Diffusion bonding at the interface of Bi₂Te₃ thermoelectric modules

In production of Bi₂Te₃-based thermoelectric modules, Bi₂Te₃ materials were coated with electroless nickel (EN), electroless palladium, and immersion gold multilayers to suppress unanticipated interfacial reactions between Bi₂Te₃ and Sn-based solder alloys. However, the coating elements reacted with the solder alloys to yield intermetallic compounds (IMCs) at the Bi₂Te₃/solder and solder/Cu electrode interfaces. Significant Cu content from the Cu electrodes contributed to the IMCs along with to the coating and Sn elements. In this study, the influence of IMC composition on bonding strength and the characteristics of the Bi₂Te₃-based material/Cu electrode interface was investigated. An EN layer was plated onto Cu electrodes to suppress Cu diffusion from the electrodes into the solder alloy. IMCs mainly consisted of Sn and Cu elements in the case of uncoated Cu electrodes. When Cu electrodes were coated by the EN layer, Ni and Sn content prevailed in IMC composition. Additionally, IMCs at both the Bi₂Te₃/solder and solder/Cu electrode interfaces with Cu or Pd were thicker than those without Cu or Pd. Meanwhile, the bonding strength of the Bi₂Te₃-based material/Cu electrode interface with the EN layer on the Cu electrodes was lower than that of the interfaces without the EN layer.