Characterization of Nanosilver Dry Films for High-Temperature Applications

Nanosilver paste seems to be one of the most promising lead-free die-attach alternatives. The emergence of this technology is mainly due to the desired characteristics of the joint, such as high electrical and thermal conductivities, low elastic modulus offering a good thermomechanical reliability, low process temperature, and high operating temperature. Compared to traditional paste sintering, dry films are more appropriate for large area, do not need a drying process, and reduce the short-circuit risks for electrodes with narrow spacing. In this paper, the effects of some of the sintering profile variables (applied pressure, sintering temperature, and holding time) as well as the joint dimensions (thickness and area) on the sintered dry films were investigated by mechanical, thermal, acoustic, and microscopic analysis. The applied pressure and the holding time at the desired sintering temperature affect closely the die shear strength. In our tests, at the highest temperature (350 °C) and lowest pressure (3 MPa), the structural and thermal properties of the joint are almost similar to those obtained at 280 °C at four times higher pressure. These results can be very useful for assembling pressure-sensitive devices. Unlike a nanosilver-paste-sintered joint, the joint dimensions do not influence the shear strength values. Otherwise, the reliability of the joint was tested during high-temperature storage at 300 °C up to 2000 h and during thermal cycles between -40 °C and 160 °C. The storage at 300 °C for 1000 h did not lead to any degradation of the die shear strength, while a continuous decrease of the shear strength is observed during thermal cycling tests for the thinnest joint. The thermomechanical reliability can be improved by increasing the joint thickness.