automotive devices: quad no-lead (qfn) technology...

External Use

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Automotive Devices: Quad No-

Lead (QFN) Technology with

Inspectable Solder Connections

FTF-SDS-F0026

A P R . 2 0 1 4

Dwight Daniels | Package Engineer

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Agenda

• Wettable Lead Ends / Definition & Alternate Names

• QFN Packages / Intro to Features & Lead End Options

• Freescale Evaluations / Inspectable Solder Joint Study

• Solder Joint Life / How Good are They?

• Conclusion

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Purpose: Wettable Lead Ends A wettable lead end on a QFN is to promote wetting of solder to the ends of the QFN terminals during PCB assembly in order to form a solder fillet which is inspectable by AOI (Automated Optical Inspection).

PROMOTE WETTING OF SOLDER – ”wettable” does not guarantee wetting

SOLDER FILLET – presence and shape are dependent on many factors of the PCB assembly process

INSPECTABLE BY AOI – assumes expertise and equipment capability at the PCB assembly facility

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Wettable Lead Ends – A.K.A. … Nomenclature:

WETTABLE FLANK (WF) – the flank (end) of the lead/terminal is created to be wettable

INSPECTABLE JOINT (IJ) – the lead/terminal ends are created to be wettable to promote a visible, inspectable joint/fillet

SOLDERABLE LEAD END – the lead/terminal end is created to allow solder to readily wet to it

SIDE SOLDERABLE – the side of the package (I.O.W. lead ends) is created to promote solder wetting

SOLDER FILLET – the resulting formation/presence of the inspectable solder joint

In the case of the 1st 4 -- A rose by any other name is still a rose.

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Agenda





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QFN Packages

(Quad Flat-pack No-leads) QFN / Package Structure Package Characteristics / Pro’s &Con’s Lead End Variants / Options & Creation

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Anatomy of a QFN Package

flip chip Chip-on-Lead configuration No pad (typically) Accommodate a larger die:package ratio

wirebond

Exposed pad Good thermal efficiency Added mechanical interface

Half-Etch Features Locking features, etc.

Terminal - Design Variations Standard – pull back e-Type - exposed lead end Wettable – plated lead end feature

Saw QFN

Small Footprint Due to no lead extension

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• Small body sizes

− <2x2mm up to ~ 12x12mm (sweet spot 3x3mm to 8x8mm)

− Relatively low lead count

• Thin package height

− ≤1mm max height is typical

• “Single sided” mold – no parting line

− Lead frame aligns with the seating plane of the package body

• Exposed flag is a standard feature (typically) − Good thermal efficiency

− Solderable feature to improve mechanical robustness & board life.

• Leads/terminals contained within the physical outline of the body − No bent/skewed leads – No Trim/Form/Excise required

− No lead structure which extends beyond the body

Small footprint

No lead compliance

Traditionally difficult to inspect solder joints after board assembly • Inspection requirements are market dependent

QFN Package Characteristics: General

This is why we’re here.

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QFN – Terminal Design Variants

“Exposed” Lead End (e-type) Increased terminal surface area Potentially wettable terminal end surface

“Standard” Lead End Plastic Mold Compound (EMC) fills in the half-etched* portion of the lead end

Wettable Lead End (WF or IJ*) Wettable terminal end surface Used for applications which require AOI (Automated Optical Inspection)

* Wettable Flank or Inspectable Joint -- or -- side solderable or …

*

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QFN – Terminal Design Variants

“Exposed” Lead End (e-type) Increased terminal surface area Potentially wettable terminal end surface

“Standard” Lead End Plastic Mold Compound (EMC) fills in the half-etched* portion of the lead end

Wettable Lead End (WF or IJ*) Wettable terminal end surface Used for applications which require AOI (Automated Optical Inspection)

* Wettable Flank or Inspectable Joint -- or -- side solderable or …

Plated Surface

Shown in Gray

*

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QFN – Wettable Lead End Variants

