stress in flip-chip solder bumps due to package warpage matt pharr es-240 project 12/9/08
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Stress in Flip-Chip Solder Bumps due to Package
Warpage
Matt PharrES-240 Project
12/9/08
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Flip Chip
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Applied Load
Circuit Board
e e eSi
Circuit Board
e e eSi
W
MTTF = 183 hrs
MTTF = 880 hrs
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Origin of Applied LoadSi – rigid,
Small CTE
Solder in Molten State
Substrate, large CTE
Cooling
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Finite Element Model
200
400
400
5000
Silicon
Bismaleimide Triazene (BT) Substrate
200200
150
Underfill Solder
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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
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Mesh
4-node linear coupled temperature-displacement quadrilateral
Fairly fine – why not? Refined near regions of interest
Edges and solder
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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
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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
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Mises Stress
Curvature agrees with intuition Slight variation (few MPa)
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σ22
Stress is ~20 MPa in Solder Bumps Slight variation (~5 MPa)
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Mises Stress Rigid Si
More variation in stress among solders
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σ22 Rigid Si
Variation in stress in solders: ~20 MPa on right-side to ~35-40 MPa near left-side
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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
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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