Effect of Substrate Preheating Treatment on Thermal Reliability and Micro-Structure of Ag Paste Sintering on Au Surface Finish

Sintering Ag paste on Au surface finished substrate (Ag-Au joint) is a hot topic due the Au surface finish is one of the mostly used surface methods in industry. However, to realize a reliable Ag-Au joint is not easy due to massive Ag-Au grain boundary diffusion being able to degrade the joint. In this work, we reported a preheating treatment of the substrate for improving the reliability of Ag-Au joints. The Ag-Au joints with the preheated substrate remained a shear strength of 16.5 MPa after 1000 h thermal aging at 250 °C, which is superior to the Ag-Au joints without the preheating treatment and other reported works. The fracture surface of both types of joints was also investigated. The preheating treatment can also drastically alleviate the diffusion of Ni-P to Au surface, which leaves black spots on fracture surface. The improvement of thermal reliability and the change of micro-structure are attributed to the decrease of Au grain boundaries, which is caused by the growth of Au grains during preheating treatment.