Wettable Lead End – methods of formation Mechanical Removal: created by removing material from the package during the assembly process, usually by mechanical saw. - Often referred to as Step Cut Etching: typically created by the half etch process during lead frame manufacturing - Often referred to as Dimple

Plating After Singulation: created after singulation in assembly by depositing wettable material on the lead end - Often performed by Electroless Sn plating

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Step Cut – 2-pass saw process Dimple – Leadframe ½-etch process

Wettable Lead End – Physical Comparison

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Step-Cut saw blade – 1st pass saw

Singulation saw blade – 2nd pass saw

Step Cut

2-pass saw process

Wettable Lead End – Step Cut Formation

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Mechanical (step cut)

Freescale’s preferred option

Design Flexibility – can be

applied to existing leadframe

designs

Created using existing QFN

assembly manufacturing

processes

IJ features are full lead width

IJ features can be greater than

half the lead frame thickness

Adds nominal cycle time to the

assembly process

Requires plating process in

assembly - even if using pre-

plated leadframes

Etched (dimple)

Created during the normal

leadframe manufacturing flow

Equally compatible with standard

Cu or pre-plated lead frames

Half-etched features are limited

to roughly half the leadframe

thickness

Features etched during

leadframe manufacturing can’t

be full lead width

Saw singulation of narrow, half-

etched features are difficult to

saw singulate cleanly

- Imposes a practical minimum

lead pitch for saw singulated

QFN package outlines

Consumes more of the surface

area of the lead/terminal

Post-Singulation (e-less)

Design Flexibility – can be

applied to existing leadframe

designs

Maximizes wettable height &

width for any given lead frame

thickness

Adds nominal cycle time to the

assembly process

Requires additive process in

assembly - even if using pre-

plated leadframes

Usually created using

electroless plating

- Not a standard QFN assembly

process

- E-less plating can be difficult to

maintain

- Can complicate handling in

assembly manufacturing.

Wettable Lead End Variants – Pros & Cons

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QFN & Wettable Lead Ends – Wrap-up

SMALL BODY, SMALL FOOTPRINT

GOOD THERMAL EFFICIENCY

VERSITILE MANUFACTURING CONFIGURATIONS

VARIOUS TERMINAL DESIGN OPTIONS – Including WF/IJ/SS options

STEP CUT – freescale PREFERRED OPTION FOR IJ/WF

– compatability with existing manufacturing – robust performance

More about this later

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Agenda





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Inspectable Joint – Step Cut Evaluation & Inspection Results

Freescale Evaluation / Design & Set-up

Representative Solder Joints / Good vs. “Bad” or Ugly?

Inspection / Results and Comparisons

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2 package outlines: 5x5mm, 32ld QFN-IJ – 8 DOE legs (step cut) + control cells with e-type leads 7x7mm, 48ld QFN-IJ – 8 DOE legs (step cut) + control cells with e-type leads & punch dimple

2 PCB pad designs for each body size: Nominal lead size + 0.05mm (under the package) + 0.3mm (extension beyond the package) Nominal lead size + 0.05mm (under the package) + 0.6mm (extension beyond the package)

2 solder paste materials: Both – SAC305 (Sn96.5/Ag3/Cu0.5), 89% metal load, ROL0, “no clean”

Aging vs. No-Aging: 16hr, 155C aging bake prior to PCB assembly vs. No aging pre-bake

• Study Factors for Board Assembly:

• Factors were chosen based on former board mount studies of QFN-IJ

(wettable lead-end) packages.

Step Cut PCB Assembly Evaluation

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Board Assembly Study: overview of the numbers

768 study samples (step cut units): plus 288 control units 5x5mm, 32ld QFN-IJ – 8 DOE legs (step cut). 2 control cells with e-type leads 7x7mm, 48ld QFN-IJ – 8 DOE legs (step cut). 2+2 control cells with e-type & punch dimple

30720 step cut solder joints: plus 12288 control solder joints 15360 - 0.3mm pad extension 15360 - 0.6mm pad extension 12288 - control parts (all with 0.6mm pad extension)

16 DOE Study Populations: plus 6 control cells:

4000+ Temp Cycles (and climbing):


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PCB Assembly Process Factors

• Successful formation of inspectable solder joints is dependent on

many factors of the PCB assembly process.

