guidelines for final finishes of electronic enclosures - ipc · ipc-a-630 draft september...

IPC-A-630

Draft September 2009January 2010

1

Guidelines for Final Finishes of Electronic Enclosures

IPC-HDBK-630 Guidelines for Design, Selection and Application of Electronic Enclosures

1 SCOPE

1.1 1.1 Introduction

Electronic enclosures are used to protect and provide form to the products which contain

printed board assemblies (PBA‟s), cable and wire harness assemblies, and other

electronics. The designer and the users of electronic enclosures for various types of

environments should be aware of the properties of various types of materials and their

interactions with electronics to protect the electronics in the end-use environment for its

design life of the PBA (or beyond). This document has been written to assist the

designers and users of electronic enclosures in understanding the design, fabrication

assembly and test of materials, sub-assemblies used within electronic enclosures.

characteristics of various materials which are used in assembly and fabrication of the

electronic enclosures. Understanding and accounting for these materials can ensure the

reliability and function of electronics.

1.2 Purpose

The purpose of this handbook is to assist in design, assembly and test of electronic

enclosures. This handbook attempts to cover from start to finish the assembly process

including: fabrication, finishing, assembly and testing. This handbook represents the

compiled knowledge and experience of the IPC Electronic Enclosure Handbook Task

Group. It is not enough to understand the properties of the various materials; the user

needs to understand what is to be achieved by set of selected materials within the end use

environment and how to verify that the desired results have been realized.

1.32 Scope

Electronic Enclosures, for the purpose of this document, is defined as a chassis, box, top

level or other designation used to describe the envelope which surrounds the electronic

assemblies or equipment in the final, end-use environment.

It is the responsibility of the user to determine the suitability, via appropriate testing, of

the selected electronic enclosure and application method for a particular end use

application. An electronic enclosure may have several functions depending on the type of

application. The most common is are:

• Tto protect electronic assembliesy from end use environment, such as moisture,

vibration, shock, EMI, and other movements detrimental to electronic assemblies.

• To reduce or eliminate electromagnetic interference (EMI) from/to the

environment

•

Formatted: Outline numbered + Level: 2 +Numbering Style: 1, 2, 3, … + Start at: 1 +Alignment: Left + Aligned at: 0" + Indent at: 0.25"

Formatted: Indent: Left: 0"

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1.3 Purpose

The purpose of this handbook is to assist in design, assembly and test the designers of

electronic enclosures. and assembly operations. This handbook describes design criteria

and assembly requirements for the manufacture of electronic enclosures. This handbook

attempts to cover topics from start to finish in the assembly process including:

fabrication, finishing, assembly and testing. final finishes of metals, fasteners, adhesives

and other methods used to install electronic assemblies into enclosures, inspection and

testing, packaging and handling, etc. This handbook represents the compiled knowledge

and experience of the IPC Electronic Enclosure Handbook Task Group. It is not enough

to understand the properties of the various materials; the user needs to understand what is

to be achieved by set of selected materials within the end use environment and how to

verify that the desired results have been realized.

1.4Applicability

1.4.1 Aerospace and Defense

2.0 Applicable Document

2.1 Referenced Documents: The following documents form a part of this standard to the

extent specified herein. In the event of a conflict between the text of this document and

the references cited herein, the text of this document takes precedence.

2.1.1 Military / Federal

MIL-DTL-5541

MIL-A-8625

MIL-PRF-23377

MIL-PRF-53039 (CARC Requirements)

MIL-PRF-64159

MIL-C-15328 (zinc chromate)

MIL-HDBK-454 General Guidelines for Electronic Equipment

MIL-HDBK-60 Threaded Fasteners – Tightening to Proper Tension

FED-STD-H28/2 Screw Thread Standards for Federal Services Section 2, Unified

Inch Screw Threads-UN and UNR Thread Forms

2.1.2 Industry

ASTM D 523

ASTM E 1499

ASTM D 1729

ASTM D 2244

TT-E-529

TT-E-479

AS 1310 Fastener Torque for Threaded Applications, Definitions of

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ISO 6789 Assembly Tools for Screws and Nuts – Hand Torque Tools –

Requirements and Test Methods for Design Conformance Testing, Quality

Conformance Testing and Recalibration Procedure

ASME B107.14M Hand Torque Tools

2.2 DEFINITIONS

The following definitions are common terms used in industry and in this document and

have been taken from AS 1310. Reference AS 1310 for terms not covered herein. IPC-T-

50.

2.2.1. APPLIED TORQUE. The torque transmitted to the fastener by the installation

tool.

2.2.2. Assembly Sequence: A method indicating sequence of assembling fasteners in a

prescribed pattern. This may also be referred to as Installation Sequence, Tightening

Sequence and Torquing Sequence.

2.2.3. Assembly Torque: This shall be the design torque applied at final assembly. It

shall include the net effect of the following:

a) The torque required to overcome kinetic friction between mating bearing

faces and between mating threads.

b) The torque required to overcome the self-locking feature (if any).

c) The torque required to apply the desired axial load to a fastener assembly.

The assembly torque may be prefixed by the words MAXIMUM or MINIMUM when

referring to the toleranced torque values of the nominal assembly torque. The assembly

torque shall be measured only in the tightening direction. This may also be referred to as

Installation Torque, Tightening Torque.

2.2.4. Breakaway Torque: For fasteners incorporating self-locking torque, this is the

torque required to initiate relative motion between male and female threads when the

self-locking mechanism is fully engaged and there is no contact between the bearing

surface of the fastener and the material being joined. Breakaway torque can be prefixed

with the words MAXIMUM or MINIMUM, and shall be measured in the loosening

direction.

