Tailored microstructure and enhanced bonding strength of in-situ surface modified Cu flakes via oxidation–reduction bonding

• In-situ surface modified Cu flakes were achieved via the transformation of premade oxide to Cu. • The growth mechanism of oxide was detailed and the dominant diffusion method was determined. • The correlation between surface microstructure of Cu flakes and sintered Cu joint strength was investigated. It is generally believed that oxidation is detrimental to sintered Cu interconnection for power electronic and should be completely removed before sintering. In this study, in-situ surface modified Cu flakes with high sinterability were fabricated via the transformation of premade oxide to Cu during oxidation–reduction bonding (ORB). The oxides formed on Cu flake surface show a morphological evolution from uniform nanograins into irregular microbulges during longer oxidation durations, which results in reduced Cu nanoparticles and Cu mixture of microhillocks and nanoparticles, respectively. The growth mechanism of oxide and its dominant diffusion method were detailed. The sintered Cu joint strength increases with increasing number of oxide grains, while excess oxides will cause coarsening of reduced Cu nanoparticles and degradation of joint strength. The reduced Cu mixture is found to be beneficial to joint strength by increasing the sintering density. Under optimal ORB conditions, the joint strength reaches 80 MPa and only degrades 34 % after 1000 h aging. This study first reveals the effect of oxide morphology on reduced Cu structure by formic acid atmosphere and sintered Cu joint strength. The proposed bonding process is compatible with the existing reflow furnace and could provide new approach to achieve sintered Cu interconnections.