PCB PAD DESIGN Pad length/extension (beyond package outline), feature clearance, consistent PCB quality of finish

SOLDER PASTE MATERIAL flux system, alloy, quality/uniformity, compatibility (with overall process)

STENCIL DESIGN & MATERIAL material, thickness, aperature, release characteristics

PCB ASSEMBLY PROCESS OPTIMIZATION placement/alignment accuracy, dispense volume, reflow profile & conditions

REFLOW CONDITIONS oven type, # of zones, temperature control, oven atmosphere, board layout

Note: PCB assembly for this study was performed at an independent, 3rd party

contract board assembly provider – no pre-assembly optimization was performed.

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PCB Assembly Process Factors – Package dimensions

Package Vehicle 1:

Body size: 5x5mm

Body Thickness: 0.85mm

Leads: 0.4 x 0.25mm @ 0.5mm pitch

Exposed Pad: 3.6 x 3.6mm

Package Vehicle 2:

Body size: 7x7mm

Body Thickness: 0.85mm

Leads: 0.4 x 0.25mm @ 0.5mm pitch

Exposed Pad : 5.05 x 5.05mm

Note: In this study the exposed pads of all packages were soldered to the PCB

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Pad & Stencil Design Details:

2 PCB pad designs for each body size: Nominal lead size + 0.05mm (under the package) + 0.3mm (extension beyond the package) Nominal lead size + 0.05mm (under the package) + 0.6mm (extension beyond the package)

PCB Pads

Under Package Exterior to Nominal Package Outline

0.275 mm wide

0.35mm + [0.55 / 0.25] mm – based on pad design

for 0.6 / 0.3 pads

0.25 mm wide Solder Paste

Package Lead e.g. 0.4 x 0.25mm


Pictorial representations are NOT to scale

0.6 mm pad extension

0.3 mm pad extension 0.45mm +

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0.3mm pad design 0.6mm pad design

PCB Assembly – Solder Fillet Formation

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0.3mm pad design 0.6mm pad design


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PCB Assembly – Solder Fillet Formation (cross-sections)

0.3mm pad

0.6mm pad

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PCB Assembly – Solder Fillet Formation (cross-sections)

20

0µm

20

0µm

20

0µm

20

0µm

2

00

µm

0.3mm pad

0.6mm pad

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comparison: Step Cut & Dimple - punch

Dimple

Step Cut

solder joint formation solder joint cross-sections

20

0µm

2

00

µm

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Solder fillet formation by cell –AND– Manual vs. AOI Inspection results


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Solder fillet formation vs. DOE Factors – & – Manual vs. AOI Inspection

results

Pad design is a

dominant factor

for consistent

fillet formation

------------

This result is

consistent with

past Freescale

fillet formation

studies

There is general

alignment between

manual & automated

inspection methods

- specifically, complete

agreement that pad

design is the key factor

------------

Presence of a fillet

and interpretation

of the shape of the

fillet are somewhat

subjective


Note: Manual inspection & AOI confirm no missing fillets on all control lots. All control lots were assembled on pads with 0.6mm pad extension.

Control cell 5-A-6-n exhibited relatively high occurrence of convex fillets

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PCB Assembly – AOI Failure Mode

Short pads are more susceptible to variation in fillet formation due to

placement &/or solder printing process tolerance.

This solder joint

is formed and reliable,

but AOI is all but

certain to interpret

the joint as “missing.”

AOI failure

AOI pass

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Poor alignment on a short pad is more likely to form a solder fillet

that AOI will reject.

PCB Assembly – AOI Failure Mode

pin 25

Note: Mitigation strategy for poor fillet formation is process optimization and

increased (longer) pad length with accompanying paste print increase.

