BGA FEA underfll study
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This document presents a simplified first-order approach to predicting the fatigue life of solder ball attachments on BGA component packages through thermal cycling analysis. Finite element analysis was used to model different underfill materials and solder compositions under thermal cycling conditions. The predicted solder ball fatigue life ranged from 125 to 300 cycles for SN63 solder and 700 to 2000 cycles for SAC305 solder. A schematic diagram shows the stress-strain behavior of SN63 solder derived from the FEA simulation over temperature changes.
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BGA FEA underfll study
- 1. Don Blanchet 3B Associates 03 April 2016
- 2. There are literally thousands of studies that have been performed to try to predict the fatigue life of solder ball attachments on BGA component packages. Many have been published. Presented here is a simplified first order approach. 2
- 3. Thermal cycle definition single cycle 20C –> 100C -> -45C –> 20C No pwb glass transition effects ¼ symmetry used to limit model size Linear solution Constant material properties over temperature Ball processing collapse included in the models Failure Criteria used : Cumulative Strain energy density – @ crack initiation From publish test data sources Solders SN63 and lead free SAC305 All under fill materials bulk modulus of elasticity, Poisson’s ratio = 0.4 determined by similarity to other organic materials. Typical crack initiation
- 5. Expansion direction X Expansion direction Y Fixed center point MLB , 1.5 x 1.5 x .100 Very large high CTE Ceramic BGA
- 9. High mesh density added In highly stressed Corner balls
- 10. part Body Material Designated Under fill Under fill Thickness, mm BGA High CTE ceramic Ablefill UF8807 0.40 BGA High CTE ceramic Loctite 3568 0.40 BGA High CTE ceramic Mereco 1650 0.40 BGA High CTE ceramic No Under fill Ball height 0.40
- 11. Material Property Pwb FR-4 30% cu Eutectic solder SN63 SAC 305 Lead free solder (flexible) (flexible) (rigid) BGA Body ceramic BGA Body Plastic BT epoxy BGA Body Flip chip Silicon CTE ppm/ C 14 24.7 23.5 250 40 28 6.7 13 3 Elastic Modulus E, psi 3.5e0 6 2.03e06 2.50e06 250 200 1.5e06 32.0e06 3.5e06 16.3e06 Poisson’s ratio 0.3 0.4 0.4 0.4 0.4 0.4 0.22 0.3 0.28 Under fill Materials
- 12. Board bending ~0.003 inches shown at 50X BGA & under fill removed for clarity
- 14. CTE Stretch Induces solder tensile stress CTE mismatch Induces solder shear stress Ref Undistorted ball
- 16. Joule/cu-meter As strain energy exposure accumulates then cracks can form.
- 18. part Body Material Designated Under fill material BGA High CTE ceramic Ablefill UF8807 300 2000 BGA High CTE ceramic Loctite 3568 125 700 BGA High CTE ceramic Mereco 1650 180 1200 BGA High CTE ceramic No underfill 200 1400
- 19. For the specified temperature range per cycle the predicted solder ball fatigue life ranges from: 125 to 300 cycles for SN63 solder 700 to 2000 cycles for SAC 305 solder The predictions are based on published test data for measured cumulative strain energy density in similar BGA’s. FYI : Published data for vibration testing indicate that eutectic solder exhibits a longer life for BGAs. The opposite of thermal cycling.
- 20. Schematic diagram of SN63 solder stress/strain behavior derived from Solidworks FEA simulation. Stress strain behavior vs. temperature and strain rate Hysteresis data from references
- 21. 20C 100C -45C Yield point Yield point Area is proportional to strain energy Per thermal cycle
- 22. Take away Faster rate stiffer Higher temp softer Much Like taffy candy
- 23. Note: the simulation performed was a non-linear visco-plastic model including creep .