Active Metal Soldering and Characterization of Soldered Joints in Cu Base Plate to Aluminum-Graphite Composites

Heat dissipation materials with high thermal conductivity (TC) can meet the high demand for improving heat dissipation in high-power IGBT modules. The current study focused on soldering Al-graphite composites (Al-Gr) with a copper (Cu) base plate using an active type Sn-Ag-Ti (SAT) solder. Ultrasonic active soldering (UAS) was performed in air at 250 °C for 30 sec. The relative spreadability rates of the direct UAS process versus conventional soldering were + 276.6% for SAT/Cu and +186.1% for SAT/Al-Gr. After direct UAS, a Cu6Sn5 layer formed at the active solder/Cu interface and Al dissolved into the active solder zone, thus forming a ternary coarse Al-Ag-Sn solid solution in the active solder region. In addition, submicron particles (e.g., Al-Ag-Sn and Ag3Sn) adsorbed on the surface of active solder/Gr interface. The calculated Gibbs free energy results indicated that both solute Ti and Ti-Sn compounds could react with C to form TiC compounds, and TiC reacted with Ti-Sn compounds to form the Ti2SnC phase, which was accelerated with the direct UAS process. The shear strengths were measured to be 31.0 ± 4.1MPa for Cu/SAT/Cu joints, 14.3 ± 3.2 MPa for Al-Gr/SAT/Cu joints, and 12.8 ± 3.8 MPa for Al-Gr/SAT/Al-Gr joints, respectively.