2.2.5. Free Running Torque: Shall be the torque required to overcome kinetic friction

between mating threads. This torque can be measured in either a loosening or tightening

direction. Free running torque shall not include any component of torque required to

overcome a self-locking feature or axial load in a fastener assembly. Free running torque

is not applicable where a selflocking mechanism is present in the fastener being torqued.

Also referred to as: No Load Torque.

2.2.6. Net Torque: This shall be the component of the installation torque available for

axially loading a fastener assembly after overcoming all frictional and mechanical forces

not inducing axial tension. (Net Torque = Installation Torque – torque required to

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overcome frictional (Prevailing Torque) and mechanical forces.) This may also be

referred to as Preload Torque.

2.2.7. Prevailing Torque: This shall be the torque required to overcome kinetic friction

of the mating threads plus the torque required to overcome the locking feature when

100% of the locking feature is engaged and fastener is unseated. The word “prevailing”

shall be understood as the average locking torque when it is not prefixed by the words

MAXIMUM or MINIMUM. Prevailing torque shall not contain any torque component

for axial load in the fastener assembly. The prevailing torque can be measured in either a

loosening or a tightening direction while the fastener is in motion. This may also be

referred to as Self-Locking Torque, Run Down Torque.

2.2.8. Seated Position: This shall be the position of the fastener when its bearing surface

contacts the material being joined, and any additional applied torque will produce a

residual axial stress in either the male (bolt) or female (nut) component.

2.2.9. Torque: The force or turning moment tending to produce rotation. It is expressed

in terms of applied load and the length of the moment arm applying it, usually in pound-

inches, pound-feet or Newton-meters (lb-in, lb-ft, Nm).

2.2.10. Torque Out: This term refers to a general type of torsional failure where any

fastener component (tool, retaining or mounting device or drive recess) reacting to the

applied torque, disengages or fails. In the case of recess drives, the driver climbs the

inclined plane of the recess as a result of the applied torque. This may also be referred to

as Cam Out Torque, Tear Out Torque, Twist out Torque and Shear Out Torque.

2.2.11. Unseated Position: This shall be the position of the fastener when the

application of removal torque disengages the bearing surface of the fastener from the

material being joined, reducing the axial stress to zero.

2.2.12. Unseating Torque: This shall be the torque required to initially move the fastener

from the assembly torque condition. The unseating torque shall always be measured in a

loosening direction. This may also be referred to as Breakloose Torque.

3.0 Materials for Electronic Enclosures

3.1 – Sheet Metal

Electrical Components

Mechanical Components

Displays

3.1.1. Selection Process

3.2 – Machined Metal

3.3 – Castings

3.4 – Plastics

3.5 – Composite

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Prohibited Material

Feature Selection

PMA – Precision Metal Forming Association

Stamping

Bending

Blanking

Deburring

Edge treatment

Sand Blast

Shotpeen

Surface Preparation – for Materials

Misc. Hardware

Components

Fasteners

Connector

Adhesives

How to hold the circuit boards

(1) Design - Material Selection a) Fabrication of Enclosure

(2) Fabrication – Mil spec E-917 Forming of Features determined by End Use

(3) Assembly

4.0 Final Finishes

Final Finishes are used in conjunction with electronic enclosures for various reasons. The

designer and the users of final finishes of electronic enclosures should be aware of the

properties of various types of final finishes to protect the electronic enclosure in the end-

use environment for the design-life. This document has been written to assist the

designers and users of final finishes in understanding the characteristics of various

coating systems and materials, as well as the factors that can influence properties when

the coatings are applied. Understanding and accounting for these materials can ensure the

reliability of electronics.

Outside Help

Many of the materials described herein are dependent on the manufacturing process and

application. It is recommended that a review of any suppliers or internal manufacturing

processes be completed. Often third parties can help mitigate some of the risks through r

equiring third party accreditations on purchase orders. A third party example which

would do accreditation for final finishes would be AC7108/1, Nadcap Audit Criteria for

Painting & Dry Film Coating.

4.1 - Wash Primer

Though the name indicates a cleaning process, wash primers are a thin coating which

help promote adhesion of subsequent coatings. Typical one should avoid use of wash

primers on non-bare metallic surfaces as they often interact with chromated or

phosphated surfaces.

Formatted: Bullets and Numbering

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4.1 – Chromate Conversion Coating (MIL-DTL-5541, MIL-DTL-81706)

Conversion coating is a very thin amorphous layer of chromate which acts as a barrier

between the substrate (typically a metallic chassis) and the external environment. It is

sometimes used as an adhesion promoting layer in paint systems. Chromate conversion

coatings are very thin materials, often less than 0.1 mil.

There exist two types of chromate. The distinction is being increasingly familiar as

companies look to transition away from hexavalent chrome to less toxic materials. The

specification MIL-DTL-5541 outlines the basic performance requirements for conversion

coatings.

4.1.1 - Type I - Hexavalent Chromium

The hexavalent chromium is used widely on aluminum alloys. The chromate coatings aid

in corrosion protection in the end use environment. Though application methods vary,

most common is immersion.

Consistency of the coating is often improperly linked to the color of the coating. Often

customers question variations in conversion coating on enclosures. The specifications

outline the coating shall be visibly discernible in daylight. The variations arise from time

in bath, concentration of the bath and many other aspects of the process. If a customer is

requesting a clear Type I coating, it must be specified on the drawing as the Class 3

should contain a yellowish hue.

4.1.2 Type II – Non-Hexavalent Containing (Trivalent Chromes, et al.)

Due to expanding global economy and differing environmental policies encountered,

hexavalent chromium is continually being limited due to its negative health impacts.

