Electrical, thermal and mechanical characterization of low temperature, pressure-less sintered silver bond interfaces

In the field of power electronics the requirements in size, weight, reliability, durability and ambient temperature are driving the operational temperature beyond the limits of current die-attach technologies. Common bonding agents such as solder and high conducting adhesives are more and more replaced by silver pastes which are pressure assisted sintered at low temperatures. The silver pastes provide low thermal resistance and increased reliability at higher temperatures. Thus, the operational temperature of the package can be elevated without losing reliability and durability. However, the pressure assisted bonding is difficult to handle for more than one chip simultaneously, so that the common parallel process changes to a serial process. In the presented investigation, novel commercial silver pastes are investigated which allow pressure-less sintering. For this purpose a silicon chip with gold metallization is bonded to a Mo30Cu heat spreader with silver metallization. The bond strength is investigated with a shear tester to be in the range from 10 MPa to 48 MPa. The electrical conductivity is measured by four-wire sensing and reaches values from 3.8 MS/m to 16.2 MS/m. The resulting thermal conductivity is calculated with the Wiedemann-Franz law to be 26 W/mK to 110 W/mK. Alternatively the thermal conductivity can be calculated from the shift of the thermal resistance of a ceramic beam with and without the silver paste, what is shown in a thermal simulation. Advantages of the presented bond process are the parallel and pressure-less manufacturing approach and the high electrically and thermally conductive bond interface with a high bond strength.