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					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




                                                               -1-
              June 19th, 2008   Nicholas Vickers
                                Overview

l Failure analysis Methods

l Failure analysis results

l Mechanical testing

l Conclusions




                                                      -2-
              June 19th, 2008      Nicholas Vickers
        Failure Analysis Techniques

l 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




                                                       -3-
               June 19th, 2008   Nicholas Vickers
                            Results


                                                 35%




**Note- Some components show more than one failure mode

                                                          -4-
          June 19th, 2008     Nicholas Vickers
Pad Cratering and Electrical Failure




                          Pad
                          Cratered

                                                  Not Pad Cratered




                                                                     -5-
        June 19th, 2008        Nicholas Vickers
          Solder Fracture Failure

l Cross-sectioned solder joint is shown to be
  cracked near the board side copper pad




                                                 -6-
            June 19th, 2008   Nicholas Vickers
           Solder Fracture Failure

l Cross-sectioned solder joint is shown to be cracked
  near the board side copper pad
l Copper trace failure also shown (left side)




                                                        -7-
             June 19th, 2008   Nicholas Vickers
    Cracking Under Pads (Cratering)

l Epoxy on the PWB board surface cracked away from
  the fibers within the board, allowing the copper pad
  to lift away from the board




                                                         -8-
             June 19th, 2008   Nicholas Vickers
    Cracking Under Pads (Cratering)

l 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-
            June 19th, 2008   Nicholas Vickers
         Input/Output Trace Failure

l I/O trace gets stretched when the copper pad lifts
  away from the PWB
l If the copper pad lifts far enough away, then ductile
  failure occurs in the copper trace




                                                          -10-
              June 19th, 2008   Nicholas Vickers
                  I/O Trace Failure

l Input/Output (I/O) traces that connect to the
  daisy-chain ‘resistor’ were often broken
l Many components had this broken trace and no
  other identifiable failure




         Component side                           Board side
                                                               -11-
             June 19th, 2008   Nicholas Vickers
I/O Trace Failure Location




                                        -12-
   June 19th, 2008   Nicholas Vickers
         Failure Mode Comparison

l I/O Trace and Daisy-chain Trace failures are both
  caused by pad cratering
l Pad cratering was present on 88% of electrically
  failed components, and is directly responsible for
  69% of electrical failures




                                                       -13-
             June 19th, 2008   Nicholas Vickers
          Failures After 10 Drops (No EB)




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)

                                                                                                             -14-
                         June 19th, 2008         Nicholas Vickers
          Failures After 14 Drops (No EB)




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-
                         June 19th, 2008         Nicholas Vickers
    Failures After 325 Drops (Epoxy EB)




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-
                         June 19th, 2008         Nicholas Vickers
   Failures After 279 Drops (Acrylic EB)




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-
                         June 19th, 2008         Nicholas Vickers
SAC305 Solder Microstructure




                                         -18-
    June 19th, 2008   Nicholas Vickers
Microhardness Testing




                                      -19-
 June 19th, 2008   Nicholas Vickers
Mechanical Testing of Boards

l Need to know material properties of
  the board to simulate using FEA
  methods.

l Tested along fiber direction using an
  Instron tensile tester.




                                            -20-
       June 19th, 2008   Nicholas Vickers
     Along Fiber Results



                                                  σ fracture

                                        ion
                                     eg
                              ti   cR
                      E   las




Fibers Unkinking




                                                               -21-
    June 19th, 2008            Nicholas Vickers
Along Fiber Strength




                                     -22-
June 19th, 2008   Nicholas Vickers
Along Fiber Modulus




                                     -23-
June 19th, 2008   Nicholas Vickers
                                Conclusions

l Pad cratering is the most common failure mode
l Pad cratering does not necessarily cause electrical
  failure, but can cause electrical failure by introducing
  other failure modes
l Dominance of pad cratering indicates that solder
  joints are not the weakest part of this lead-free
  assembly




                                                             -24-
              June 19th, 2008       Nicholas Vickers
               Conclusions (Cont.)

l Tougher board material is needed to increase
  reliability
l Majority of failures occurred on the cable side of the
  board when DAQ cable is attached
l First failures usually occur in the corners of the CSPs
l Edge-bonding is effective at reducing pad cratering
  problems




                                                            -25-
              June 19th, 2008   Nicholas Vickers
             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




                                                  -26-
           June 19th, 2008   Nicholas Vickers
             Acknowledgements
l Project Sponsors:




l Office of Naval Research (ONR)
   Through California Central Coast Research Park (C3RP)
l Society of Manufacturing Engineers Education Foundation
l Surface Mount Technology Association Silicon Valley



                                                            -27-
            June 19th, 2008   Nicholas Vickers
          Thank You.
         Any Questions?




                                     -28-
June 19th, 2008   Nicholas Vickers
Supplementary Slides




                                     -29-
June 19th, 2008   Nicholas Vickers
        Dye Stained Solder Fractures

l 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




                                                                   -30-
               June 19th, 2008   Nicholas Vickers
Failures After 10 Drops (No EB)




                                          -31-
     June 19th, 2008   Nicholas Vickers
Failures After 14 Drops (No EB)




                                          -32-
     June 19th, 2008   Nicholas Vickers
Failures After 325 Drops (Epoxy EB)




                                            -33-
       June 19th, 2008   Nicholas Vickers
Failures After 279 Drops (Acrylic EB)




                                             -34-
        June 19th, 2008   Nicholas Vickers
     Test Vehicle Drop Orientation

l Test vehicle is always mounted with components
  face down




                                                   -35-
           June 19th, 2008   Nicholas Vickers
           Component Locations

l JEDEC defined component numbering
  • The DAQ cable attaches near component C6 (in between
    components C1 and C11)




                                                           -36-
            June 19th, 2008   Nicholas Vickers
    Blank PWB – No Cable vs Cable


                                 1500G Input Acceleration




• Symmetry of acceleration peaks has shifted (C7 vs C9)
• Maximums greatly reduced by cable (C3, C13, C8)


                                                              -37-
               June 19th, 2008             Nicholas Vickers
    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)



                                                                -38-
               June 19th, 2008             Nicholas Vickers
           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


                                                                    -39-
               June 19th, 2008             Nicholas Vickers
          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


                                                                    -40-
               June 19th, 2008             Nicholas Vickers

				
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