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

Flip Chip

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

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

Finite Element Model Silicon Bismaleimide Triazene (BT) Substrate UnderfillSolder

Material Properties Material Young’s Modulus, E (GPa) Poisson’s Ratio ( ) Thermal Exp. Coefficient (10 -6 /K) Sn-3.5Ag Solder Underfill Silicon chip Bismaleimide Triazene (BT)

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

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

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

Mises Stress  Curvature agrees with intuition  Slight variation (few MPa)

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

Mises Stress Rigid Si  More variation in stress among solders

σ 22 Rigid Si  Variation in stress in solders: ~20 MPa on right-side to ~35-40 MPa near left-side

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

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