Improvement of Sn-3Ag-0.5Cu Soldered Joints Between Bi0.5Sb1.5Te3 Thermoelectric Material and a Cu Electrode

A Bi0.5Sb1.5Te3 thermoelectric (TE) element was directly soldered to a Cu electrode using Sn-3Ag-0.5 Cu alloy. The interface was sound and the bonding strength was satisfactory (8.6 MPa). However, the solder layer was exhausted quickly during high-temperature storage (HTS) tests at 150 degrees C for 300 h and 600 h, and the bonding strength drastically decreased to 1.5 MPa. The consumption of the solder was prevented by electroplating a Ni barrier layer on the TE element, though a low bonding strength of 1.9 MPa resulted. Adding a Sn-rich thin film and a Ni barrier layer onto the Bi0.5Sb1.5Te3 element led to a high bonding strength of 12.1 MPa, which decreased only slightly after HTS reliability tests at 150 degrees C for 1000 h. The sound interfaces of the Bi0.5Sb1.5Te3/Cu joints maintained their stability even after HTS at 175 degrees C for 1000 h.