ANALYSIS OF THE EFFECT OF COPPER SURFACE INTERFACE TREATMENT ON THE MORPHOLOGY AND PHASE GROWTH IN COPPER-TIN REACTIONS

Abstract

This study investigates the influence of preliminary surface modification of copper on the formation, morphology, and defect structure of the Cu–Sn contact zone after thermal annealing. The research focuses on the development of intermetallic phases and porosity at the Cu/Cu₃Sn interface, which is one of the critical regions determining the stability and reliability of copper–tin contact systems. Several types of surface preparation were used, including surface mechanical attrition treatment, chemical tin deposition, stationary copper electrodeposition, and non-stationary copper electrodeposition regime. After surface modification, the working surface of the samples was treated with flux and heated to 130 °C to activate the flux and remove oxide films. The samples were then immersed in molten tin at 300 °C for 1 s and quenched in water. Subsequent thermal annealing was carried out for 190 h at 210 °C in an argon atmosphere. The morphology of the contact zones was studied using scanning electron microscopy.

The results show that thermal annealing leads to the formation of a reaction zone consisting mainly of Cu₃Sn and Cu₆Sn₅ intermetallic phases. The thickness and morphology of these phases depend significantly on the initial state of the copper surface. The most pronounced growth of the intermetallic layer was observed for the sample prepared using non-stationary electrodeposition regime followed by chemical tin deposition. In this case, the total thickness of the Cu₃Sn + Cu₆Sn₅ layer was considerably higher than that of the reference sample, mainly due to the intensive growth of the Cu₆Sn₅ phase. Samples treated by a combination of SMAT and chemical tin deposition also demonstrated enhanced interfacial reactions, although the effect was less significant than in the case of non-stationary electrodeposition.

Special attention was paid to the Cu/Cu₃Sn boundary, where dark regions corresponding to pores, voids, and local discontinuities of contact were observed. These defects were mainly localized along the interphase boundary and often formed discontinuous chains. Such morphology suggests that pore formation is associated with diffusion-controlled growth of intermetallic phases and the accumulation of vacancies near the Cu/Cu₃Sn interface. The observed porosity may therefore be considered as a manifestation of vacancy-related degradation processes, which can reduce the effective contact area and negatively affect the mechanical and electrical stability of the joint.

The obtained results demonstrate that the structural and chemical state of the copper surface before interaction with tin is a key factor controlling the kinetics of phase formation, the balance between Cu₃Sn and Cu₆Sn₅, and the development of interfacial defects. Thus, controlled surface modification can be considered an effective tool for regulating the morphology and reliability of Cu–Sn contact zones.