Common alternatives are trivalent chromiums, fluoride compounds and others.

4.2 - Anodization

Anodization is commonly used throughout the aerospace industry due to its high

durability. The electrolytic process grows a columnar oxide structure on the exposed

areas of the base material.The oxidation layer is typically grown on aluminum but can

also be used on zinc, titanium and magnesium alloys.

3.4.1 MIL-C-8625

3.4.1.1 Type I – Chromic Acid

3.4.1.2 Type II – Sulfuric Acid

3.4.1.3 Type III – Sulfuric Acid Hardcoat

3.4.2 Industry Specification??

4.3 - Plating (solicit help from plating possibly Milt Stevenson, Anoplate)

4.3.1 Sn

4.3.2 SnBi

4.3.3 SnPb

4.3.4 Ni

4.4 - Primer

Suggest

others to

write.

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MIL-PRF-23377 (chromates)

MIL-PRF-23377 (Type N)

MIL-C-15328 (zinc chromate)

TT-E-529

TT-E-479

4.5 - Top Coatings

4.5.1 Liquid

4.5.2 Powder

4.5.3 – Materials

4.5.3.1 – Epoxies

Epoxy Paints are a thermosetting resin. Epoxies come most commonly as a two part

which hardens when mixed. The epoxy paints are characterized by outstanding water

resistance, tough, heat and abrasion resistant, high alkali resistance, above average cost

and chalking in sunlight

4.5.3.1.1 - MIL-C-22750 – Chemical Agent Resistant Coating (CARC) - Interior Use

As referenced in the MIL-53072, the epoxies performance requirements of MIL-C-22750

meet those of chemical agent resistant coating (CARC) but is limited to use in internal

environments.

4.5.3.2 –Urethanes

Urethanes/Polyurethane paints are made by the reaction of polyols with a multifunctional

isocyanate. They are characterized by superior overall properties, excellent mar

resistance, flexible, with high gloss possible, outstanding durability, relatively high cost.

4.5.3.2.1 - Chemical Agent Resistant Coating (CARC) Painting System

MIL-DTL-64159 – Water Bourne

MIL-DTL-53039 – Solvent Bourne

4.5.3.3 – Acrylics

Acrylic – Excellent weather resistance, chemical resistance, high heat stability, above

average cost, hard, abrasion resistant, slightly brittle

4.5.3.4 – Alkyds

Alkyd – very good overall properties, average cost, air dry curing, extremely heavy use in

paint dues to their fine performance, formulation widely variable

4.5.3.5 – Vinyls

Vinyls – Low in cost, extraordinary acid resistance, outstanding water resistance,

excellent exterior durability, degrades in heat applications. Due to porous film properties

allows water to move through coating without blistering, etc.

4.6 - Color

From brand recognition to meeting customer requirements, the color of a product can

play an important role. When met these requirements are easily overlooked but when

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rejections occur, trying to solve these issues is sometimes laborious and costly. In this

section tips will be provided to ensure proper controls are established and feasible goals

and expectations have been established for controlling colors on products.

4.6.1 Federal Standard Colors

Federal standard colors are often used for products which are intended for the defense

industry. The specification, FED-STD-595 contains a large number of colors to choose

from with a descriptive coding for each. The coding helps to decipher many aspects of

the color. The first digit indicates the category of a finish; 1 – full gloss (80-100); 2 –

semigloss (30-45); 3 – lusterless (0-6). The second digit indicates the selected color

classification group. The final three numbers indicate the specific color which is being

specified.

4.6.2 - Customer Defined

Customers frequently have defined colors which represent their branding on products.

Color is therefore crucial to ensure matching with other suppliers and meet all

requirements of the engineering drawing. The customer often portrays the color

requirements in various methods. The recommended method is to obtain the CIE *L*a*b

values (explained in section 4.6.5 herein). These colors are sometime portrayed through

color chips provided by the customer, sometimes requiring a fee. Color chips come with

their own risks associated with storage conditions, spectrometer calibration, the material

the chip is made of, etc.

4.6.3 - Color Chips

4.6.4 - Color Tolerance Sets

Seven panel color tolerance sets are a further definition of a color than that depicted by a

single chip. The sets include variations in three aspects of the coloring of a product.

4.6.5 - CIE Lab Values (ASTM D 1729, ASTM D 2244)

Control the color based on variations from the standard color, denoted as a Delta

E.

4.6.6 - Specular Gloss (ASTM D 523)

Gloss represents the reflectance from a surface of light. The rougher the surface the less

reflectance you get and the smoother the surface more reflectance is observed. The gloss

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requirements of a painted product should be defined to ensure compliance with customer

requirements.

Flat, commonly referred to as lusterless, is defined as having low reflectance. When

measured at an angle of 60º the reflectance would be 15 or lower. Semi-gloss is the

median range of gloss commonly ranging in the 15-25 reflectance range. Gloss has

higher reflectance with requirements often stating a minimum of 80 or 90.

5.0 – Electronic Enclosure Assembly & Materials

5.1 – Adhesives / Sealants

5.2 - Gasketing

5.3 – Hardware

5.4 - Torque Requirements - This section establishes threaded fastener torque values

and the design requirements for determining and specifying threaded fastener torque

values.

5.4.1 LIMITATIONS. This standard does not address all the design aspects of joints

using threaded fasteners. It is the design engineers responsibility to ensure a threaded

fastener joint meets all design requirements. Refer to MIL-HDBK-60, FED-STD-H28/2

and MIL-HDBK-454 for information regarding bolt preloads, length of engagement,

strength factors and other design considerations.

