Intermetallic Phase Growth and Microhardness of Sn42Bi58 Solder Joints on Silicon Solar Cells

The temperature sensitivity of new solar cell technologies forces to use solder alloys with lower melting temperatures for interconnection [1].Our study explores the dynamics of intermetallic phase growth and microhardness in Sn42Bi58 solder joints applied to low-temperature silver metallization on silicon heterojunction (SHJ) solar cells. Through rigorous experimentation and analysis, a comprehension of the influence of these factors on the mechanical and material properties of the solder joint is achieved. Microstructural changes in Sn42Bi58 solder compared to conventional tin-lead solder are investigated through microscopy of cross sections, revealing enlarged intermetallic particles and phase boundary growth. These changes, attributed to lower homologous temperature of the low-melting solder, are expected to negatively impact the mechanical strength of the solder joint. For the intermetallic phase Ag3Sn simulations predict a potential layer thickness of 20 µm after 25 years SHJ module operation. Our results show that the Ag3Sn phase has a significant impact to the microhardness. Following the aging process, the nano hardness of the low temperature Ag metallization experiences a twofold increase, shifting from 660 ± 53 N/mm² to 1367 ± 411 N/mm². This strengthening is primarily attributed to the prevailing influence of the Ag3Sn intermetallic phase.