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-1-Nicholas VickersJune 19th, 2008
SMTA Silicon Valley Chapter, June 19th, 2008
Board Level Failure Analysis of Chip Scale Packages
Nicholas Vickers, Kyle Rauen, Andrew Farris, Michael Krist, Ronald Sloat, Jianbaio Pan, Ph.D.
California Polytechnic State University at San Luis Obispo
-2-Nicholas VickersJune 19th, 2008
OverviewOverview
Failure analysis Methods
Failure analysis results
Mechanical testing
Conclusions
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Failure Analysis TechniquesFailure Analysis Techniques
2 techniques used:
• Dye Penetrant• Exposes cracks cause by drop testing
• Crack area, and direction
• Cross sectioning• Locates the layer in which the crack occurs
• Identifies composition of layer cracks occur
-4-Nicholas VickersJune 19th, 2008
ResultsResults
35%
**Note- Some components show more than one failure mode
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Pad Cratering and Electrical FailurePad Cratering and Electrical Failure
Not Pad Cratered
Pad Cratered
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Cross-sectioned solder joint is shown to be cracked near the board side copper pad
Solder Fracture FailureSolder Fracture Failure
-7-Nicholas VickersJune 19th, 2008
Solder Fracture FailureSolder Fracture Failure
Cross-sectioned solder joint is shown to be cracked near the board side copper pad
Copper trace failure also shown (left side)
-8-Nicholas VickersJune 19th, 2008
Cracking Under Pads (Cratering)Cracking Under Pads (Cratering)
Epoxy on the PWB board surface cracked away from the fibers within the board, allowing the copper pad to lift away from the board
-9-Nicholas VickersJune 19th, 2008
Epoxy on the PWB board surface cracked away from the fibers within the board, allowing the copper pad to lift away from the board
Cracking Under Pads (Cratering)Cracking Under Pads (Cratering)
-10-Nicholas VickersJune 19th, 2008
Input/Output Trace FailureInput/Output Trace Failure
I/O trace gets stretched when the copper pad lifts away from the PWB
If the copper pad lifts far enough away, then ductile failure occurs in the copper trace
-11-Nicholas VickersJune 19th, 2008
Input/Output (I/O) traces that connect to the daisy-chain ‘resistor’ were often broken
Many components had this broken trace and no other identifiable failure
I/O Trace FailureI/O Trace Failure
Board sideComponent side
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I/O Trace Failure LocationI/O Trace Failure Location
-13-Nicholas VickersJune 19th, 2008
I/O Trace and Daisy-chain Trace failures are both caused by pad cratering
Pad cratering was present on 88% of electrically failed components, and is directly responsible for 69% of electrical failures
Failure Mode Comparison
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Failures After 10 Drops (No EB)
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failureSolder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)
-15-Nicholas VickersJune 19th, 2008
Failures After 14 Drops (No EB)
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failureSolder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)
-16-Nicholas VickersJune 19th, 2008
Failures After 325 Drops (Epoxy EB)
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failureSolder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)
-17-Nicholas VickersJune 19th, 2008
Failures After 279 Drops (Acrylic EB)
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failureSolder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)
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SAC305 Solder MicrostructureSAC305 Solder Microstructure
-19-Nicholas VickersJune 19th, 2008
Microhardness TestingMicrohardness Testing
-20-Nicholas VickersJune 19th, 2008
Mechanical Testing of BoardsMechanical Testing of Boards
Need to know material properties of the board to simulate using FEA methods.
Tested along fiber direction using an Instron tensile tester.
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Elastic
Region
Fibers Unkinking
σ fracture
Along Fiber Results
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Along Fiber StrengthAlong Fiber Strength
0
50
100
150
200
250
300
350
10 mm/min 30 mm/min 50 mm/min
Stre
ngth
(MPa
)
-23-Nicholas VickersJune 19th, 2008
Along Fiber ModulusAlong Fiber Modulus
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
10 mm/min 30 mm/min 50 mm/min
Elas
tic
Mod
ulus
(M
Pa)
-24-Nicholas VickersJune 19th, 2008
ConclusionsConclusions
Pad cratering is the most common failure modePad cratering does not necessarily cause electrical
failure, but can cause electrical failure by introducing other failure modes
Dominance of pad cratering indicates that solder joints are not the weakest part of this lead-free assembly
-25-Nicholas VickersJune 19th, 2008
Conclusions (Cont.)Conclusions (Cont.)
Tougher board material is needed to increase reliability
Majority of failures occurred on the cable side of the board when DAQ cable is attached
First failures usually occur in the corners of the CSPsEdge-bonding is effective at reducing pad cratering
problems
-26-Nicholas VickersJune 19th, 2008
AcknowledgementsAcknowledgements
Cal Poly: Michael Krist, Kyle Rauen, Micah Denecour, Andrew Farris, Ron Sloat, and Jianbaio Pan, Ph.D.
Flextronics: Dongkai Shangguan, Ph.D., Jasbir Bath, David Geiger, Dennis Willie
Henkel: Brian Toleno, Ph.D., Dan Maslyk
-27-Nicholas VickersJune 19th, 2008
Acknowledgements
Project Sponsors:
Office of Naval Research (ONR)
Through California Central Coast Research Park (C3RP)Society of Manufacturing Engineers Education FoundationSurface Mount Technology Association Silicon Valley
-28-Nicholas VickersJune 19th, 2008
Thank You.Thank You.Any Questions?Any Questions?
-29-Nicholas VickersJune 19th, 2008
Supplementary SlidesSupplementary Slides
-30-Nicholas VickersJune 19th, 2008
Dye Stained Solder Fractures
Dye stained solder fractures were found• Partial solder fracture (left) was not completely fractured
before the component was removed
• Complete solder fracture (right) was fully fractured before the component was removed
-31-Nicholas VickersJune 19th, 2008
Failures After 10 Drops (No EB)
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Failures After 14 Drops (No EB)
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Failures After 325 Drops (Epoxy EB)
-34-Nicholas VickersJune 19th, 2008
Failures After 279 Drops (Acrylic EB)
-35-Nicholas VickersJune 19th, 2008
Test Vehicle Drop Orientation
Test vehicle is always mounted with components face down
-36-Nicholas VickersJune 19th, 2008
Component Locations
JEDEC defined component numbering• The DAQ cable attaches near component C6 (in between
components C1 and C11)
-37-Nicholas VickersJune 19th, 2008
Blank PWB – No Cable vs Cable
• Symmetry of acceleration peaks has shifted (C7 vs C9)
• Maximums greatly reduced by cable (C3, C13, C8)
1500G Input Acceleration
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Populated PWB – No Edge Bond
• Dampening due to the cable seems less significant than with blank PWB (both graphs are more similar)
1500G Input Acceleration
-39-Nicholas VickersJune 19th, 2008
Epoxy Edge Bonded CSPs
• Stiffer board with edge bonding has less symmetry disturbance
• Overall accelerations are significantly reduced vs no edge-bond
1500G Input Acceleration
-40-Nicholas VickersJune 19th, 2008
Acrylic Edge Bonded CSPs
• Stiffer board with edge bonding has less symmetry disturbance
• Overall accelerations are significantly reduced vs no edge-bond
1500G Input Acceleration