5.4.2. TORQUE VALUES

5.4.2.1 STANDARD TORQUE VALUES. The standard torque values to be used for

certain fastener materials, part numbers and joint configurations shall be in accordance

with the tables in Appendix A. The torque values in Appendix A are Net Torque values

and do not include the prevailing torque. When design requirements dictate that a fastener

not covered in Appendix A is to be used, the design engineer shall ensure all joint design

requirements have been met and that the torque and tolerance to be used comply with

5.4.2.2 and 5.4.2.3.

5.4.2.2. NONSTANDARD TORQUE VALUES. When design requirements dictate that

the values in Appendix A are not acceptable, the design engineer may use a torque which

deviates from this standard. In cases where a nonstandard torque is used, the responsible

Manufacturing Engineer shall be consulted to determine if the torque is achievable using

the current tool set or if new tools or processes are required.

5.4.2.3. TORQUE TOLERANCES. Every torque shall have a tolerance assigned to it.

The standard tolerance shall be plus or minus 10% of the assembly torque value. This

tolerance is based on torque wrench industry standards ASME B107.14M - 1994 and ISO

6789. When design requirements dictate that a tolerance other than plus or minus 10% be

used, the responsible Manufacturing Engineer shall be consulted to determine if the

tolerance is achievable using the current tool set or if new tools and processes are

required.

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5.4.2.4 PREVAILING TORQUE. When a fastener contains a self-locking mechanism

the prevailing torque shall be added to the standard torque values tabulated in Appendix

A. Prevailing torque shall be determined in accordance with the assembly procedures in

4.3.

5.4.2.5 LUBRICANTS. The torque values tabulated herein were derived assuming that

no lubrication will be used. Refer to the assumption section included with each table in

Appendix A for the assumptions made with regards to friction.

5.4.2.6 MANUFACTURER’S SUGGESTED TORQUE. When the manufacturer of a

threaded fastener provides a suggested torque value the torque value specified by the

manufacturer normally should take precedent over the values in Appendix A. If design

requirements dictate that the manufacturer‟s suggested torque is not acceptable, the

design engineer may specify a torque which deviates from the manufacturers suggested

torque in accordance with 4.1.2. The tolerance for a manufacturer‟s suggested torque

shall comply with 4.1.3.

5.4.2.7 BASIC TORQUE DETERMINATION. To determine the required preload and

torque for a basic bolted joint, the following method may be used. Note that this is an

extremely simplified method using approximated formulas. Care and engineering

judgement should be exercised in its use. This method does not address all the design

aspects of joints using threaded fasteners. It is the design engineer‟s responsibility to

ensure a threaded fastener joint meets all design requirements. Use the equations in

section 4.1.8.

1. Use equation (1) to determine the bolt preload. Inputs are material yield strength,

stress area (based on thread size), and percentage factor. The percentage factor is

how much of the stress the designer will place on the bolt. Typical values are

50% to 80% of the yield strength.

2. Using the preload found in step 1, plug into equation (2) to determine the torque

required to achieve the preload. Note that nut factor (k), which is related to the

friction coefficient, will vary for materials and lubrication used. This should be

determined experimentally. For most kinds of fastener materials and finishes the

nut factor is typically between 0.1 and 0.3. This is a disadvantage to the industry

standard torque tightening method, since there is potentially a high degree of

variability in the applied preload.

Bolted joints are usually designed so that the bolt breaks before the threads strip. It is

easy to detect a bolt breaking failure at assembly than a thread stripping failure. An

assembly may seem to be acceptable if the threads are weakened but haven‟t sheared

completely. This situation risks a defective product in service.

Thread stripping strength can be calculated using equation (3). The inputs are the

Ultimate Shear strength of the material and the Shear Area. Note that there are two

possible shear areas: ASn (if the nut / internal threads are weaker) or ASs (if the bolt /

external threads are weaker. Use equations (4) or (5), respectively. The inputs to

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equations (4) and (5) are based on the fastener size and the length of engagement (LE).

The LE is very important in preventing a shear failure. The longer the LE, the stronger

the joint is against shear forces. Electronic enclosures are typically made from a lighter

material, such as aluminum. However, for strength, fasteners are usually steel or

stainless steel. An external steel thread torqued into an internal aluminum thread can

easily strip the weaker aluminum internal threads without a proper LE. Helical coil

inserts are usually used in this situation. They can be made from steel, and will provide

additional strength to the aluminum threads, allowing for strong joints with short LE‟s.

The shear strength of a particular material may not be available. A typical shear ratio

(ultimate shear strength to ultimate tensile strength) for steel or aluminum is 0.6. This

value should be determined experimentally.

Note that the designer should use minimum strength and LE for worst case scenarios.

To determine the bolt breaking strength, use equation (6). This is similar to equation (1),

however, the Ultimate Tensile Strength is used in place of Yield Strength, and there is no

percentage factor.

The thread shear strength and bolt breaking strength can be compared to ensure the

design fails first by the bolt breaking.

These equations can also be used to determine if a particular applied torque will cause a

joint to fail. This is particularly useful if a particular torque tool is found out of

calibration for a certain period of time. The manufacturing engineer can determine if the

potentially applied torque (from an “tool out of calibration report”) would cause a

potential issue in the field.

Analysis Method - Overtorquing

Calculate the force from the potentially applied torque (per the tool report), using

equation (1).

Note that the nut factor, which represents friction between the threads, can be taken as 0.1

for all calculations if overtorquing is thought to be an issue. This value is usually chosen

for lubricated fasteners (i.e. lower friction). Since the calculation is attempting to show

worst case, the lower the friction, the higher the force (i.e. the closer it might be to the

breaking / stripping force).

