Modeling the failure of intermetallic/solder interfaces

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 due to high current density. Previously, the prevalent failure mode in a solder joint was the ductile thermo-mechanical fracture of solder material due to repeated thermal cycling. In addition to this mode of failure the joints were also found to fail by brittle fracture near the solder–intermetallic interface. To predict and reduce the failure of solder joints, a model that incorporates both plastic damage in bulk solder and solder–intermetallic interface failure is timely and useful to the electronics industry. Based on cohesive fracture theory, a 3D finite element model has been developed to predict the interfacial damage of Sn–Ag–Cu eutectic lead-free solder interconnects. Unified creep-plasticity theory is incorporated in the model considering the creep and hysteresis effects in solder bulk. Using the unified creep-plasticity-cohesive finite element model, the intermetallic layer growth effect has been researched for different solders. The failure of solder is a complex, coupled thermo-electric-mechanical problem. To understand better the phenomenon, a thermo-electric finite element analysis has been conducted to predict the electrical concentration and joule heating effects on the failure of Sn–2.5Ag–0.8Cu–0.5Sb 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 has been predicted. Pronounced temperature and electrical current concentration are observed near the crack tip. Although the Pb-free solder is usually regarded as having a higher melting temperature compared with traditional Pb–Sn eutectic solder, the concentration of heat at the crack tip caused by joule heating may still melt the solder material under high current density stressing.