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Comments on Common Solder Joint Inspection Methods:

• AOI (Automated Optical Inspection)

− Faster and more repeatable than human eye or X-Ray

− Desirable for high reliability applications, such as automotive

− Relatively complex set-up and programming for interpretation of lighting

• X-Ray

− Relatively slow

− Preferred for identifying voids and defects located under the package

Voids do not correlate well with early solder joint life failures

Defects (such as electrical shorts) under the package are not common

• Human Eye Inspection

− SLOW! And subject to fatigue, distraction, boredom, hunger, restlessness, etc.

− Better at judging if a solder joint is formed when the fillet is not optimal

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PCB Assembly Evaluation Results - Wrap Up

Board assembly of 1000+ units, in 22 distinct populations, was performed using common industry methods & pre-selected BOM, for the purpose of studying inspectability & reliability of QFN packages with step cut IJ feature.

GOOD PROCESS YIELD – especially considering the process wasn’t optimized >99% process yield – “hard” failures (opens/shorts due to print errors)

ALIGNMENT BETWEEN HUMAN-EYE INSPECTION & AOI agreement that we achieved 100% solder fillet formation on long (0.6mm) pads agreement that all discrepent or marginal fillets occurred on short (0.3mm) pads poor agreement on which solder joints (fillets) were considered visual fails

SUCCESSFUL AUTOMATED OPTICAL INSPECTION capable of performing relatively fast inspection requires expertise to set-up and program properly likely to reject reliable solder joints if board assembly process exhibits significant variation

SOLDER WETTING & WETTED HEIGHT excellent wetting to the lead ends wetted solder height consistently >140um increased variability in solder fillet formation/shape on shorter (0.3mm) pads

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Agenda





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Solder Joint Life Joint Life / Study Results

Feature vs. Feature / Where does the weakness lie

Fillet formation / How this factor influences outcome

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Solder Joint Test Study Conditions

Solder Joint Life Testing (SJRT) performed at internal Freescale lab.

DAISY CHAIN CONFIGURATION – ALL UNITS/BOARDS

AIR-to-AIR TEMPERATURE CYCLE Temperature range = -40C / 125C approx 1 hour cycles – 15minute dwells & 15minute transitions

INSITU MONITORING real-time event detection/recording.

ALL BOARDS TEMP CYCLED SIMULTANEOUSLY All SJRT printed circuit boards are cycling in the same chamber

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Note: e-type control cells show similar SJRT performance (all control cells = 0.6pads & no bake).

3 of 4 e-type control cells have 0 failures at 4333 cycles.

1 control cell (e-5-A-3-n) has 9 failures – 1st fail at 2796 cycles.

5x5mm 32ld:

Step Cut Solder Joint Reliability Testing – Results to date

7x7mm 48ld:

7x7mm Step cut DOE - SJRT 4333 cycles

CELL paste pad ext aging Cell 1st Fail # Failed % Failed

Step Cut 71 A 0.6 No 71 3593 4 13%

Step Cut 72 A 0.6 Yes 72 2304 3 9%

Step Cut 73 A 0.3 No 73 3233 8 25%

Step Cut 74 A 0.3 Yes 74 1589 13 41%

Step Cut 75 B 0.6 No 75 0 0%

Step Cut 76 B 0.6 Yes 76 3438 1 3%

Step Cut 77 B 0.3 No 77 3668 4 13%

Step Cut 78 B 0.3 Yes 78 4152 2 6%

5x5mm Step cut DOE - SJRT 4333 cycles

CELL paste pad ext aging Cell 1st Fail # Failed % Failed

Step Cut 51 A 0.6 No 51 2638 10 31%

Step Cut 52 A 0.6 Yes 52 2798 7 22%

Step Cut 53 A 0.3 No 53 1540 7 24%

Step Cut 54 A 0.3 Yes 54 1474 12 38%

Step Cut 55 B 0.6 No 55 3553 2 6%

Step Cut 56 B 0.6 Yes 56 3157 1 3%

Step Cut 57 B 0.3 No 57 3459 1 3%

Step Cut 58 B 0.3 Yes 58 0 0%

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Solder Joint Reliability Test results – as of 4333 cycles

5-A-6-n

4145

2638

3222

3419

3435

3524

3697

3728

3960

4002

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

32 . . .

