Solder joint failure modes in crystalline Si PV modules operated in Tsukuba, Japan for 10 years

There are many kinds of solder joint failure modes in the crystalline Si modules which have been operated in Tsukuba, Japan for 10 years. The modules including mono crystal, multi crystal and hetero junction structure were supplied from four different companies. The samples sliced from the modules are analyzed by using optical microscope, X-ray transmission image, SEM and SEM-EDX. The cross section of module structure beneath the busbar electrode and the finger electrode of the sample is observed by SEM and SEM-EDX. The distribution of Sn, Pb, Ag, Cu, and Si in the cross section region is analyzed by SEM-EDX images. The malfunction positions of the module are specified by blackening in the electrode area in visual inspection as well as identified by EL image inspection. It indicates that there are many types of failure modes such as melting Cu interconnection ribbon, Cu leaching into solder, Ag leaching into solder, long cracks at the interface between solder joint and Ag electrode over 50% of contact region, melting finger line, voids caused by poor wettability of the solder at the interface of Ag electrode, voids caused by excess flux coating, increasing the grain size of solder. It suggests that the crack induces the high electric resistance region and hot spot. Some of cracks may cause the DC arcing which melts Cu ribbon, solder and Si. It was confirmed that soldering process is the most important to enhance the reliability of crystalline Si PV modules.