Interfacial reaction between n- and p-type thermoelectric materials and SAC305 solders

• Three layers with different morphologies occur in solder/p-type TE material couple.
• A self-forming diffusion barrier layer is formed by solder/n-type material couple.
• Scallop-type IMCs appear in solder/n-type material couple for long reaction time.
• The growth rate of solder/n-type material couple is much lower than that of p-type.

Sn96.5Ag3.0Cu0.5 (SAC305) is one of the most promising lead-free solder alloys and bismuth telluride-based compounds are the best known thermoelectric materials. Interfacial reactions of the SAC305/Bi0.5Sb1.5Te3 and SAC305/Bi1.8Sb0.2Se0.15Te2.85 couples at 260 °C are studied to investigate the effect of reaction on the efficiency and reliability of thermoelectric device. Initially, the major products in SAC305/Bi0.5Sb1.5Te3 couple reaction for a short time are a strip-type Ag-rich IMC layer and a dense SnTe–SbSn mixture layer. With the reaction time increasing, a two-phase layer, which is SnTe–SbSn mixture with pores with 200–500 nm size which are occupied by solder originally, appears next to the dense layer. However, the reaction products do not change significantly. It is observed that the growth rate of reaction phase layer is about 4.05 μm/min. As for the SAC305/Bi1.8Sb0.2Se0.15Te2.85 couple reaction, besides SnTe phase, Bi–Te phase is also formed at the solder/Bi1.8Sb0.2Se0.15Te2.85 interface. As the reaction time increases, the morphology of IMC layer becomes rough, and then the stable scallop-type IMC appears; the growth rate of reaction phase layer is about 0.39 μm/min.