Improvement of bonding strength of a (Pb,Sn)Te–Cu contact manufactured in a low temperature SLID-bonding process

•SLID bonding at low temperature has been employed for the (Pb, Sn)Te thermoelectric modules.•The TE/Ni interface strength was improved up to 10.9 - 13.2 MPa by a pre-coated Sn film.•The modified SLID bonding process will not degrade the module’s thermoelectric properties. A (Pb,Sn)Te thermoelectric (TE) material coated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode at low temperatures in the range of 175–250°C with an In thin film. The In film reacted initially with the Ag layer to form a double layer of Ag3In and Ag2In intermetallic compounds, which then completely reacted with the Ag layer to form a Ag3In intermetallics layer having a melting point of 690°C. The resulting module can thus be applied at the optimized operation temperature of (Pb,Sn)Te material (400°C). The bonding strengths ranged from 6.3 to 7.8MPa for various bonding temperatures at 30min, and shear tests revealed that the specimens fractured along the interface between the (Pb,Sn)Te thermoelectric element and the Ni barrier layer. The adhesion of the TE/Ni interface was improved through pre-electroplating a 1μm Sn film on the surface of the thermoelectric element and heating at 250°C for 3min. In this case, the bonding strengths increased to values between 10.9MPa and 13.2MPa for a bonding time of 30min at temperatures ranging from 175 to 250°C, and in shear tested TE modules, fracture occured in the interior of the TE elements. After high-temperature storage at 400°C for 200h, the shear strengths of TE/Cu joints bonded at various temperatures for 30min decreased to values of 10.5–10.8MPa, and the fracture occurred in the Ni3Sn4 intermetallics at the (Pb,Sn)Te/Ni interfaces.