Bonding copper ribbons on crystalline photovoltaic modules using various lead-free solders

The requirement for environmentally friendly technology has led to considerable efforts to develop photovoltaic (PV) modules. The copper (Cu) ribbon interconnects of PV modules require alternative bonding materials to replace the current solder that contains lead. Several candidate lead-free solder materials have been developed that are inexpensive and reliable. In this study, we evaluate Cu ribbon interconnection bonding with Sn-0.7Cu and Sn-48Bi-2Ag solder materials to address their feasibility for use in PV modules by investigating material degradation during thermal cycling. After thermal cycling, in the Sn-0.7Cu solder, considerable intermetallic compound (IMC) growth occurred at both interfaces, while Bi-rich grains coarsened with IMC growth in Sn-48Bi-2Ag solder. However, the degradation of the electrical performance of the PV modules was similar for those containing both solder materials. Cracks that appeared at the interface propagated from the fractures of the Ag3Sn IMC layers. This suggests that preventing IMC growth at the interface between the Cu ribbon and Ag sintered electrode is important to improve the reliability of interconnects in PV modules. The use of Pb-free solder in PV modules opens up a novel opportunity to realize high reliability at low cost.