use bronze terminals.

o Brass (copper/zinc) alloys.

• When a brass terminal is used it shall be plated with a barrier layer of copper or nickel of 3 µm to 10 µm.

Note-1: A barrier layer is necessary on brass items to prevent the diffusion, and subsequent surface oxidation, of zinc.

14

Note-2: It is good practice to use a copper barrier layer on brass terminals because nickel is magnetic and has poor solderability.

• Terminals with coatings on the mounting surface shall be rejected if the coatings loosen in subsequent soldering operations.

5.6.2Typeofterminal• Terminals on PCBs shall not be tin, silver or gold plated.

• Tin, silver or gold-plated finishes shall be replaced using pretinning.

5.6.3Shapeofterminals

Bifurcated and turret terminals shall have ledges or grooves to allow both the accurate location of connecting wires

and the flow of solder.

5.7 Wires• Wire shall be selected from Declared Material List of respective centre.

• Chemical stripping materials shall be completely neutralized and be cleaned such that there are no residues from the stripping, neutralizing, or cleaning steps.

• The enamel shall not be visually contaminated by the stripping process.

5.8 PCBs

5.8.1 Boards

Boards shall be made of materials, and manufactured, according to the requirements of ISRO-PAX-300.

5.8.2Goldfinishonconductors• De-golding of conductors shall be in accordance with para 6.3.2

5.8.3Classificationofboards• Printed circuit boards and substrates shall be selected from the classes given in Table5‑3• The class of board selected shall have a coefficient of thermal expansion (CTE) characteristic compatible with

the CTE of the devices.

• The warp and twist of the printed circuit multilayer board shall be in accordance with ISRO-PAX-304.

Table5‑3:Classificationofprintedcircuitboardsandsubstrates

Class Description CTE (10‑6/ 0C) Remarks

1 Non-compensated printed board 14 – 17 Epoxy-woven glass and

polyimide-woven glass

2 Ceramic 5 –7 Alumina and Aluminium Nitride

3 Compensated printed board 11 – 13

Epoxy / Polyimide resin with low CTE fibers such as aramid, quartz or carbon

15

4 Compensated printed board 9 – 11

CTE compensated boards use standard construction and are compensated with materials such as distributed plane consisting of low CTE material

5 Compensated printed board 5 – 9

CTE compensated boards use standard construction and are compensated with materials such as low CTE substrate or cores. Typical cores are copper plated invar and copper plated molybdenum

5.9 Adhesives(pottingcompounds&heatsinking),Encapsulants&Conformalcoatings

• Limited shelf life items shall be stored and controlled in accordance with the material manufacturer’s recommendations or in accordance with the manufacturer’s documented procedures for controlling shelf life and shelf life extensions where permitted.

• Adhesives shall be dispensable, non-stringing, and have a reproducible dot profile after application.

• The uncured (tack) strength shall be capable of holding devices in place during handling prior to cure.

• Adhesives, encapsulants and conformal coatings shall be non-corrosive to devices and substrates.

• No materials that emit acetic acid, ammonia, amines, hydrochloric acid and other acids shall be used. Such compounds can cause stress corrosion cracking of part leads.

• Adhesives, encapsulants and conformal coatings shall conform with the outgassing requirements ISRO DML requirements.

• Shrinkage of resin during cure and repair shall not degrade the coated articles.

• Materials covered by this clause shall be individually assessed in accordance with DML, when flammability requirements are applicable.

• Stress relief of device leads shall not be reduced by the encapsulant or conformal coating.

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6 COMPONENT MOUNTING

6.1 Principlesofreliablesolderedconnections

The following are the general principles to ensure reliable soldered connections:

• Reliable soldered connections are achieved by using proper design, having control of tools, selecting the right materials, trained & qualified personnel, applying processes with precaution in a controlled work environment and taking into account inspection requirements.

• The basic design concepts to ensure reliable connections and to avoid solder joint failure are as follows:

o Stress relief is an inherent part of the design, which reduces detrimental thermal and mechanical stresses on the solder connections.

o Where adequate stress relief is not possible, a method of solder-joint reinforcement is incorporated.

o Materials are selected such that the mismatch of thermal expansion coefficients is a minimum at the constraint points in the component-mounting configuration.

o Materials and processes which result in the formation of brittle intermetallics, such as soldering to gold using tin-lead alloy, are avoided.

o The assembled substrates are designed to allow inspection.