Will the threads be stripped or will the bolt break due to this force? Both need to be

checked.

Calculate the thread stripping force, as above, using equations (3), and (4) or (5).

Calculate the bolt breaking force, using equation (6).

Compare the force applied (calculated in step 1) with the thread stripping force and the

bolt breaking force (calculated in step 2). If the force applied from the potential

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overtorque situation is greater than the worst case scenario for thread stripping or bolt

breaking, there may be a potential impact in the field.

Analysis Method - Undertorquing

Calculate the force from the potentially applied torque (per the tool report), using

equation (1).

Note that the nut factor, which represents friction between the threads, can be taken as 0.3

for all calculations if undertorquing is thought to be an issue. This value is usually

chosen for non-lubricated fasteners (i.e. higher friction). Since the calculation is

attempting to show worst case, the higher the friction, the lower the force (i.e. more

applied torque, and therefore preload, is lost due to friction).

How does this potential undertorque compare to forces the joint may likely see in the

field? Calculate this from equation (7). The inputs are mass of the part being fastened,

number of fasteners holding the part, and the environmental acceleration level. Assume a

= 100g. This is a worst case scenario, and gives a margin of safety for the calculations.

Compare the force applied (calculated in step 1) with the outside force (calculated in step

2). As a rule of thumb (which should be used with caution, based on engineering

judgment), if the outside force is much less than 50% of preload then the joint may be ok.

5.4.3 BASIC TORQUE EQUATIONS.

Bolt Preload:

ZAYF ss (1)

where, F = Preload

Ys = Yield Strength of Bolt

As = Stress Area

Z = Percentage Factor

Torque (Nut Factor Method):

kFDT (2)

where, T = Torque

k = nut factor

F = Force

D = Diameter

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Thread Stripping:

xASUSSF (3)

where, F = Stripping Strength

USS = Ultimate Shear Stress

ASx = Shear Area, where x = n (nut / internal thread weaker) or s (bolt / external thread

weaker)

Shear Area (nut / internal thread weaker):

max2minmin 57735.0

2

1Dd

nnLEdAS n

(4)

or

Shear Area (bolt / external thread weaker):

max1min2max1 57735.0

2

1Dd

nnLEDAS s

(5)

where, n = number of threads / inch

LE = Length of Engagement

D1max = maximum minor diameter of internal thread

D2max = maximum pitch diameter of internal thread

dmin = minimum major diameter of external thread

d2min = minimum pitch diameter of external thread

Note: Equations 4 and 5 from ASME B1.1 Appendix B

Bolt Breaking:

sAUTSF (6)

where, F = Force

UTS = Ultimate Tensile Strength

As = Stress Area

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Outside Force:

an

mF

(7)

where, F = Outside Force per fastener

m = mass of part secured

n = number of fasteners used to secure part

a = acceleration likely to be seen from the environment

5.4.4. DRAWING REQUIREMENTS

5.4.4.1. MINIMUM REQUIREMENTS. Each threaded fastener used in a design shall

have a torque and torque tolerance specified on the engineering drawing. When a locking

mechanism is used with a threaded fastener, the engineering drawing shall state that the

prevailing torque shall be added to the specified (net) torque.

5.4.4.2. ADDITIONAL REQUIREMENTS. When torque striping or use of lubricant is

required, the materials shall be specified on the parts list. The individual fasteners that

require torque striping or the use of lubricant for installation shall be identified on the

engineering drawing.

5.4.4.3. DRAWING NOTES. The drawing shall contain standard notes pertaining to

fastener torquing. Standard notes may be similar to the following.

Torque [insert part description, including find no. from parts list], to [insert torque value,

including units and tolerance] in addition to prevailing torque, in accordance with [insert

process / manufacturing standard].

E.g.: Torque nut find no. 99 to 50 ± 10% in-lbf in addition to prevailing torque, in

accordance with manufacturing process specification 12345.

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5.4.5. FASTENER ASSEMBLY

The assembly and torquing of threaded fasteners shall be in accordance with the

following.

5.4.5.1 Work Environment

The fume hood, work area, workstation and tools shall be maintained in a clean and

orderly condition. All dirt and spills shall be promptly cleaned up. Preparation and

application of marking compounds and adhesives shall be performed under local exhaust

ventilation.

5.4.5.2 Health, Safety and Environment

Always wear safety glasses in the required areas. Electro Static Discharge (ESD)

protective equipment must be worn when torquing static sensitive units.

Operators shall review the applicable Material Safety Data Sheets (MSDS) for all

materials before handling. Avoid inhaling, ingesting or directly contacting any of the

materials. Skin contact should be avoided by wearing nonporous gloves. Eye protection

shall be worn at all times.

Use materials in a well ventilated area with the exception of zinc chromate. Zinc

chromate has been identified as a confirmed human carcinogen. It may only be used in

designated areas under local exhaust ventilation, not at operators‟ workbenches. Zinc

chromate must not be handled by operators unless trained by the Health, Safety and

Environment department.

Keep chemicals away from excessive heat, sparks or open flame. Avoid prolonged

breathing of vapours or mists. Avoid contact with skin and wash thoroughly after using

and before smoking or eating. Flammable chemicals shall be stored in flammable

chemical safety cabinets.

In the event of a chemical spill operators shall call the site emergency number. If

directed and/or safe to do so, operators may shut off the leak, remove sources of ignition,

place absorbent materials from the spill kits around the spill, ventilate, or evacuate. In

the event of a splash or spill onto an operator the emergency eyewash or shower stations

shall be used to rinse the affected area.

