Microstructural Stability of Au-Sn SLID Joints for Harsh Environments

Solid liquid inter diffusion (SLID) is an interconnection technique for electronic packaging, particularly beneficial for high power and harsh environments conditions. It consists of the bonding of two materials with different melting points at a low processing temperature to achieve a high melting point interconnection. The materials investigated in this work are a gold-tin bond attaching a SiC diode to an AlN direct-bond-copper (DBC) substrate. Gold (Au) is the high melting point constituent while the eutectic gold-tin (80 wt.% Au-20 wt.%Sn) offers the low melting point (280°C). This work is aimed at the microstructural evaluation of the joints at different bonding and aging conditions in an effort to get the insights of this interconnection technology from a metallurgical perspective. Four different bonding conditions were used: 315°C-5min, 315°C-10min, 340°C-1min and 340°C-5min; from which a base-line as built condition was assessed by means of metallographical analysis. Furthermore, the samples were aged at 250°C from 1000 to 4000 hours in increments of 1000hrs to study and quantify the microstructural stability and intermetallic (IMC) growth at the interface. This aging experiment has been designed to obtain accelerated information on the kinetics of this reaction so that predictive models can be developed for the real application conditions. The samples were diced, polished and analyzed following standard metallographical techniques; both optical and electronic microscopy (SEM-EDS) was employed. The as-built samples, for the four bonding conditions, presented differences in IMC growth with the thickest layers appearing at the harshest processing conditions. After aging the IMC kept growing and the formation of a new IMC layer was discovered and investigated, furthermore, cracks started to show in some of the samples. It was observed that after 4000 hours some of the cracks extended across the whole interface.