6.2 Preparatoryconditions

6.2.1Facilitycleanliness• Personnel facilities shall be separated from the soldering areas.

• Furniture shall be arranged to allow thorough cleaning of the floor.

• Areas used for soldering shall be kept free from contaminants.

• Working areas shall be kept free from any tools or equipment not used for the current task.

• Working surfaces shall be covered with an easily-cleaned hard top or have a replaceable surface of clean, non-corrosive, silicone-free paper.

• Tools used during soldering operations shall be free of visible contaminant.

• However overall clean room requirement shall be as per para 3.

6.2.2PreparationofComponentsleads,conductors,terminalsandsoldercups

6.2.2.1 Stripping tools

Stripping tools or machines shall be in accordance with section (tools).

6.2.2.2 Damage to insulation

• The remaining conductor insulation shall not be damaged by the insulation removal process.

• Conductors with damaged insulation shall not be used.

• Insulation damage includes nicks, cuts, crushing and charring.

• The operation of mechanical stripping tools can leave slight pressure markings in the remaining conductor insulation. This effect is considered to be normal.

• The insulation material shall not be charred by thermal stripping.

• However Discoloration of the insulation material after thermal stripping is normal.

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6.2.2.3 Damage to conductors

• The conductor shall not be damaged by the insulation removal process.

• Conductor damage includes twisting, ringing, nicks, cuts or scores.

• Part leads and other conductors that are reduced in cross-sectional area by the insulation removal process shall not be used.

• Copper visibility shall not be accepted.

6.2.2.4Maximuminsulationclearance

The maximum insulation clearance, measured from the solder joint, shall be as stated in Table6‑1

In the case of the assembly of coil winding wires, maximum insulation clearances may be exceeded provided that

electrical clearances are maintained.

6.2.2.5 Minimum insulation clearance

For PTFE-insulated wire, the minimum distance between the insulation and the solder fillet shall be 1 mm.

The minimum clearance distance for PTFE insulation accommodates cold flow.

The minimum insulation clearance shall not result in insulation imbedded in the solder joint.

The minimum insulation clearance shall not obscure the contour of the conductor at the termination end of the

insulation.

This table is not applicable for high voltage cables.Table 6‑1: Clearances for insulation

Wirediameter (AWG)

Conductor diameterwithoutinsulation,d(mm)

Insulationclearance (minimum)

Insulationclearance (max.)

32 to 24 0.200 to 0.510 d 4 × d

22 to 12 0.636 to 2.030 d 3 × d

≥ 10 ≥ 2.565 d 2 × d

6.3 Surfacestobesoldered

6.3.1Cleaning

Before assembly, devices, wire, terminal and connector contacts shall be visually examined for cleanliness, absence of

oil films and freedom from tarnish or corrosion.

Conducting surfaces to be soldered shall be cleaned using approved solvents specified in 5.4.1

Abrasives shall not be used for surface preparation except in the case of gold-plating on substrates and devices.

Abrasives can include pumice-impregnated erasers.

6.3.2De‑goldingofgold‑platedleadsandterminals

63/37 Tin-lead solders shall not be used for soldering to gold finish. Recommended solders for gold plated surfaces

are listed in para 5.2.2. Also use anti wicking tweezers wherever possible to avoid thermal damage.

18

6.3.3Methodsfordegolding

6.3.3.1 Solder bath method

• Solder bath for de-golding process is described in para 4.3.3.• Gold-plated component leads and terminals shall be dipped into solder bath 1 for 2 to 3 seconds.

• Unless otherwise specified, solder bath contamination shall be monitored periodically (Once in 6 months)

6.3.3.2 Soldering iron method

• Solder shall be melted onto the conductor using a heated soldering iron.

• Solder shall be wicked-out using stranded wire/ wicking tape.

6.3.3.3 Solder cup method: to dissolve the gold plating

• Solder shall be melted within the gold-plated solder cup. The liquid solder dissolves the gold plating.

• The liquid solder shall be wicked-out using stranded wire/ wicking tape.