In the event of a fire or explosion operators shall call the site emergency number. If

directed and/or safe to do so, operators may power all equipment down (including the

oven), remove sources of ignition, extinguish the fire using the extinguishers provided,

and/or evacuate.

Hazardous waste shall be collected and disposed of according to proper procedures.

Operators shall collect hazardous materials in designated containers. Disposal shall be

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conducted through the Chemical Stores department. Chemicals shall NOT be washed

down personal hand wash sinks provided.

To prevent Repetitive Strain Injury (RSI) while torquing, please read: “Guidelines for the

Prevention of RSI‟s Associated with Torquing Activities.”

External supplier safety: All external processors shall establish their own safety

procedures, including chemical handling procedures, hazardous waste disposal, and RSI

prevention.

5.4.5.3 Parts

Parts include, but are not limited to, threaded fasteners (nuts, bolts, etc.), washers and

other components in the joint that are to be torqued, printed boards, cable assemblies, and

end unit assemblies.

All parts must be checked beforehand for damage or wear. All paint, dirt and corrosion

products shall be removed from threads prior to installation and torquing. Mating parts

and surfaces shall be smooth and clean. The fasteners, if found damaged, must be

discarded and replaced.

5.4.5.4 ESD

ESD Control shall be in accordance with ANSI/ESD S20.20, MIL-STD-1686, or

equivalent.

5.4.6.1 MATERIALS & EQUIPMENT

5.4.6.2 Materials

Epoxy polyamide primer system

Zinc chromate primer

Dry Lubricant

Torque Stripe Material

Masking tape

Swabs

Fine camels hair brush

Kimwipes

Isopropyl alcohol

5.4.6.3 Equipment

Calibrated torque wrenches / screwdrivers

Calibrated torque tools capable of measuring or compensating for prevailing torque

Torque tool verification system

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Sockets

Adapters

Extensions

Calibrated torque control power drivers

Various nut drivers

Various screw drivers

Oven, re-circulating air type

Fume hood

5.4.6.4 Torque Value

All fastening applications shall have a corresponding torque value.

Prior to torquing, the operator shall review the drawing/SHOP FLOOR PROCEDURE to

verify the required torque value.

If the torque value is not on the SHOP FLOOR PROCEDURE, please contact your

manufacturing engineer (ME) to help select the proper torque value. The operator shall

not use any torque value that has not been reviewed and approved by the ME.

5.4.6.5 Threadlocker

When specified on the engineering drawing, fasteners shall be retained in position using

adhesive compounds. Cyanoacrylate adhesives shall be applied and cured per X.

5.4.6.6 Lubricant

If lubricant for threads is required, the drawing / SHOP FLOOR PROCEDURE will call

out the part number to apply.

NOTE: Lubrication to be performed under vented fume hood conditions.”

Also, unless specified otherwise, a small amount of Isopropyl Alcohol (IPA) may be

applied to self-locking fasteners as a lubricant prior to installation in order to prevent

galling.

5.4.6.7 Corrosion Protection

Fasteners that form dissimilar metal couples in the installed configuration shall be

protected in accordance with X, using either epoxy polyamide primer or zinc chromate

primer. The epoxy primer and zinc chromate are types of corrosion protection. In either

case, the appropriate callout will be made on the engineering drawing and SHOP FLOOR

PROCEDURE.

5.4.6.8 Prevailing Torque

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Definition: The torque required to overcome kinetic friction of the mating

threads plus the torque required to overcome the locking

feature when 100% of the locking feature is engaged and

fastener is unseated.

When it is indicated on the engineering drawing / SHOP FLOOR PROCEDURE, the

prevailing torque of the fastener combination must be measured because a larger force

will be required to torque the fastener properly. This is usually required when there is a

type of locking fastener in the joint.

Some examples of locking fasteners used are locknuts (with distorted threads or nylon

inserts), lockscrews (with non-metallic strips), and locking helicoils. See Figures 4, 5,

and 6, below.

Figure 1: Locknut with Nylon Insert

Figure 2: Lockscrew

Figure 3: Locking Helicoil

The torque required to overcome the friction of the fastener shall be determined using an

indicator style torque tool, or a tool with a built in functionality that can, when

programmed correctly, compensate for prevailing torque.

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5.4.6.9 Manual Method:

The fastener shall be tightened to 1/8 inch of the fully seated position (2 or 3 threads).

Slowly tighten the fastener at least one half turn and note the peak torque value.

This peak torque is the prevailing torque, which will vary considerably for each fastener

of each size and type.

The prevailing torque shall be added to the torque value given in the SHOP FLOOR

PROCEDURE or drawing. The sum of these two torque values shall be the torque

applied to properly install a fastener with a locking device.

5.4.7.0 Proper Torque Tool Selection

The operator shall verify that they have the correct torquing tools for the fastener

installation as specified in the Standard Shop Procedure (SHOP FLOOR PROCEDURE).

In some cases the specific torque tool to use is written in the SHOP FLOOR

PROCEDURE. In this case this specific tool or tool type will be used.

Selecting the proper torque tool involves knowledge of the torque required on the

engineering drawing or SHOP FLOOR PROCEDURE. It also involves knowing the

torque tolerance associated with the tool. The tolerance is usually expressed in terms of a

plus or minus percent, and is dictated by the tool manufacturer.

The following table contains common tools and their torque tolerances (from

manufacturer‟s documentation). Most torque tool manufacturer‟s quote standards

conforming to ASME-B107.14M or ISO 6789 (for hand torque tools).

