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 Pbfree 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.

No abstract

Photovoltaic cells, thermal cycling durability, Pb-free solder, The thermal cycling durability of large-area Pb-free viscoplastic finite element analysis, energy partitioning (Sn3.5Ag) solder interconnects on photovoltaic (PV) solar fatigue model laminates, has been studied and benchmarked against existing Sn36Pb2Ag interconnects, using a combination of accelerated NOMENCLATURE testing and physics-of-failure (PoF) modeling. Accelerated thermal cycling tests conducted on photovoltaic laminates of & series ~~~i~t~~~, ohms both solder compositions, show that the interconnect k,, shunt ~~~i~t~~~~, ohms resistance (measured from dark I-V curves) show that Ph-free w, cyclic plastic work it^, m~/mm3 laminates outperform Sn37Pb laminates with significantly wC cyclic creep work it^, m~/mm3 different response history. Linear extrapolation of the trends N, Cycles to Failure from the first 1000 cycles, suggests that Sn3.5Ag N,, cycles to ~~i l~~~ from plasticity interconnects are 3.5 times more durable than Sn36Pb2Ag N,~ cycles to ~~i l~~~ from creep interconnects. Due to nonlinearities in the damage growth qr ~~~~i l~ shear stress, pa rate, this estimate may be non-conservative. Post failure $I, y Plastic Tensile Strain analysis shows cracks close to the interface between the solder Y Total Shear Strain and the Ag ink used on the Si wafer. Distributed solder QActivation Energy, Jtmol damage is also evident in Sn36Pb2Ay specimens. ~c c e k a t i o n factors were estimated based on a two m T T m A n n 7 F T m A~l 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-free) 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.

No abstract

No abstract