Durability of Pb-Free Solder Connection between Copper Interconnect Wire and Crystalline Silicon Solar Cells - Experimental Approach

The thermal cycling durability of large-area Pb-free (Sn3.5Ag) solder interconnects on photovoltaic (PV) solar cells, has been studied and benchmarked against existing Sn36Pb2Ag interconnects, using a combination of accelerated testing and physics-of-failure modeling. This paper reports the accelerated testing tasks and the modeling tasks are reported elsewhere in the literature. Accelerated thermal cycling tests have been conducted on photovoltaic laminates of both solder compositions, to characterize the increase in interconnect resistance due to fatigue damage. Interconnect resistance is measured up to 1000 cycles, from dark I-V curves for PV single-cell laminates. Resistance measurements show that single-cell Pb-free laminates outperform the Sn36Pb2Ag laminates. The Sn36Pb2Ag configuration shows a steady increase of resistance while the Pb-free configuration shows a bilinear increase as a function of thermal cycles. The resistance increase rate at 1000 cycles is higher for the Sn36Pb2Ag interconnects than for the Sn3.5Ag interconnects. A simple linear extrapolation of the trends observed during the first 1000 cycles, suggests that the Sn3.5Ag interconnect is 3.5 times more durable than the Sn36Pb2Ag interconnect. However, due to nonlinearities in the damage growth rate, this ratio is expected to be non-conservative and somewhat lower in practice. Post failure analysis of the PV laminates shows cracks predominantly close to the interface between the solder and the Ag ink used as contact electrodes on the silicon wafer. These results from the accelerated test results will be combined in a future paper with acceleration factors obtained from physics of failure modeling, to predict the thermal cycling durability under field conditions