Bonding Strengths and Thermal Degradation of Photovoltaic Module Ribbon Solder Joints

In this study, solar ribbon solder joints were investigated to ensure the reliability of photovoltaic (PV) modules. Ribbon joints comprising two different solder compositions (wt. %: 60Sn40Pb, 62Sn36Pb2Ag) were used to perform thermal aging tests at three different temperatures (150 °C, 120 °C, and 90 °C) during a 1000-h period to analyze the resultant thermal degradation; shear tests were also performed to measure bond strengths. Initial bond strengths were 271.3 MPa and 241.3 MPa for 62Sn36Pb2Ag and 60Sn40Pb solder joints, respectively. Bonding strength decreased as a result of thermal aging, but was maintained at a value of around 130 MPa regardless of solder composition. Fracture surfaces from the shear test were observed to analyze for the constant bond strength phenomenon. It was verified that the shear fracture surface changed from the solder/sintered Ag interface and the sintered silver (Ag)/silicon (Si) wafer interface during thermal aging. Thus, the bond strengths and bonding characteristics of PV ribbon solder joints decreased under a thermal load, which could be attributed to a weakening of the bonding characteristics for sintered Ag silicon interfaces as opposed to a degradation of solder metallurgy.