6.3.3.4 Constraints on degolding and pretinning methods

• The maximum temperature rating of the component, stated by the manufacturer, shall not be exceeded.

• Thermal shunts, in accordance with para 4.1.8 may be used.

• Components having glass-to-metal lead seals shall be preformed with tools as per para 4.• Liquid solder shall not come into contact with the component body or its glass meniscus.

• The limit of the pretinned coating shall not be less than 0.75mm from any lead-to-glass seal of the component package.

6.3.4Pretinningofstrandedwires• Solder shall penetrate to the inner strands of stranded wire.

• Solder shall not obscure the wire contour at the termination end of the insulation.

• Anti-wicking tools in accordance with para 4.1.5 may be used.

• Pretinning shall not degrade the characteristics of the wire.

• Flow of solder (wicking) beyond the insulation can reduce the flexibility of the wire hence not acceptable

• The insulation shall not be damaged by the pretinning.

• Flux shall be removed by means of a cleaning solvent (Refer para 5.4.1).

6.3.4.1 Solder bath method

• Solder baths for pretinning shall be in accordance with para 4.3.3• The insulation shall be removed in accordance with para10.7• Rosin Mildly Activated (RMA) flux shall be applied to the end of the strands.

• The fluxed end of the wire shall be dipped into solder bath 2 for a time between 2 and 3 seconds.

• Pretinning promotes Solderability and prevents untwisting or separation of stranded wires.

6.3.4.2 Soldering iron method

• Stranded wires may also be pre-tinned by applying solder to the wire using a heated soldering-iron tip.

• Solder shall be melted onto the conductor using a heated soldering iron.

6.3.5Pre‑tinningofComponentleadsandsolid‑wireconductors

6.3.5.1 Solder bath method

Solder baths for pretinning shall be in accordance with para 4.3.3. Component leads with unacceptable Solderability

19

in accordance with the component procurement specification and solid wires shall be pre-tinned by dipping into

solder bath 2 for a period between 2 and 5 seconds. Also use anti wicking tweezers wherever possible to avoid

thermal damage.

• It is good practice to observe an immersion period between 3 and 4 seconds.

• A slow, vertical and smooth withdrawal of the component lead from the bath promotes an even coating.

• The cross-sectional area of conductors shall not be reduced by dissolution into the solder bath.

The component shall cool before cleaning. Rapid cooling by contact with cleaning solvents can crack packages or

glass-to-metal seals.

6.3.5.1 Soldering‑iron method

Solder shall be melted onto the conductor using a heated soldering iron.

6.3.6Preparationofthesolderingbit

6.3.6.1 Bit

• The bit shall be fitted in accordance with the equipment manufacturer’s specification.

• Oxidation products shall be removed from the bit. Build up of oxidation products can reduce the ability of the tip to transfer heat.

• Plated tips shall be examined for cracking. Cracked platings allow the liquid solder to alloy with and erode the underlying copper, forming intermetallics which reduce heat transfer and lead to unacceptable joints.

• Prior to soldering, solder present on the surface shall be removed when the iron is hot by wiping the bit with moist, lint-free, sponge material.

• Bits with cracked platings shall be removed from the soldering area.

6.4 Storage

6.4.1 Components• Storage facilities shall protect components from contamination and damage.

• Storage boxes and bags shall be made of materials which do not degrade the solderability of the components.

• Storage materials shall not contain amines, amides, silicones, sulphur or polysulphides.

6.4.2 PCBs

PCBs shall be stored in controlled environment or desiccators.

6.4.3StorageofwiredPCBs• The baking process shall be carried out again according to Table6‑2 when assembled PCBs are stored in

ambient conditions for more than 24 hours prior to soldering.

• Dry nitrogen, dry air, vacuum or desiccants may be used to extend the storage period.

• Additional baking if required may be done as and when required

6.5 PreparationofPCBsforsoldering• PCBs shall be cleaned using Approved cleaning solvent

• PCBs shall be demoisturized in accordance with Table6‑2

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Table 6‑2 : Baking conditions

Sr. No. DescriptionBakingCondition

BarePCB(PWB) AssembledPCB(PWA)

1. Double sided / Multilayer PTH PCB

93°C, 4Hrs. 65°C, 4 Hrs.

