Optimization of Copper Thermocompression Diffusion Bonding under Vacuum: Microstructural and Mechanical Characteristics

Abstract

The optimization of the autogenous diffusion copper bonding via thermocompression at vacuum environment was investigated. The influence of various bonding parameters on the interdiffusion efficiency was studied in detail at the micro (SEM-EBSD) and nano (TEM) scales. Bonding at 1000 °C for 90 min under pressure (10 MPa) presented optimum structural and mechanical results. Under these conditions, interdiffusion phenomena were observed at a significant extent through the swelling transformation of existing fine grains or the formation of equiaxed copper grains with an orientation parallel to the bond interface. Transmission electron microscopy revealed the importance of the grain size of the base material on the bond quality. In the regions with fine-sized copper grains, the formation of small equiaxed recrystallized twins was observed. Their length within the bonding zone was in the order of 200 and 400 nm. On the contrary, in the regions with coarse grains the interdiffusion was poorer. The processing temperature and duration presented a significant effect on the bonding strength (BS). BS exceeded 100 MPa in case of processing conditions of T ≥ 850 °C and t ≥ 60 min, while the maximum BS value achieved (≈180 MPa) was comparable with the respective value of the base material. The microhardness of the optimum bond reached 55 HV—slightly higher in comparison to the hardness of the initial copper material. The results indicated that the proposed thermocompression process is appropriate for the production of Cu-Cu bonded structures that can be potentially used as electrical components under mechanical stress.