Solder Bond Degradation of Fielded PV Modules: Climate Dependence of Intermetallic Compound Growth

Solder bond degradation is typically ranked in the top two degradation modes observed in the field deployed modules. One of the main reasons for the solder bond degradation is the formation of intermetallic compounds (IMC) at the solder-cell metallization interfaces. The growth of IMC layer is dictated by the microclimate factors in which the modules are being exposed over several years. This paper presents the characterization and analysis of IMC layer of the modules, with identical model/type (Siemens M55), retrieved from two different field climates – Arizona (hot and dry) and Florida (hot and humid) along with an unexposed control module. The modules were tested with current-voltage (I-V), electroluminescence imaging and infrared thermography. The series resistance (Rs) and other electrical parameters of individual cells were obtained from dark I-V curves. The field induced degradation of Cu ribbon-solder and solder-Ag busbar interfaces were examined with scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS) profiling. The IMC thickness is calculated, which exhibits a good correlation with cell Rs. Arizona modules operating at elevated operating temperatures suffered from thermomechanical fatigue, leading to the detachment of soldered ribbon from both top and rear cell contacts, while Florida modules showed higher degradation predominantly at the rear contacts due to the moisture transport and attack through the backsheet.