The last column, the Range Where Tolerance Valid, indicates which percentage of the

tool settings are guaranteed to maintain the tolerance listed. The standard is 20 – 100 %.

That means that from 20% of maximum setting all the way to 100% of the maximum

setting, the tool can be used with a measure of confidence. So, say a tool‟s listed range is

from 0 – 20 in-lbs. That means that the tool can be used from 4 in-lbs. (20%) to 20 in-

lbs. (100%). Below 4 in-lbs. the torque is not guaranteed to be accurate to the tolerance

standard. Another tool must be used in place of the example tool if torquing anywhere

below 4 in-lbs.

Also required are the proper sockets, adapters, ratchets, screwdrivers, etc. to complement

the calibrated torque tool. These depend on the type of fastener used and the accessibility

of the fastener in the unit.

5.4.7.1 Calibration Check

All torque drivers and wrenches used for final installation of torqued fasteners shall be

calibrated and identified with a calibration label. Always check the calibration label prior

to use to ensure that the tool is not passed its calibration due date.

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5.4.7.2 Verification Check

Torque tools shall be verified at least once per month. The X torque verification system

is used as the primary tester, and instructions are found in X. Designated cell reps are

responsible for the verification testing, and have been trained on the use of the specific

equipment.

5.4.7.3 Three Stage Torquing

An appropriate screw driver/nut driver shall then be used to tighten the fastener. If the

selected torquing tool is of an adjustable type, the torque shall be set to the value given on

the drawing / SHOP FLOOR PROCEDURE.

5.4.7.4 Stage 1: Run Down to Just Seating

The operator shall start all fasteners by hand. Torquing the fasteners to just seating

requires the bearing surfaces to contact each other but no additional torque is applied. If

free-running hardware (i.e. no locking feature) is used, this step can be done by hand. If

locking hardware is used, prevailing torque must be measured, as above.

5.4.7.5 Stage 2: First Pass and Torque Sequence

The objective of this stage is to ensure that the resulting assembly is evenly loaded. An

evenly loaded assembly is:

more resistant to hardware loosening due to vibration

necessary where minimum thermal resistance is desired

to allow for “relaxation” in fasteners and parts

The “first pass” and torque sequence help to achieve this objective.

First Pass

All torqued fasteners shall first be torqued to a percentage of the final torque, at least

50%, but not the final torque value.

5.4.7.6 Torque Sequence

Unless otherwise specified, where the installation of a part involves tightening a group of

3 or more fasteners, the proper torquing sequence shall be followed.

The torquing sequence shall follow two key concepts:

1) Work from the inside out.

2) Alternate to the opposite fastener around the pattern

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Figure X illustrates a typical torquing sequence. When an operator is not clear on the

torquing pattern to be followed, the operator shall consult with a ME. Also, the SHOP

FLOOR PROCEDURE occasionally has the torque sequence specified.

Stage 3: Apply Final Torque

Using the same sequence as in Stage 2, final torque all fasteners to 100% of the specified

torque value.

Torqued bolts, screws, and nuts shall seat with mating surfaces, with no gap under bolt

head, screw head, or nut.

Over-Torqued Fasteners

Fasteners that have been tightened beyond the maximum torque value specified on the

engineering drawings or SHOP FLOOR PROCEDURE shall be removed and discarded.

Over-torqued fasteners shall not be backed off and re-torqued.

Post-Torquing Material Application

Identification of Torqued Fasteners (Torque Striping)

When specified on the drawing / SHOP FLOOR PROCEDURE, all torqued fasteners

shall be identified by means of a torque stripe witness mark.

All materials used for torque striping shall be applied and cured in accordance with

manufacturer‟s instructions.

5.4.7.7 Identification shall:

consist of a continuous stripe of sealant approximately 1/16 inch wide extending across

the fastener at least 50%;

extend onto the adjacent substrate;

be aligned with the center-line on the fastener;

be applied to the nut wherever practicable. Otherwise, the head of the bolt or screw shall

be marked.

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The striping compound shall be specified in the drawing, and shall be applied using a

brush, swab or other suitable applicator.

NOTE: Fasteners shall be torque striped immediately after torquing and before the

application of any strapping compounds.

5.4.7.8 Torque Strapping

Epoxy adhesives shall be applied and cured per manufacture‟s recommended procedures.

The adhesive compound shall be specified on the engineering drawing. Strapping

compounds shall provide at least 25% circular coverage. The adhesive shall not

contaminate any other surrounding components.

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5.4.7.9 Inspection

Fasteners shall be torqued to the value specified on the engineering drawing or SHOP

FLOOR PROCEDURE.

Torqued bolts, screws and nuts shall seat with mating surfaces, with no gap under the bolt

head, screw or nut.

Fasteners shall exhibit no signs of cross-threading, detrimental, or hazardous burrs or

mutilation. There shall be no worn or burred edges on screwhead slots.

Flathead screws shall be flush with, or slightly below the mounting surface.

Countersinking shall be free of chattermarks and shall be concentric with the hole. The

countersink shall have the same angle as the screwhead.

Re-torquing is not permitted when a fastener is over-torqued.

If at any time the identification stripe is found to be miss-aligned, indicating movement

of the fastener following final torquing, the fastener shall be backed-ff one full turn and

re-torqued per the drawing requirements.

In cases where subsequent painting/coating will obliterate the torque striping, re-marking

shall not be required provided that the existing marking was intact prior to coating.

Unless specified otherwise,

a male threaded fastener shall project through its female fastener by a minimum of 1.5

threads (3 threads for strapped connections)

a male threaded fastener installed into a tapped hole shall have a minimum thread

engagement consistent with the strength of material into which the threads are cut.

