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Stress in Flip-Chip Solder Bumps due to Package Warpage

Stress in Flip-Chip Solder Bumps due to Package Warpage. Matt Pharr ES-240 Project 12/9/08. Flip Chip. Circuit Board. MTTF = 183 hrs. Si. W. e. e. e. e. e. e. Circuit Board. Si. MTTF = 880 hrs. Applied Load. Si – rigid, Small CTE. Solder in Molten State. Cooling.

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Stress in Flip-Chip Solder Bumps due to Package Warpage

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  1. Stress in Flip-Chip Solder Bumps due to Package Warpage Matt Pharr ES-240 Project 12/9/08

  2. Flip Chip

  3. Circuit Board MTTF = 183 hrs Si W e e e e e e Circuit Board Si MTTF = 880 hrs Applied Load

  4. Si – rigid, Small CTE Solder in Molten State Cooling Substrate, large CTE Origin of Applied Load

  5. 5000 400 Silicon 200 Bismaleimide Triazene (BT) Substrate 400 150 Underfill Solder 200 200 Finite Element Model

  6. Material Properties

  7. Mesh • 4-node linear coupled temperature-displacement quadrilateral • Fairly fine – why not? • Refined near regions of interest • Edges and solder

  8. Loading Conditions • Step 1: 221°C – melting point of solder • Step 2: 23°C • Coupled temp-disp steady state • x-Symmetry Condition on Right End • Step 3 (Attempted): 1A current through solder • Coupled thermal-electric • Inputted thermal properties of materials • Did not converge • Not sure why

  9. Loading Conditions (cont.) • Step 3: Solder and underfill at 100°C; linear variation in substrate and Si to ambient temp of 70°C • Used subroutine to define this temp field • Study 2: Ran same procedure except that it was assumed that the Si was very rigid and hence could not deform in the vertical direction

  10. Mises Stress • Curvature agrees with intuition • Slight variation (few MPa)

  11. σ22 • Stress is ~20 MPa in Solder Bumps • Slight variation (~5 MPa)

  12. Mises Stress Rigid Si • More variation in stress among solders

  13. σ22 Rigid Si • Variation in stress in solders: ~20 MPa on right-side to ~35-40 MPa near left-side

  14. Discussion • Curvature seems physically intuitive • Variation in solder location seems to have minimal effect on stress • Only ~5 MPa for σ22 • I guessed it would be larger but that was assuming Si is perfectly rigid • If we make Si completely rigid, we get larger variation in stress among solders

  15. Lessons Learned about FEA • FEA has advantages (over experiments): • Relatively easy • Easy to change material parameters • Do not assume FEA can handle everything • Model could be wrong • Solution may not converge

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