Material Testing & Mitigation Techniques for Pad Crater ... · Material Testing & Mitigation...

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Material Testing & Mitigation Techniques for Pad Crater Defects

John McMahon P.Eng.

Agenda

• Introduction

• Pad Crater Defects

• Bend Test Methods

• Process Strain Limits

• Laminate Susceptibility

2

• Laminate Susceptibility

• Package Compliance

• Observations

• Mitigation Techniques

Pad Crater Defects

Crack initiation in the PWB resin system between the top surface and the first layer of glass reinforcement

•Pb-free solders are less compliant

•IMC interfaces are now more resistant to fracture

–Packaging houses have converted to direct solder on Copper processes

•“Phenolic” cured resins systems replace “Dicy” cured resins

•Micro clays and ceramic particles are added to resin

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•Micro clays and ceramic particles are added to resin systems to reduce Z-axis expansion.

Pad Crater is now reported as the dominant mechanically induced defect in PCBA

assembly

Courtesy of the Intel pad crater group

Bend Test Comparison

• 4 point Bend

–Unidirectional strain and constant bending moment between anvils

• Spherical bend

–Based on ring on ring stress test.

–Drives biaxial strain

• Proposed specification 9707 for

4

120 mm

• Proposed specification 9707 for

spherical bend

SMTAI – Oct . 2011

Spherical BendTest Geometry

Mode 1

Opening

Mode 3

Out-of-Plane Shear

Mode 2In-Plane Shear

Three Modes of Crack Loading

Crack Loading & Path

• Crack path is determined by two primary

factors.

• Stress profile – relative proportions of

mode 1 & mode 2

• Stiffer packages generate mode 1

dominant sytems.

• Type of material – filled or unfilled

• Filled material creates many

opportunities for crack to turn

Pad crater

Cohesive & Adhesive

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opportunities for crack to turn

Pad Peel or Pad Lift

Cohesive only

Typical Telecomm product

There is only 10 to 20 micron between

Pad Peel & Pad Crater

Characteristic crack paths

Strain Limit Specifications• IPC-9704 process window for strain vs. strain rate based on failure in solder

joint

• Revised guidelines published in 2005 – Courtesy of Keith Newman

• Current proposed revision of IPC 9704 moves strain vs. strain graph to

white paper to make updating easier

6

1000

Str

ain

(u

e)

Celestica Internal Strain Limits Test Program to Validate “Safe Working Strain” Limits for Pb-free

compatible materials.

•Spherical bend Test geometry

•Step stress approach to

testing stiff systems.

•Test program:

– 3 strain rates 1000, 3000, 6000

– 2 board thicknesses 0.100 & 0.130

Typical curves for a single board thickness

7

0

500

100 10000

Log Strain rate (ue/s)

SnPb Guideline

"Diag 0.1% 90%CL

"P1 0.1% 90%CL"

"Survival"

"Failure"

– 2 board thicknesses 0.100 & 0.130

– 2 surface finishes

– 4 solder sphere alloys

– Primary attach & forced rework

•Below 2000 ue/s the results seem to

be in good agreement with previous

guidelines.

•Above 2000 ue/s The difference

becomes significant.

Laminate SusceptibilityMultiple Suppliers build to a single design data package

Probability – Weibull 2P

Comparisons are based on:Nominal Principal Strain rate = 3000 µe/sTV - 20 layer – 2.54 mm (0.100) (0.104)32 mm CBGA • With minor exceptions

the slopes (Beta) are very similar and we understand now what to expect from a new lot.

Probability-W eibull

Fab 1\Lam AWeibull-2PRRX SRM MED FMF=13/S=27

Data PointsProbabi lity L ine

Fab 3\Lam DWeibull-2PRRX SRM MED FMF=14/S=14

Data Points

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lot.

• The test method does define statistical differences between lots.


Fab 3\Lam GWeibull-2PRRX SRM MED FMF=19/S=9


Fab 4\Lam CWeibull-2PRRX SRM MED FMF=12/S=20


Fab 4\Lam HWeibull-2PRRX SRM MED FMF=10/S=14


Fab 5\Lam B OSPWeibull-2PRRX SRM MED FMF=32/S=16


Fab 6\Lam EWeibull-2PRRX SRM MED FMF=16/S=16


Fab 6\Lam FWeibull-2PRRX SRM MED FMF=11/S=17


Failure, F(ue)

micro strain, (ue)

1.000Fab 1\Lam AWeibull-2PRRX SRM MED FMF=13/S=27

Data PointsReliability Line




Comparisons are based on:Nominal Principal Strain rate = 3000 µe/sTV - 20 layer – 2.37 mm (0.100) (0.104)32 mm CBGA

Laminate SusceptibilityMultiple Suppliers build to a

single design data package

Reliability – No Damage Plot

• This view is perhaps

more easily read in terms

of risk / reliability

• There is some question

about extrapolation to

very low values when the

slope is different.

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F=19/S=9Data PointsReliability Line










Data Points

micro strain, (ue)

slope is different.

