Effect of electroplated Au layer on bonding performance of Ag pastes

In the silver sinter joining process, not only the bonding materials but also the metallization layer play important roles in bonding reliability. Compared with Ag, Au is utilized less often due to its higher cost, however Au plating is the inevitable choice for many applications which require a multiple reflow processes. In our research, the influence of the sintering process on the bonding strength of joints using Ag paste, as well as the influence of the electroplated Au layer on the joining property of SiC chips bonded on a printed circuit board (PCB) substrates with various types of Ag pastes was investigated. Electroplated layers of Ni/Pd/Au were prepared as a substrate metallization for this research. The shear test results revealed that higher shear strength was achieved on substrates with 0.8 μm-thick Au layer compared to those with 0.15 μm and 0.3 μm-thick Au layers. Fracturing inside the porous Ag near the substrate side was the main fracture mode for all joints in this research. The cross-section morphologies revealed that a weak bonding layer would form on the substrate side which was caused by the migration of Ag towards Au plating during the sintering process. The migration rate, which is in inverse proportion to the grain size of exterior Au plating, would impact the weak bonding layer, and thereby influence the bonding strength of joints. According to our results, higher bonding strength can be achieved on Au plating with larger grain size.