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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
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• 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
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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
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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.
Data PointsProbabi lity L ine
Fab 3\Lam GWeibull-2PRRX SRM MED FMF=19/S=9
Data PointsProbabi lity L ine
Fab 4\Lam CWeibull-2PRRX SRM MED FMF=12/S=20
Data PointsProbabi lity L ine
Fab 4\Lam HWeibull-2PRRX SRM MED FMF=10/S=14
Data PointsProbabi lity L ine
Fab 5\Lam B OSPWeibull-2PRRX SRM MED FMF=32/S=16
Data PointsProbabi lity L ine
Fab 6\Lam EWeibull-2PRRX SRM MED FMF=16/S=16
Data PointsProbabi lity L ine
Fab 6\Lam FWeibull-2PRRX SRM MED FMF=11/S=17
Data PointsProbabi lity L ine
Failure, F(ue)
micro strain, (ue)
1.000Fab 1\Lam AWeibull-2PRRX SRM MED FMF=13/S=27
Data PointsReliability Line
Fab 3\Lam DWeibull-2PRRX SRM MED FMF=14/S=14
Data PointsReliability Line
Fab 3\Lam GWeibull-2PRRX SRM MED FMF=19/S=9
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
Fab 4\Lam CWeibull-2PRRX SRM MED FMF=12/S=20
Data PointsReliability Line
Fab 4\Lam HWeibull-2PRRX SRM MED FMF=10/S=14
Data PointsReliability Line
Fab 5\Lam B OSPWeibull-2PRRX SRM MED FMF=32/S=16
Data PointsReliability Line
Fab 6\Lam EWeibull-2PRRX SRM MED FMF=16/S=16
Data PointsReliability Line
Fab 6\Lam FWeibull-2PRRX SRM MED FMF=11/S=17
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%
Fab 3\Lam DWeibull-2PRRX SRM MED FMF=14/S=14
95%
Fab 3\Lam GWeibull-2PRRX SRM MED FMF=19/S=9
95%
Fab 4\Lam CWeibull-2PRRX SRM MED FMF=12/S=20
95%
Fab 4\Lam HWeibull-2PRRX SRM MED FMF=10/S=14
95%
Fab 5\Lam B OSPWeibull-2PRRX SRM MED FMF=32/S=16
95%
Fab 6\Lam EWeibull-2PRRX SRM MED FMF=16/S=16
95%
Fab 6\Lam FWeibull-2PRRX SRM MED FMF=11/S=17
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
Data PointsProbability Line
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
Data PointsProbability Line
Folio2\Lam_G cerWeibull-2PRRX SRM MED FMF=19/S=9
Data PointsProbability Line
Folio2\Lam_E cerWeibull-2PRRX SRM MED FMF=20/S=16
Data PointsProbability Line
Folio2\Lam_B cerWeibull-2PRRX SRM MED FMF=51/S=0
Data PointsProbability Line
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
99.000Probability-Weibull
Comino_20L_PA_no_poly\Data 1Weibull-2PRRX SRM MED FMF=20/S=24
Data PointsProbability Line
Comino_20L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=15/S=13
Data PointsProbability Line
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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%
Comino_26L_PA_no_poly\Data 1Weibull-2PRRX SRM MED FMF=38/S=30
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
99.000Probability-Weibull
Comino 26L_PA_poly\Data 1Weibull-2PRRX SRM MED FMF=13/S=15
Data PointsProbability Line
Comino_26L_PA_no_poly\Data 1Weibull-2PRRX SRM MED FMF=38/S=30
Data PointsProbability Line
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