Influence of electroless Ni-P and Pd-P plating on the bonding strength of n-type Bi-Te thermoelements

In this study, the effect of electroless Ni-P and Pd-P plating on the bonding strength of n-type Bi-Te based thermoelectric module was investigated. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric element without electroless plating, and many defects such as pores were present around these compounds. When electroless Ni-P plating was applied to the surface of the thermoelectric element, the bonding strength of the thermoelectric module approximately doubled. The electroless Ni-P plating layer effectively functions as a diffusion-preventing layer to prevent the interdiffusion of the thermoelectric element and the solder from forming a brittle SnTe intermetallic compound. On the other hand, application of Pd-P plating onto the electroless Ni-P plated layer also enhanced the bonding strength by 20% compared to the application of electroless Ni-P plating alone. The electroless Pd-P plating enhanced the solder spreadability due to the rapid diffusion of Pd into the solder layer, and the plating forms a thick Pd-Sn diffusion layer on the bonding interface below the Pd-P plating layer.