stress in flip-chip solder bumps due to package warpage
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Stress in Flip-Chip Solder Bumps due to Package
Warpage
Matt PharrES-240 Project
12/9/08
Flip Chip
Applied Load
Circuit Board
e e eSi
Circuit Board
e e eSi
W
MTTF = 183 hrs
MTTF = 880 hrs
Origin of Applied LoadSi – rigid,
Small CTE
Solder in Molten State
Substrate, large CTE
Cooling
Finite Element Model
200
400
400
5000
Silicon
Bismaleimide Triazene (BT) Substrate
200200
150
Underfill Solder
Material Properties
Material Young’s Modulus, E
(GPa)
Poisson’s Ratio ()
Thermal Exp. Coefficient (10-6/K)
Sn-3.5Ag Solder 50 0.3 23
Underfill 6 0.35 30
Silicon chip 131 0.3 2.8
Bismaleimide Triazene (BT)
26 0.39 15
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)
σ22
Stress is ~20 MPa in Solder Bumps Slight variation (~5 MPa)
Mises Stress Rigid Si
More variation in stress among solders
σ22 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 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
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