The ZnSb intermetallic compound may have thermoelectric applications because it is low in cost and environmentally friendly. In this study, a Zn4Sb3 thermoelectric element coated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode using a Ag/Sn/Ag solid–liquid interdiffusion bonding process. The results indicated that a Ni5Zn21 intermetallic phase formed easily at the Zn4Sb3/Ni interface, leading to sound adhesion. In addition, Sn film was found to react completely with the Ag layer to form a Ag3Sn intermetallic layer having a melting point of 480°C. The resulting Zn4Sb3 thermoelectric module can be applied at the optimized operation temperature (400°C) of Zn4Sb3 material as a thermoelectric element. The bonding strengths ranged from 14.9 MPa to 25.0 MPa, and shear tests revealed that the Zn4Sb3/Cu-joints fractured through the interior of the thermoelectric elements.

A (Pb, Sn)Te thermoelectric element plated with a Ni barrier layer and a Ag reaction layer has been joined with a Cu electrode coated with Ag and Sn thin films using a solid–liquid interdiffusion bonding method. This method allows the interfacial reaction between Ag and Sn such that Ag3Sn intermetallic compounds form at low temperature and are stable at high temperature. In this study, the bonding strength was about 6.6 MPa, and the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. Pre-electroplating a film of Sn with a thickness of about 1 μm on the thermoelectric element and pre-heating at 250°C for 3 min ensures the adhesion between the thermoelectric material and the Ni barrier layer. The bonding strength is thus increased to a maximal value of 12.2 MPa, and most of the fractures occur inside the thermoelectric material. During the bonding process, not only the Ag3Sn intermetallics but also Cu6Sn5 forms at the Ag3Sn/Cu interface, which transforms into Cu3Sn with increases in the bonding temperature or bonding time.

For the production of thermoelectric modules, Bi0.5Sb1.5Te3 was sequentially electroplated with a 4-μm Ni barrier layer and a 10-μm Ag layer and then diffusion soldered with the Cu electrode, which was also electroplated with 4-μm Ag and 4-μm Sn layers. The Bi0.5Sb1.5Te3 and Ni interface with no sufficient chemical bonds resulted in a bonding strength lower than 3 MPa. Through the pre-coating of a 1-μm Sn thin film on Bi0.5Sb1.5Te3 and heating at 250 °C for 3 min before the electroplating of the Ni barrier layer, the bonding strengths of Bi0.5Sb1.5Te3/Cu assemblies increased to a maximal value of 10.7 MPa. The intermetallic compounds formed at various interfaces in the Bi0.5Sb1.5Te3/Cu modules after diffusion soldering at temperatures ranging from 250 to 325 °C for 5-60 min and their growth kinetics was analyzed.

Directly coating a GeTe(Pb) thermoelectric device with a Ni barrier layer and an Ag reaction layer and then diffusion soldering with a Cu electrode coated with Ag and Sn leads to breakage at the GeTe(Pb)/Ni interface and low bonding strengths of about 6 MPa. An improved process, precoating with 1 μm Sn film and heating at 250°C for 3 min before electroplating with Ni and Ag layers, results in satisfactory bonding strengths ranging from 12.6 MPa to 19.1 MPa. The precoated Sn film leads to the formation of a (Ni,Ge)3Sn4 layer between the GeTe(Pb) thermoelectric material and Ni barrier layer, reducing the thermal stress at the GeTe(Pb)/Ni interface.