Modeling the Mechanical Properties of Intermetallic/Solder Interfaces

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Solder joint integrity is recognized as a key issue in the reliability of flip chip and ball grid arrays in integrated circuit packages. Significant reductions in the solder-joint interconnect size results in both the increased volume fraction of brittle intermetallics in the joint and joule heating and electromigration failure due to high current density. Based on cohesive fracture theory, a 3D computational model has been developed to predict the crack nucleation, propagation and interfacial damage of interconnects. Unified creep-plasticity theory and a cohesive zone model were incorporated to predict the creep and hysteresis effects on fatigue crack propagation in solder and the interfacial behavior between the solder bulk and the intermetallic layer, respectively. A thermo-electric numerical analysis was conducted to predict the electrical concentration and joule heating effects on the failure of solder under different applied current densities. The temperature and current density distribution in a solder joint with a crack that propagates near the interface of the bulk solder and intermetallic layer was predicted. An approach based on phase transformation theory, micromechanics, and fracture mechanics has been developed to treat fatigue crack propagation in both lead rich and lead free eutectic solders. The predicted fatigue crack propagation rate using phase transformation theory was compared with experimental data for Sn-3.5Ag and Sn-37Pb eutectic solders. Reasonable agreement between theoretical predictions and experimental results was obtained. With the reduction of the size of electronic devices, the current density is increasing rapidly and the electromigration effect becomes more critical to the failure of solder interconnects. The mechanism of electromigration effect to void propagation caused interconnect failure was studied in the research. A kinetic mass diffusion model was developed to predict void width and propagation speed near the interface of intermetallic and solder caused by electromigration. The model gives reasonable prediction to the void width and propagation velocity compared with experimental results.

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  • 09/19/2018
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