2. Polyimide / Flex-Rigid MLB PCB

120°C, 4 Hrs. 65°C, 4 Hrs.

3.

PWA : Printed Wiring Assembly

PWB : Printed Wiring Board

Vacuum baking at 3mm of Hg / 3 torr may be used for PWAs at 650C, 2.5 Hours

6.6 PartsMounting

6.6.1Generalrequirements

Parts, terminals, and conductors shall be mounted and supported as prescribed herein. Dimensions provided in this

chapter are for acceptance and/or rejection criteria only. Unusual environmental applications require special design

measures to provide necessary environmental survival capability. Such measures shall be detailed on the appropriate

engineering documentation. Engineering documentation shall prescribe which alternative approach is selected, as well

as potting compounds and conformal coating requirements. They shall also detail any special mounting arrangements

or design requirements not fully covered herein.

6.6.2StressRelief

Stress relief shall be incorporated into all leads and conductors terminating in solder connections to provide freedom

of movement of part leads or conductors between points of constraint. Leads shall not be temporarily constrained

against spring-back force during solder solidification so that the joint is subject to residual stress.

6.6.3Stressreliefofcomponentswithbendableleads• Stress relief shall be incorporated into:

o Soldered leads and conductors,

o Interfacial connections.

o Stress relief provides freedom of movement for component leads or conductors between points of constraint.

o Stresses can arise between points of constraint due to mechanical loading or temperature variations.

o Stress relief methods, shown in Figure6.1 o The assembly of TO-39, TO-59 and CKR-06 packages shall be performed in accordance with Figure6‑2

when assembled without stress relief.

o Stress relief designs shall not damage the assembly.

o Long lead lengths or large loops between constraint points can vibrate and damage the assembly.

o Leads shall not be temporarily constrained against spring-back force during soldering so that residual stresses are not produced in the lead material or solder joint.

21

o Solder fillets shall not cover the stress relief bends.

o CKR-06 and similar packages shall be adhesively potted in accordance with Figure6‑2. o TO 39 and TO 59 packages may have an underfill as shown inFigure6‑2

(e) Alternative methods

Pad

C

C SR

(a) Clinched lead (b) Stud-mounted lead

(c) Offset lap joint

C SR

C

C C

SR

C

C

SR

SR

(d) Stud-mounted leads

C

C SR

Transistor mounting pad

Plated-through hole

Figure 6.1 : Methods for incorporating stress relief with components having bendable leads

6.6.4Dualin‑linepackage

A Dual in line package (DIP) used in conjunction with printed wiring assemblies shall be mounted in accordance

with the following requirements.

22

Figure 6.2 : Assembly of under filled TO-39 and TO-59, and adhesively staked CKR06

Figure 6.2 : Assembly of under filled TO-39 and TO-59, and adhesively staked CKR06

DIP devices up to DIP 24 may be assembled without additional stress relief, provided that the tapered portions of

the leads are clear of the component-side lands of the plated-through holes.

• The base of the device shall be spaced from the surface of the printed wiring board a minimum of 0.25 mm and a maximum of 2.0 mm.

• The base of the device shall be parallel to the surface of the printed wiring board within one percent of the length of the DIP and shall not be greater than 0.2 mm.

• DIP devices shall not be mounted in sockets or other plug in devices, which rely upon contact pressure for part retention. Leads of the DIP device shall be soldered in place.

• The lead-to-body seals of mounted devices shall not be damaged. Body chip outs that extends to or into the glass seal and chip outs that expose a normally encased area of lead are unacceptable. Hairline cracks in either the seal or the body are not acceptable as in Figure6‑3.

• In order to achieve acceptable stand-off, a shim can be used.

23

Figure 6.3 : Not acceptable body and seal conditions Figure 6.3 : Not acceptable body and seal conditions

6.6.5PartPositioning

Parts shall be positioned in compliance with the engineering documentation and mounted in accordance with the

requirements specified herein.

Parts shall be mounted so that terminations of other parts are not obscured. When this is not possible, interim

assembly inspection shall be carried out to verify that the obscured solder joints meet the requirements herein.

Parts having conductive cases mounted over printed conductors or which are in close proximity with other

conductive materials shall be separated by insulation of suitable thickness. Insulation shall be accomplished so that

part identification markings remain visible and legible.

