Intermetallic Phase Growth and Reliability of Sn-Ag-Soldered Solar Cell Joints

Soldering is the most common way to interconnect solar cells into strings to form a PV module. The most prevalent and standard solder material is the SnPbAg-solder. Since lead is a hazardous material and the RoHS-guideline is expected to prohibit its further use in PV, the industry is looking for an alternative. In electronics there are a number of different alloy options. Promising candidates are the SnAg-solders, which are already in use by several PV manufacturers. The growth of intermetallic phases in the joint, both during soldering and lifetime, affects the quality of the joint and its reliability in terms of mechanical behavior. In this work we show the growth of different intermetallic phases, investigate their detrimental effects on the long term stability and compare them to a standard leaded solder. We produce solder joints and let them undergo an accelerated aging at elevated temperatures in an inert nitrogen atmosphere. We use different optical instruments including microscopy, SEM and EDX, as well as peel tests to investigate the diffusion phases and their influences. We show that intermetallic compounds (IMC) form due to diffusion in the solder joints with different intensity for both solders at elevated temperatures. Our study also poses that SnAg3.5 outperforms the leaded solder in terms of adhesion and durability. The results indicate that the failure mode and the adhesion are depending on the amount of diffused tin into the metallization.