Physics-of-degradation-based life prediction of solder interconnects of long-life solar arrays in low-earth orbit

Exact prediction of lifespans of solder interconnects is critical for designing reliable solar photovoltaic systems that operate for a long time across a wide range of temperatures in low earth orbit, but pertinent prediction methodology has rarely been investigated. Here, a systematic approach for analyzing the fatigue life of the solder interconnects was developed concurrently using experimental, computational, and analytical methods. Physics of failure was examined using thermal cycling, thermal aging, and materials characterization. Solder interconnect geometry as well as strain distribution and evolution were determined through equilibrium liquid surface simulation and finite element simulation. Microstructure evolution modeling, constitutive equation fitting, and fatigue model calibration were conducted to reliably predict solder interconnect lifetime. Using this integrated approach to busbar-to-wire solder interconnects in photovoltaic systems, we have demonstrated that increasing the stand-off height of solder interconnects can delay the onset of the fatigue cracks and prolong the total fatigue life. The interconnects with large solder amount go through long life due to slow crack propagation with low averaged plastic work, but they possess nearly the same number of cycles to fatigue crack initiation because of relatively fixed plastic strain range under thermal cycling. The approach proposed would be useful to inspire the design of robust interconnects in photovoltaic systems that survive harsh thermal cycling (−90 to + 130 °C).