6.6.6VisibilityofMarkings

Where possible, parts shall be mounted in such a manner that markings pertaining to value, part type, etc., are visible

and has same orientation of left to right / bottom to top). For parts marked in such a way that some of the marking

will be hidden regardless of the orientation of the part, the following shall be the order of precedence for which

markings shall be visible.

• Polarity

• Traceability code (if applicable).

• Piece part value and type.

6.6.7Heavycomponents• Components weighing more than 7g per lead shall be supported by either of the following methods:

o Adhesive compounds in accordance with Chapter 5 and12.

o Mechanical methods such as Lacing.

• The support method shall not impose stresses that result in functional degradation or damage to the part or assembly.

• The support method shall not damage stress relief designs.

• Component which requires fastening shall be fastened first then soldered and not vice versa.

6.6.8Metal‑casecomponents• Metal-case components shall be electrically insulated using space-approved material under the following

conditions:

24

o Mounted over printed conductors.

o In contact with a conductive material which in turn provide electrical connection to other elements.

Metal-cased components shall not be mounted over soldered connections.

Component identification marks shall not be obscured by the insulation.

6.6.9 Glass Encased Parts

Glass encased parts such as diodes, thermistors, or resistors shall be covered with transparent resilient sleeving

or other approved material when epoxy material is used for potting, conformal coating, or encapsulating or where

damage from other sources is likely. The epoxy material shall not be applied directly to glass. When using heat

shrinkable sleeving, extreme care should be taken to prevent part damage due to excessive heat or shrinkage of the

sleeving.

Table 6‑3 : List of material used for isolation

Device Materialforsleeving/isolation

Glass diodes / Glass bodied components Polyolefin sleeves /Kynar sleeves / RTV

Glass-encased parts shall be enclosed with sleeving when epoxy material is used for potting, conformal coating or

encapsulating. Polyurethene Epoxy material shall not be applied directly to the glass.

Glass-encased components may be enclosed in resilient transparent sleeving or in heat-shrinkable sleeving. Heating

and shrinkage of sleeving can damage glass-encased components. Hence end of the sleeve may be shrunk with

soldering iron tip to arrest the slippage of sleeve.

When silicon based conformal coating is used, glass bodied components need not be sleeved.

6.6.10Hookup/JumperWire

Hookup wire (single strand) / multi strand jumper wire shall be supported by a means other than the solder

connections or conformal coating if wire length exceeds 2.54cm (1 inch). Attachment to a surface by potting is

considered adequate support.

• Hook-up wire shall be supported at intervals not exceeding 25 mm.

• The support shall be provided by Potting.

• The wire shall be covered with shrinkable sleeve if wire cross over the conductor pattern.

• Use PTFE insulated jumper wires

6.6.11LeadBendingandCutting

During bending or cutting, part leads shall be supported on the body side to minimize axial stress and avoid damage

to seals or internal bonds. The distance from the bend to the end seal shall be approximately equal at each end

of the part. The minimum distance from the part body or seal to the start of the bend in a part lead shall be 2

lead diameters for round leads and 0.5mm (0.020 inch) for ribbon leads Ref. Figure6.4. The stress relief bend

radius shall not be less than the lead diameter or ribbon thickness. The direction of the bend should not cause the

25

identification markings on the mounted part to be obscured. Where the lead is welded (as on a tantalum capacitor)

the minimum distance is measured from the weld.

• Part leads shall be formed so that they may be installed into the holes in the PWB without excessive deformation that can stress the part body or end seals.

• Soldered terminations shall not be cut after the soldering operation

• All leads shall be tinned and formed before mounting the part. Where possible, part leads that is subject to stress corrosion cracking (e.g. kovar leads), shall be preformed and trimmed prior to tinning.

• Whether formed manually or by machine, part leads shall not be mounted if they show evidence of nicks or deformation. Smooth impression marks (base metal not exposed) resulting from tool holding forces shall not be a cause for rejection.

• Tempered leads (sometimes referred to as pins) shall not be bent nor formed for mounting purposes since body seals and connections internal to the part may be damaged. Tempered leads or leads with a diameter of 1.27mm (0.05 inch) or more shall not be cut with diagonal cutters or other tools that impart shock to connections internal to the part.

