1. 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. Overview Failure analysis Methods Failure analysis results
Mechanical testing
Conclusions

3. Failure 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. Results **Note- Some components show more than one failure mode 35%
Modify graph maybe make into a pareto chart
**Note- Some components show more than one failure mode

5. Pad Cratering and Electrical Failure
Pad Cratered
Not Pad Cratered
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6. Solder Fracture Failure
Cross-sectioned solder joint is shown to be cracked near the board side copper pad

7. Solder 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. 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. 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

10. Input/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. I/O Trace Failure
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
Component side
Board side

12. I/O Trace Failure Location

13. Failure Mode Comparison
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

14. Failures After 10 Drops (No EB)
No edge bonding, 10 drops, black (pad cratering) and red (solder failure)
Only 7 solder failures, almost all components have pad cratering after few drops
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 41
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
Solder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)

15. Failures After 14 Drops (No EB)
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 44
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
Solder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)

16. Failures After 325 Drops (Epoxy EB)
More solder failures, some components do not have pad cratering after many drops
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 21
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
Solder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)

17. Failures After 279 Drops (Acrylic EB)
Only observed component side failure (solder fracture near IMC) on component 11
Component 10 is completely failed but unknown reason?
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 36
Component: Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
Solder Joints: Black – pad crater, Red – solder fracture (board side), Yellow – solder fracture (csp side)

18. SAC305 Solder Microstructure

19. Microhardness Testing

20. Mechanical 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.

21. Along Fiber Results
σ fracture
Elastic Region
Fibers Unkinking

22. Along Fiber Strength

23. Along Fiber Modulus

24. Conclusions Pad cratering is the most common failure mode
Pad 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. 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 CSPs
Edge-bonding is effective at reducing pad cratering problems

26. Acknowledgements
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. Acknowledgements Project Sponsors: Office of Naval Research (ONR)
Through California Central Coast Research Park (C3RP)
Society of Manufacturing Engineers Education Foundation
Surface Mount Technology Association Silicon Valley

28. Thank You. Any Questions?

29. Supplementary Slides

30. 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. Failures After 10 Drops (No EB)
No edge bonding, 10 drops, black (pad cratering) and red (solder failure)
Only 7 solder failures, almost all components have pad cratering after few drops
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 41

32. Failures After 14 Drops (No EB)
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 44

33. Failures After 325 Drops (Epoxy EB)
More solder failures, some components do not have pad cratering after many drops
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 21

34. Failures After 279 Drops (Acrylic EB)
Only observed component side failure (solder fracture near IMC) on component 11
Component 10 is completely failed but unknown reason?
Green – no failure, Blue – transitional failure, Orange – full failure, Red – complete failure
board number 36

35. Test Vehicle Drop Orientation
Test vehicle is always mounted with components face down

36. Component Locations JEDEC defined component numbering
The DAQ cable attaches near component C6 (in between components C1 and C11)

37. Blank PWB – No Cable vs Cable
1500G Input Acceleration
also symmetry on C11 and C15
Symmetry of acceleration peaks has shifted (C7 vs C9)
Maximums greatly reduced by cable (C3, C13, C8)

38. Populated PWB – No Edge Bond
1500G Input Acceleration
Dampening due to the cable seems less significant than with blank PWB (both graphs are more similar)

39. Epoxy Edge Bonded CSPs
1500G Input Acceleration
Stiffer board with edge bonding has less symmetry disturbance
Overall accelerations are significantly reduced vs no edge-bond

40. Acrylic Edge Bonded CSPs
1500G Input Acceleration
Stiffer board with edge bonding has less symmetry disturbance
Overall accelerations are significantly reduced vs no edge-bond