5-A-6-b

4087

2798

2830

3528

3557

3758

3816

5-A-3-n

1540

2521

3118

3892

3946

4037

4051

5-A-3-b

4200

4255

1474

2633

3170

3231

3235

3629

3855

3873

3993

4058

5-B-6-n

3553

3939

5-B-6-b

3157

5-B-3-n

3459 *

5-B-3-b

*The single solder joint from cell 57, labeled as “missing” based on human-eye

inspection, survived at least 3459 cycles during board life testing (-40/125C).

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Board Life Charts – 5x5mm study cells (w/enough fails to chart)

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Outlier early life failure – probable manufacturing “artifact” due to non-optimized PCB assembly process

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Solder Joint Reliability Test results – as of 4333 cycles

7-A-6-n

4217

3593

3746

4208

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

32 . . .

7-A-6-b

2304

3027

3218

7-A-3-n

4136

4228

4251

3233

3420

3577

3752

4001

7-A-3-b

1589

2283

3074

3225

3233

3416

3476

3658

3722

3735

3832

3849

4046

7-B-6-n 7-B-6-b

3438

7-B-3-n

4223

7-B-3-b

4152

4266

3668

3812

3898

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Outlier early life failure – probable manufacturing “artifact” due to non-optimized PCB assembly process

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All PCB Assembly cells

Solder: A vs. B

64 11

Pad: 0.6 vs. 0.3

28 47

Aging: y vs. n

39 36

Body size: 5x5 vs. 7x7

40 35

Solder Joint Reliability vs. DOE Factors (based on failures after 4.3K cycles)

Solder paste system shows

the largest impact on solder

joint life

------------

Pad size may be a factor,

but likely due to board

assembly variation

rather than actual weakness

related to the presence or

“quality” of the fillet.


Note: e-type control cells show similar SJRT performance and response to DOE factors.

3 of 4 e-type control cells have 0 failures at 4333 cycles.

1 control cell (e-5-A-3-n) has 9 failures – 1st fail at 2796 cycles.

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Agenda





• CONCLUSION

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Conclusion Freescale IJ Offering / Preferences & Availability

Practicality / In the hands of the customers

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Freescale Inspectable Joint (IJ) Features

The step cut (mechanical removal by saw) is Freescale’s primary

solution for QFN-IJ (QFN packages with inspectable joint lead-end features)

DEMONSTRATED MANUFACTURABILITY Repeatable results for our customers as well as for Freescale

ROBUST SOLDER JOINT RELIABILITY SJR >2000+ cycles (-40/125C) – assumes a well designed/optimized PCB assembly process

SUPERIOR AVAILABILITY/COMPATABILITY Adaptable to the widest array of QFN outlines and manufacturing lines

The dimple (etched) IJ feature is offered by Freescale as an

alternative QFN-IJ solution for use cases which cannot use the step

cut due to specific application requirements.

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Consistent formation of inspectable solder joints requires a well

designed and optimized board assembly process.

WF / IJ FEATURE TYPE: Doesn’t really matter Step-cut or half-etched or e-type – all offer excellent solder joint life, Step-cut or half-etched or e-type – all have good potential for formation of inspectable joints Wettability of the lead end does not guarantee wetting to the lead end or an inspectable joint

DESIGN: Matters One of the more significant factors in producing good, consistent inspectable solder joints PCB pad design, feature clearances, finish quality, PCB construction, etc.

PROCESS : Matters Many factors of the board assembly process influence successful formation of inspectable solder joints Material choices consistently prove to be a significant factor in successful formation of inspectable solder joints as well as solder joint life. Choose a good quality, reliable solder paste material system & supplier

AOI: Customer’s choice Capability & repeatability are highly dependent on set up, programming, lighting, etc. Likely to reject reliable solder joints if PCB design and assembly process aren’t optimized

Practical Considerations – What Matters

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THANK YOU THANK YOU

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automotive devices: quad no-lead (qfn) technology...

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