Understanding the Influence of Copper Substrate Oxidation on Silver Pressure Sintering Performance

Recently, due to the promising properties of silver, the low-temperature silver sintering joining technology has become increasingly attractive in power electronics. Reliable bonding quality of sintered silver depends not only on the microstructure, but also on the interfacial properties to the joining materials. Sintering on precious metal surfaces such as silver or gold works very well due to the good interdiffusion behavior of these elements. Non-precious metal surfaces such as copper are preferred to reduce manufacturing costs as no additional surface metallization layer is required. Sintering on copper surfaces is usually performed in a nitrogen atmosphere, as they can potentially oxidize during sintering in an oxygen -containing atmosphere. The resulting copper oxide is proposed to act as a diffusion barrier, resulting in weak bonding of the silver compound to the copper surface. In this work, we observed that not only the substrate oxidation during sintering, but also the initial Cu oxide layer affects the bonding performance. Therefore, different copper oxidation methods were performed to produce different types of copper oxides and thicknesses on active metal brazed (AMB) substrates. X-ray photoelectron spectroscopy (XPS) measurements of the samples indicate the presence of two types of copper oxides, which further affect the bond strength of sintered Ag differently. Pressure sintering with micron Ag paste on a nearly oxide-free copper surface produces lower bond strength than sintering on oxidized substrates. The highest bond strength can be achieved on CuO, but the presence of Cu20 also contributes to the bonding of silver to the copper surface. If the oxide layer becomes too thick, bonding is no longer guaranteed. Higher sintering temperature and pressure results in a denser sintered layer and stronger bonding on copper substrates.