Life Prediction of Solder Interconnects under Harsh Thermal Cycling from Microstructural Degradation

Designing dependable solar PV systems that last a long time and function in a wide variety of temperatures in low earth orbit requires precise forecast of solder interconnect lifespans, yet useful prediction methodology has rarely been studied. Here, a strategy for measuring the solder interconnects' fatigue life was established using experimental, computational, and analytical methodologies. Thermal aging, material characterization, and thermal cycling were used to explore the physics of failure. Equilibrium liquid surface simulation and finite element modeling were used to establish the geometry of the solder interconnects, and to obtain the strain distribution. To accurately predict the lifespan of solder interconnects, microstructure evolution modeling, constitutive equation fitting, and fatigue model calibration were performed. We have demonstrated that increasing the stand-off height of solder interconnects can delay the onset of fatigue cracks and prolong total fatigue life in photovoltaic systems using this integrated approach to busbar-to-wire solder interconnects. Interconnects with large solder quantity have a long life due to slow crack propagation with low averaged plastic work, but they have about the same number of cycles to fatigue crack initiation due to relatively stable plastic strain range during heat cycling. The proposed approach could be used to motivate the design of strong interconnects in photovoltaic systems that can withstand rigorous thermal cycling.