Recommended minimums shall be as follows:

Table 1: Minimum Thread Engagement

Material Min. Thread Engagement

Steel or Aluminum with

Solid Steel Inserts

1.5 diameter

Aluminum or Magnesium 2.5 diameters

a male threaded fastener installed into a blind tapped hole shall have adequate thread

engagement per Table 2 without the possibility of bottoming in the hole at adverse

tolerance extremes.

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5.4.8 General Torquing Practices

5.4.8.1 Tool Alignment and Torque Application

5.4.8.2 General

5.x Always ensure that the „drive‟ part of the tool and any accessories are „in line‟ with

the fastener and are the correct size. Grip the tool by the indicated grip area (usually

knurled). Apply torque with a slow and steady pressure. Misalignment and jerky

movements will affect accuracy. All torque wrenches are subject to Moment-of-Inertia

errors. It should be noted that click style wrenches are highly affected by this.

5.x Whenever practicable, when torquing a bolt/nut combination, the bolt shall be held

stationary and the nut turned. This prevents damage to the screw recess.

5.4.8.3 Wrenches

5.x If a wrench is used, the body shall be parallel to the mating surface, and the torquing

force applied at approximately 90 degrees to the handle of the torque wrench and at its

full length.

5.4.8.4 Drivers

5.x If a driver is used, the body shall be perpendicular to the mating surface, and the

torquing force applied with a firm grip on the handle of the torque tool.

5.4.8.5 Posture and Positioning

5.x Make sure that your workplace is firmly held or fixed on a bench before commencing

work. Also ensure you have a secure footing. Try to avoid balancing in awkward

positions.

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5.4.8.6 Viewing Angle

5.x Torque indicating dials and scales shall be read from directly above the indicator.

Reading an indicator from an angle will result in an inaccurate torque reading (i.e.

parallax error).

5.4.8.7 Corrections for Extensions and Adapters

5.x A torque correction calculation must be made when using adapters or lever type

extensions with torque wrenches. If you extend the length of the torque wrench to reach

a fastener, then the torque value will be more than is labeled on the tool. Please let your

ME know about this and the proper steps can be taken to adjust the torque value.

5.x In order to determine what the scale reading should be for the torque value required,

the following formula shall be used:

5.4.8.8 Special Documentation / Instructions

5.x Always read the tooling documentation carefully before using torque equipment.

They will show you how to correctly use your tools and simplify operation. Also be sure

to follow instructions given on the SHOP FLOOR PROCEDURE or assembly traveler

for assembly operations.

5.4.8.9 Tool Care

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5.4.9.1 Any measuring instrument, used for any purpose other than what it was

designed for, will usually experience premature failure or become suspect in its accuracy

(i.e. dropping or using it as a pry bar or a hammer). If a torque tool is dropped it must be

recalibrated prior to use.

5.4.9.2 Storing adjustable click style tools above the lowest setting will cause premature

main spring wear. That is, at the end of the workday, reset adjustable click torque tools

to zero.

5.4.9.3 Ratchets, sockets, bits, adapters, etc. should be inspected for wear and replaced if

found damaged.

Operator Training

5.4.9.4 Torquing of threaded fasteners shall only be performed by operators who have

been trained regarding the requirements of this specification. Operators shall have a good

knowledge of the procedures and all pertinent safety precautions. The team leader shall

be responsible for requesting the training for an operator and to ensure that the operator

has received the training before performing the task. The team leader shall maintain the

operator‟s training and re-training records. Re-training is required every 24 months.

Trained operators are responsible for their re-training schedule. All training and re-

training activities are documented; copies are maintained by the team leader and the

training administrator.

5.4.9.5 Training shall consist of practical demonstration and written examination. Re-

training shall consist of written examination only. Operators shall get a minimum of 80%

on the written examination. All exams and exam results are maintained by the trainer.

5.4.9.6 Inspectors shall be trained as operators.

5.4.9.7 External supplier training: All external processors shall establish their own

training procedures and their operators and inspectors shall be trained to the requirements

of this specification.

5.5 QUALITY ASSURANCE PROVISIONS

5.5.1 Inspection - Each torqued fastener shall be visually inspected after installation for

compliance with the Workmanship requirements of this document.

5.5.2 Calibration – All torque drivers, wrenches, and measuring equipment shall be

calibrated and identified with a calibration label in accordance with the Standards Lab

calibration schedule.

5.5.3 External supplier calibration system: All external processors shall have a

documented calibration system in accordance with a National or International Standard.

Torque drivers, wrenches, and measuring equipment shall be calibrated at regular

intervals based on the type of tool and records of the tool‟s calibration. Procedures and

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records shall be maintained that document calibration. Tools shall be labeled to indicate,

at minimum, date of calibration, calibration due date, and tool identification.

5.5.4Test Methods

5.x Once each month, all torque tools shall have their torque settings verified using and

approved torque tester capable of testing within the torque tool‟s range. The torque tool

verification information shall be recorded by the system and uploaded and stored in a

database. As an alternate, the verification information shall be recorded in a Torque

Verification Log. Logs shall be maintained and be available for review.

5.x External supplier test methods: All external processors shall perform torque tool

verification testing to their own internal procedures on a regular basis.

5.5.5 Statistical Process Control

5.x Process control shall be in accordance with X.

5.x External supplier process control: All external processors shall establish and follow

their own process control.

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5.x Shelf Life Control

5.x Shelf life control of age sensitive materials shall be in accordance with X.

5.x External supplier shelf life control: All external processors shall establish and follow

their own shelf life control.

5.6 – Fasteners