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

2. Flip Chip

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

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

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

6. Material Properties Material Young’s Modulus, E (GPa) Poisson’s Ratio ( ) Thermal Exp. Coefficient (10 -6 /K) Sn-3.5Ag Solder50 0.323 Underfill6 0.3530 Silicon chip131 0.32.8 Bismaleimide Triazene (BT) 26 0.3915

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. σ 22  Stress is ~20 MPa in Solder Bumps  Slight variation (~5 MPa)

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

13. σ 22 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 σ 22 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