Effect of durability testing on solder joint adhesion and failure mode of front side metallizations

The development of front side solar cell metallization pastes is triggered by the potential to enable higher cell efficiencies. The performance of such metallization pastes with respect to cell efficiency is driven by contact resistance and finger resistivity. The essential properties for module integration are the adhesion of the metallization on the wafer and the solderability. In this work we compare a new front side metallization paste with previous generations with respect to joint adhesion. A thorough investigation at the solder joints was carried out by means of adhesion tests and metallographic analyses. This qualification program includes testing of joints after isothermal heat storage to simulate longer periods of intermetallic diffusion processes. Furthermore, we selected the best performers in terms of adhesion and solderability to manufacture small modules that were exposed to accelerated aging tests. We were able to indicate the relation between the failure mode and the responsible properties of the paste. In general, strong diffusion processes in the metallization are known to weaken the joints and lead to reduced adhesion. The evaluated novel metallization paste exhibits such a reduced diffusion activity and the measured adhesion outperforms the other 4 generations