Figure 6.4: Minimum lead bend

6.6.12 Coated Parts• Parts shall be mounted so that the insulating coating meniscus applied by the manufacturer on the leads does

not enter the mounting hole or soldered connection.

6.6.13Splices• Broken or damaged conductors, part leads, or printed wiring conductors shall not be spliced.

6.6.14Location• Part bodies shall not be in contact with soldered terminations.

6.7 PartsMountedtoPWB’s

Solder terminations shall be visible for inspection after soldering. In the cases where visual inspection cannot be

accomplished, a non destructive method of inspection shall be performed (e.g., X-ray, endoscope or fiberscope or

suitable apparatus).The non destructive method of inspection to be used shall be documented and approved by QA,

ISRO Center prior to use.

26

6.7.1AxialLeadMounting

Axial leaded parts shall be mounted as follows:

6.7.1.1 Horizontal Mount

Parts intended for horizontal mounting shall be parallel to, and in contact with, the mounting surface

(see Figure6.6), or as specified in the assembly documentation. Slight angularity is permissible. When assemblies

are to be conformal coated with silicon, a small gap, say 0.3 to 0.5mm is acceptable.

Figure 6.5 : Horizontal Mount

6.7.1.2 Radial Lead Mounting

Platedthrough‑hole: The part body shall be mounted with at least 0.5mm (0.020 inch) to a maximum of 1.27mm

(0.050 inch) above the PCB and shall allow inspection of the solder joint. The part body includes any extension such

as coating meniscus, solder seal or weld bead (see Figure6.6A).

Non‑plated‑through‑hole: The part body may be mounted flush with the PCB surface and terminated with an

off-the-pad lap solder joint (See Figure6.6B).

Figure 6.6 : Radial Leaded Parts

6.7.1.3 Hole Obstruction

Parts shall not be mounted such that they obstruct solder flow or prevent cleaning of the topside termination

areas.

27

Figure 6.7 : Obstruction of solder flow (Not acceptable)

Figure 6.8 : Stress Relief Part Termination

Figure 6.7 : Obstruction of solder flow (Not acceptable)

6.7.1.4PartswithLeadsTerminatingonOppositeSides

Stress relief shall be provided in the part lead between the part body and solder terminations (Figure6.8). The lead

may be terminated by clinch, straight-through, or lap configuration.

Figure 6.7 : Obstruction of solder flow (Not acceptable)

Figure 6.8 : Stress Relief Part Termination

Figure 6.8 : Stress Relief Part Termination

6.7.1.5PartswithLeadsTerminatingontheSameSide

Stress relief shall be provided by forming the part leads at a bend angle to the PWB of not more than 95°nor less

than 45° (Figure6.9).

2d min.

45° to 95°

Figure 6.9 : Bend Angle

Figure 6.9 : Bend Angle

28

6.7.2PartsLeadTerminations,PrintedWiring

6.7.2.1 Part Lead Terminations

Part leads shall be of the lap, clinched, or straight-through configuration as defined by the engineering documentation

and shall be terminated in accordance with paragraphs 6.7.2.1 through 6.7.2.2 No more than one item, whether

conductor or part lead, shall be inserted in any one hole.

6.7.2.1.1 Lapped Terminations

Lapped terminations consist of both round and flat ribbon leads. It is preferred that leads be seated in contact with

the termination area for the full length of the foot. Separation between the foot of the lead and the surface of the

termination area shall not exceed 0.25mm (0.010 inches) (see Figure6.10).

Figure 6.10 : Lapped Lead Height above Board

Figure 6.10 : Lapped Lead Height above Board

6.7.2.1.2 Lapped Round Leads

The round lead shall overlap the solder pad a minimum of 3.5 times the lead diameter to a maximum of 5.5 times

the lead diameter, but in no case shall the length be less than 1.27mm (0.050 inch). The cut-off end of the lead

shall be no closer than ½ the lead diameter to the edge of the solder pad. Only that portion of the lead extending

to the part body or to another soldered connection shall be beyond the solder pad (Figure6.11A). For lapped

terminations where the part body is on the same side of the PWB as the termination area, a heel fillet is mandatory

(Figure6.11B).

