Effect of surface roughness and electroless Ni-P plating on the bonding strength of Bi-Te-based thermoelectric modules

In this study, electroless-plating of a nickel-phosphor (Ni-P) thin film on surface-controlled thermoelectric elements was developed to significantly increase the bonding strength between Bi-Te materials and copper (Cu) electrodes in thermoelectric modules. Without electroless Ni-P plating, the effect of surface roughness on the bonding strength was negligible. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric elements and defects such as pores were generated at the bonding interface owing to poor wettability with the solder. However, defects were not present at the bonding interface of the specimen subjected to electroless Ni-P plating, and the electroless Ni-P plating layer acted as a diffusion barrier toward Sn and Te. The bonding strength was higher when the specimen was subjected to Ni-P plating compared with that without Ni-P plating, and it improved with increasing surface roughness. As electroless Ni-P plating improved the wettability with molten solder, the increase in bonding strength was attributed to the formation of a thicker solder reaction layer below the bonding interface owing to an increase in the bonding interface with the solder at higher surface roughness.