The shear strength of nano-Ag solders and the use of Ag interconnects in the design and manufacture of SiGe-based thermo-electric modules

Thermo-electric modules can be used to convert heat into electricity by utilizing the Seeback effect. It is now possible to buy BiTe thermo-electric modules that can operate up to temperatures of around 300°C. However, many applications, such as the harvesting of exhaust gas from large vehicles or gas turbine heat, may occur at higher temperatures Therefore, new materials and manufacturing processes need to be developed to produce packaged TEM that can operate at a maximum operating temperature of 650°C. Two critical areas in the manufacture of a SiGe TEM are the choice and strength of materials used to both solder the TE material to the rest of the module and the metal used for the interconnects. The interconnection material needs to be sufficiently strong to withstand large temperature fluctuations while maintaining a low contact resistance, as well as being compatible with the nano-Ag solder. Shear force tests of the sintered thermo electrical leg material showed that the joints are brittle when sintered to W metallized AlN substrates are used and ductile fracture behavior when sintered to Cu metallized AlN substrates using the NanoTach K nano silver paste. Almost all of the joints were found to be brittle when using the NachTach X nano silver paste. Shear testing of the solder joints showed that the X paste joints were variable in strength and stiffness, having a typical Young´s modulus between 10 and 100 MPa at room temperature. The K paste joints were stiffer, but had a similar strength as compared to the X paste joints.