Eta

Beta

500.000 1750.000750.000 1000.000 1250.000 1500.0001 .000

36.000

8 .000

15.000

22.000

29.000

Fab 1\Lam AWeibull-2PRRX SRM MED FMF=13/S=27

95%


95%


95%


95%


95%


95%


95%


95%

17/02/20112:06:55 PM

95% Confidence Contours

Laminate Results

SF SF SF SF SF SF SF SF

A B C D E F G H

Failed

Survived

D & P

Estimated Principle Strain

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SF SF SF SF SF SF SF SF

• There is no notable difference in the structures that would account for a difference of 100% between laminate B & C.

• The “hidden factor” is the resin system .

SMTAI – Oct . 2011

Package Compliance

• Selected Laminates

• No change in rank order.

• More compliant pkg. produces

an increase in survivable

strain.

• In this case 14%-17% Eta.

• Exception is at the high end of

the range. Distributions

converge as mixed failure 50.000

90.000

99.000Probability-Weibull

Folio2\Lam_G meta lWeibull-2PRRX SRM MED FMF=17/S=7

Data PointsProbability Line

Folio2\Lam_B meta lWeibull-2PRRX SRM MED FMF=14/S=16


Monolithic

Ceramic

Built up organic

w/ metal lid

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converge as mixed failure

mode is more probable.

microstrain, (ue)

Failure, F(ue)

1.000

5.000

10.000

50.000Probability Line

Folio2\Lam_E meta lWeibull-2PRRX SRM MED FMF=16/S=8


Folio2\Lam_G cerWeibull-2PRRX SRM MED FMF=19/S=9


Folio2\Lam_E cerWeibull-2PRRX SRM MED FMF=20/S=16


Folio2\Lam_B cerWeibull-2PRRX SRM MED FMF=51/S=0


Eta

Beta

700.000 2000.000960 .000 1220.000 1480.000 1740 .0003.000

40.000

10.400

17.800

25.200

32.600

Contour

Folio2 \Lam_G meta lWeibull-2PRRX SRM MED FMF=17/S=7

95%

Folio2 \Lam_B meta lWeibull-2PRRX SRM MED FMF=14/S=16

95%

Folio2 \Lam_E meta lWeibull-2PRRX SRM MED FMF=16/S=8

95%

Folio2 \Lam_G ce rWeibull-2PRRX SRM MED FMF=19/S=9

95%

Folio2 \Lam_E ce rWeibull-2PRRX SRM MED FMF=20/S=16

95%

Folio2 \Lam_B ce rWeibull-2PRRX SRM MED FMF=51/S=0

95%

Observations on mechanical systems that

create Pad Crater Defects

• The dominant mechanical failure mode in Pb-free compliant materials is Pad Crater and there are no non-destructive methods to identify crack initiation or even significant cracking.

• The strain rate dependency defined in IPC 9704 should be modified for Pb-free.

• The board thickness dependency has been validated.

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• Significant differences have been identified in laminate materials.

• Package stiffness is also a significant factor and could be incorporated into a model generated for a specific program.

Primary drivers for a detailed model:

Board Thickness / Strain rate / Laminate Material / Package Compliance

Pad Crater MitigationSMD Corner Pads• Corner pads are 30.6% larger than NSMD but

solderable area is reduced by 9%

• Force distribution on pad is changed due to change in

ratio between pad size and solderable area

• Reinforcing effect of solder mask has not been

quantified

Results

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• Board tested to mixed fail range shows

failure at IMC.

• Yellow dye cross section of same tested

board shows pad crater traveling below

first layer of glass bundle.

• Crack at IMC will almost certainly

propagate to failure under thermal cycling

loads.

• Crack into the first weave presents an

unknown risk.

Beta

40.000

16.600

24.400

32.200

Contour

Comino_20L_PA_no_poly\Data 1Weibull-2PRRX SRM MED FMF=20/S=24

90%

Comino_20L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=15/S=13

90%

Pad Crater Mitigation TV Stack Design withHigh Modulus Polyimide Layer

Weibull Analysis

• 20 Layer Primary Attach

• Failure mode converts to IMC interface.

• No significant increase in survivable

strain

Unreliability F(ue)

5.000

10.000

50.000

90.000




Comino_20L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=15/S=13


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Eta

Beta

800.000 2000.0001040.000 1280.000 1520.000 1760.0004.000

30.000

9.200

14.400

19.600

24.800

Contour

Comino 26L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=13/S=15

90%


90%

100.000 5000.0001080.000 2060.000 3040.000 4020.0001.000

8.800

• 26 layer Primary Attach

• Failure mode converts to IMC interface.

• 15% increase in survivable strain

• However we did find some artifacts in

the “un-failed / surviving” samples

Strain (ue)

1.000

Strain (ue)

1.000

5.000

10.000

50.000

90.000


Comino 26L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=13/S=15




Brian GrayCelestica Inc8/15/201112:44:13 PM

Flat Section ResultsPolyimide Lot

• Destructive analysis of survivingsamples.

• Separation at the polyimide / epoxy interface is evident in

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some samples.

• No connection to atmosphere.

• There is some possibility that the separation has been increased by the sectioning process.

Flat Section ResultsReference Lot

• Destructive analysis of surviving samples.

• We did find an example where separation had occurred between the resin and the reinforcing glass.

• No connection to atmosphere.

• This observation must be referenced back to our original assumptions and reservations on crack initiation.

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assumptions and reservations on crack initiation.

Thank youJohn McMahon P.EngJohn McMahon P.Eng

[email protected]

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