EVALUATION OF STRESS ON CELLS DURING DIFFERENT INTERCONNECTION PROCESSES

Abstract

During the last years, new stringing techniques with low stress have been developed, such as the use of conductive adhesives (1), low temperature solders (2) or laser soldering. The aim of this work is to evaluate the stress introduced in the cells during the soldering process, making a comparison between different techniques and solder materials that exist nowadays in the market. The interconnection technologies under investigation have been: low- temperature snap-cure conductive adhesives, induction and infrared soldering and laser spot soldering. For the soldering approaches different alloys and materials were evaluated, such as low temperature solders, low CTE alloys and copper ribbons of very low yield strength. Electroluminescence imaging and mechanical tests have been used to quantify the stress experienced by the cells. In addition, the possible propagation of microcracks during thermal cycling has been investigated. The interconnection process is a critical step in the fabrication of crystalline silicon PV modules, in particular in the last few years, when the thickness of the wafers has been drastically reduced and the cell area has been increased. The stress applied on cells during the stringing process can cause microcrack formation and therefore increase the breakage rates in industrial production lines. Three processes and three solder materials have been evaluated in terms of stress: conductive adhesives, induction and laser spot soldering, low temperature alloys, low CTE materials and low yield strength copper ribbons.