Challenges for Solder Interconnection pushed by High-Efficiency Solar Cell Developments

The solder interconnection to connect solar cells in series is a key process for the manufacturing of reliable high-performance solar modules. The fast development of the cell technologies, interconnector number and the throughput requirements by industrial production lines underlines the necessity of detailed evaluation of the process challenges. Within this work, the developments of the solar cell metallization are addressed by precise evaluation of the metallized area available for the solder joint. The soldering of round wire interconnectors onto small area pads or even directly onto the contact fingers for busbarless designs reveals to become challenging for precise handling and interconnector alignment. Drastic reduction of the size of the solder pad area to realize silver reduction results in poor mechanical and electrical quality of the solder joint. Every cell technology features requirements when it comes to soldering temperatures according to sensitive layers. Therefore, we present the solder alloys available according to their melting temperature. Concerning the criticality of materials potential lead-free alloys are presented. We performed soldering processes with PERC, TOPCon and SHJ solar cells at an industrial stringer machine to evaluate temperature homogeneity of the process. A temperature discrepancy of at least 14 K for SHJ and up to 90 K for PERC solar cells was measured on the cell during interconnection. The challenges and increasing sensitivities arising from developments on solar cell level reveal the need of constant evolution of the soldering tools as well as of the soldering processes.