Modelling and verification of mechanical stress induced by soldering of wires for multi busbar interconnection

The Multi Busbar metallization approach changes the interconnection architecture on silicon solar cells essentially. Compared to a standard H-pattern design with three continuous soldered busbars, between 7 and 15 round copper wires are soldered onto more than 200 solder pads on the front side of the solar cell. Due to the different coefficients of thermal expansion of copper and silicon, mechanical stress is induced after the interconnection. In this work we simulate the mechanical stress induced by soldering with a finite element model and experimentally verify the simulated results. The characterization of the stress distribution in the solder joints of a 78 by 10 mm² cell section shows that the cell bow, which is a visible indicator for mechanical stress, rises approximately from 4 to 9 mm when the wire diameter is increased from 250 to 430 mm. Pre-stretching the interconnecting wires up to 1% before soldering causes a slight increase of the cell bow. The verified simulation model allows the identification of reasons for potential defects of solder joints after reliability testing and the determination of the areas where such defects most likely occur. In addition, the model enables a comparison of the thermo-mechanical stress distribution of different interconnection concepts.