Lead-free Solders for Ribbon Interconnection of Crystalline Silicon PERC Solar Cells with Infrared Soldering

We report about the analysis of Pb-free, low-temperature solders for the ribbon-interconnection of PERC solar cells with an industrial infrared stringer. Five solders (SnPb, SnBi-A, SnBi-B, SnBiAg and a proprietary lead-free, low-temperature (PLFLT) alloy) are characterized with differential scanning calorimetry to determine the melting and solidification temperature. It is found that SnBi-B, SnBiAg, and the PLFLT composition melt in a temperature range between 137 degrees C to 171 degrees C instead of a single temperature. Solidification occurs at a 3K to 11K lower temperature (undercooling). Mono-crystalline silicon PERC cells are contacted using an industrial stringer. The microstructure of the solder bonds is investigated with scanning electron microscopy. For the SnBi-A-solder, large and brittle Bi-phases are identified. The SnBi-B, SnBiAg and PLFLT solder show a finer grain structure. The added Ag in SnBiAg forms an intermetallic compound of Ag3Sn close to the Cu-core of the ribbon. The peel strength of the connected solar cells with the Pb-free solders is on average 1N mm(-1) or slightly higher. Some bonds show low adhesion. The observed fracture mode is mainly failure at the busbar metallization to solar cell irrespective of the solder type. However, the occasionally observed solder residues on the metallization clearly reveal brittle fracture for the Pb-free solders, which is not observed for SnPb. First reliability tests show similar degradation of 1% to 2% for all solders.