Lead-free solders and isotropically conductive adhesives in assembling of silicon solar cells - preliminary results

Most often the electrical interconnection of crystalline silicon solar cells within a photovoltaic module is made by soldering of tin-lead coated ribbons to the screen-printed busbar. During nearest years, the lead-tin solders, most often used for this purpose, should be replaced by alternative interconnection materials - Pb-free solders or electrically conductive adhesives. This paper presents the electrical properties and long-term stability of joints between conductive tracks, screen-printed on alumina or on silicon, and proper flat ribbon in dependence of joint material. Joints were made from typical tin-lead solder (reference joint), lead-free solder or isotropically conductive adhesives. It was proved that tested lead-free solder could be applied for assembling of silicon solar cells even with typical tin-lead coated Cu ribbon. The adhesive joints are not so stable in this configuration. Probably this is connected with oxidation or galvanic corrosion of base metal during ageing at elevated temperature and humidity. But promising results appeared for silver-plated ribbon.