29

Figure 6.11 : Lapped Round Termination Figure 6.11 : Lapped Round Termination

6.7.2.1.3 Lapped Ribbon Leads

The ribbon lead shall overlap the solder pad a minimum of 3 lead widths to a maximum of 5.5 lead widths. Only

that portion of the lead extending to the part body or to another soldered connection shall be beyond the pad. The

cut-off end of the lead shall be a minimum of 0.25mm (0.010 inch) from the end of the pad. One edge of the lead

may be flush with the edge of the solder pad. There shall be sufficient area around two of the three lead edges to

accommodate solder filleting (see Figure6.12).In instances where ribbon leads are less than 0.5mm (0.020 inch) in

width, ribbon overlap shall be no less than 1.27mm (0.050 inch). For lapped terminations where the part body is on

the same side of the PWB as the termination area, a heel fillet is mandatory (Figure6.12).

6.7.2.1.4 Clinched Lead Terminations

The length of the clinched portion of conductors and part leads shall be at least ½ the largest dimension of

the solder pad or 0.78mm (0.031 inch), whichever is greater. Lead overhang shall not violate minimum electrical

spacing requirements. The lead shall be bent in the direction of the longest dimension of the solder pad. If the

pad dimensions are not sufficient, the lead shall be bent in the direction of the printed wire path (Figure6.13).

There shall be sufficient solder pad area extending beyond the sides of the lead to accommodate solder filleting.

Fully clinched leads are defined as leads bent between 20°and 40° from a horizontal line parallel to the PWB

(Figure6.14). Non bendable leads shall not be clinched.

30

Figure 6.12 : Lapped Ribbon Leads

Figure 6.12 : Lapped Ribbon Leads

Figure 6.13 : Clinched Termination

Figure 6.14 : Lead Bend

Figure 6.13 : Clinched Termination

31

Figure 6.13 : Clinched Termination

Figure 6.14 : Lead Bend

Figure 6.14 : Lead Bend

6.7.2.2 Straight‑Through Lead Terminations

Part leads terminated straight through the PWB shall extend a minimum of 0.5mm (0.020 inch) and a maximum

of 2.29mm (0.090 inch) (Figure6.15) .The minimum lead length shall be determined prior to soldering (actual

measurement is not required except for referee purposes). Straight-through leads may be bent up to 30° from a

vertical plane to retain parts during the soldering operation(Figure6.16). Non-bendable leads shall not be bent.

Figure 6.15: Straight-Through Termination

Figure 6.16: Straight-Through Lead Retention

Figure 6.15: Straight-Through Termination

Figure 6.15: Straight-Through Termination

Figure 6.16: Straight-Through Lead Retention Figure 6.16: Straight-Through Lead Retention

32

6.7.2.3 Consideration for Conformal coating and encapsulation

Coatings compounds shall not bridge stress relief loops or bends at terminations in component leads or

connecting wires.

Stress relief of device leads shall not be impaired by encapsulants or conformal coatings.

6.7.3Leadbendingrequirements

6.7.3.1 Conductors terminating on both sides of a non‑plated‑through hole

Stress relief shall be provided in the component lead on both sides of the PCB in accordance with Figure6.17(a)

When a solid hook-up wire is used to interconnect solder terminations on opposite sides of a PCB, stress relief shall

be provided in the wire between the two terminations in accordance with Figure6.17(b)

C

C C

C

SR

SR SR

C

(a) (b)

Figure 6.17: Leads with solder termination on both sides Figure 6.17: Leads with solder termination on both sides

6.7.4MountingofterminalstoPCBs

Swage-type terminals, designed to have the terminal shoulder soldered to printed conductors, shall be secured to

single-sided PCBs by a roll swage in accordance with Figure6.18(a).

Swage-type terminals that are mounted in a plated-through hole shall be secured to the PCB by an elliptical funnel

swage in accordance with Figure6‑18(b).

An elliptical funnel swage enables complete filling of the plated-through hole with solder.

The PCB shall not be damaged by the swaging process.

After swaging, the terminal shall be free from circumferential splits or cracks.

After swaging, the terminal may have a maximum of three radial splits or cracks, provided that the splits or cracks

do not extend beyond the swaged area of the terminal and are a minimum of 90° apart.