Abstract:

The thermal cycling durability of large-area Pb-free (Sn3.5Ag) solder between silicon semiconductor and copper interconnects in photovoltaic (PV) cells is assessed and compared to benchmark results from Pb-based (Sn36Pb2Ag) PV cells. Accelerated thermal cycling tests have been conducted on PV cells of both solder compositions, and the increase in series resistance due to interconnect damage has been characterized using in situ dark I–V measurements. Both the Pbfree and Pb-based cells show a steep initial rise followed by a steady rate of increase in degradation histories, with the Pb free cells showing a more pronounced ‘knee’ in the degradation curves. Extrapolation of the degradation data for both solders suggests that Pb-free cells are four times more durable than the Pb-based cells at the test condition. This superior thermal cycling fatigue durability of Pb-free cells was also confirmed with physics of failure (PoF) analysis, consisting of nonlinear finite element (FE) stress analysis and an energy-partitioning (E-P) solder fatigue model. FE models error-seeded with manufacturing voids in the solder interconnect predicted a significant reduction in the thermal cycling durability with increasing solder void density. However, even the most voided Pb-free cells modeled are predicted to be twice as durable as void-free Pb-based cells, under the accelerated temperature cycle used in the test. The acceleration factor (AF) predicted by the PoF analysis for a typical service environment is three times higher for Pb-free cells than that for Pb-based cells.

Key words: lead-free solder; solar cells; thermal cycling fatigue; dark I–V testing; solder fatigue; acceleration factor; field load; accelerated load

Complete article available to CALCE consortium members