Swelling Phenomena in Sintered Silver Die Attach Structures at High Temperatures: Reliability Problems and Solutions for an Operation above 350°C

Abstract

A very effective method to bond an electronic device to a substrate is the pressure assisted sintering of a sub-micron silver paste at temperatures between 150°C and 300°C. This technique-sometimes called “Silver-Sintering” or “Low Temperature Joining Technique (LTJT)” is already used in many power electronics industry applications. It provides die attach layers with excellent pull-strength of more than 100 MPa at room temperature and 30 MPa at 300°C. Additionally the electrical and thermal conductivity is nearly as good as it is in pure silver. Such sintered die attach layers are said to be stable up to temperatures above 500°C, but no detailed investigations in this temperature range are available up to now. Our recent investigations by shear- tests on monometallic chip/substrate-samples and dilatometer measurements showed a non-linear thermal expansion of the sintered structures above 350°C including a strong irreversible expansion of the whole sintered structure after long term heat treatments. Beyond the investigations on the swelling effect itself we present studies on the relationship between the expansion of the sintered layer and the resulting layer properties. Finally we discuss possibilities to take advantage out of this effect and additionally analyze options to reduce the swelling by adding SiC-particles to the silver paste. As a result, we demonstrate that swelling of the silver paste has to be taken into consideration when silver sintering is to be used for device operation temperatures above 350°C.