Comprehensive Study of Intermetallic Compounds in Solar Cell Interconnections Including Lead-Free, Low Melting Point Solders
Intermetallic compounds (IMC) in solder bonds are commonly considered critical for the reliability of interconnections. The microstructure and thermal aging characteristics of solder bonds of crystalline silicon solar cells are investigated, whereby two solders, Sn60Pb40 and a lead-free, low melting point alternative Sn41Bi57Ag2 are considered. Solder bonds of front side busbars are formed with a semi-automatic tool and cross sections are prepared, which are characterized with confocal laser microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy (EDX). The kinetic of IMC growth is modeled with a diffusion model, and the parameters are obtained with systematic thermal aging studies and the Arrhenius relationship. The model is capable of processing non-isothermal temperature profiles and is used to simulate IMC growth during PV module reliability tests like thermal cycling 600 (TC) and damp heat for 3000 h. Furthermore, the thicknesses of the IMCs are estimated for a PV module in an outdoor location. It is found that solder joints undergo strong ripening processes during thermal aging. In particular, Sn41Bi57Ag2 solder joints show faster IMC growth, grain coarsening and Sn-penetration into the busbar than Sn60Pb40 bonds. Sn-penetration is observed already in the as-soldered condition and EDX confirms the existence of significant amounts of diffused Sn around cavities in the busbar. Large portions of the busbar are consumed by the Ag3Sn IMC after 155 h at 130 °C and further aging of the Sn41Bi57Ag2 joint leads to Bi segregation and, in some cases, complete depletion of Sn from the solder matrix. The simulation of IMC growth shows a final IMC thickness after TC 600 of 1.2 m and 1.5 m; furthermore, a thickness after 3000 h at 85°C of 2.6 m and 3.7 m respectively for Ag3Sn in Sn60Pb40 and Ag3Sn in Sn41Bi57Ag2 bonds. The prognosis of IMC growth in a PV module in the outdoor location estimates an Ag3Sn thickness of 1.2 m and 1.3 m for Sn60Pb40 and Sn41Bi57Ag2 respectively due to the rare occurrence of temperatures above 60 °C.