Durability of Pb-Free solder connection between copper interconect wire and crystalline silicon solar cells

Abstract

The thermal cycling durability of large-area Pb-free (Sn3.5Ag) solder interconnects on photovoltaic (PV) solar laminates, has been studied and benchmarks against existing Sn36Pb2Ag interconnects, using a combination of accelerated testing and physics-of-failure (PoF) modeling. Accelerated thermal cycling tests conducted on photovoltaic laminates of both solder compositions, show that the interconnect resistance (measured from dark I-V curves) show that Pb-free laminates outperform Sn37Pb laminates with significantly different response history. Linear extrapolation of the trends from the firs 1000 cycles, suggests that Sn3.5Ag interconnects are 3.5 times more durable than Sn36Pb2Ag interconnects. Due to nonlinearities in the damage growth rate, this estimate may be non-conservative. Post failure analysis shows cracks close TO the interface between the solder and the Ag ink used on the Si wafer. Distributed solder damage is also evident in Sn36Pb2Ag specimens. Acceleration factors were estimated based on a two dimensional viscoplastic finite element analysis and damage predictions based on an energy-partitioning fatigue model. Error-seeded models reveal that process-induced voids, commonly encountered in this architecture can be detrimental to thermal cycling durability. Results suggest that even the worst case (highest void density) Pb-free specimen has a higher durability than the best case {void-tree} Sn37Pb specimen. For the worst void density configuration, accelerated test simulations predict that the Sn3.5Ag interconnects are 1.8 times as robust as the Sn36Pb2Ag interconnects. PoF modeling also shows that the Pb-free solder PV laminates have a higher acceleration factor than the Sn37Pb solder laminates