INFLUENCE OF COPPER SUBSTRATE TREATMENT ON THE POROSITY OF THE CU₃SN INTERMETALLIC PHASE DURING REACTIVE DIFFUSION IN THE Cu-Sn SYSTEM

Abstract

This paper presents a comprehensive experimental investigation into the influence of copper substrate surface preparation on the porosity and microstructure of the intermetallic Cu₃Sn phase formed during reactive diffusion in the Cu-Sn binary system. The motivation for this study stems from the growing importance of solder joint reliability in microelectronics, where the formation of intermetallic phases at the Cu-Sn interface – particularly Cu₃Sn and Cu₆Sn₅ – plays a critical role in determining the mechanical strength and electrical conductivity of connections. One of the key degradation mechanisms in these intermetallic layers is the formation of Kirkendall voids, especially within the Cu₃Sn phase, which can significantly impair device performance.

Three different types of copper substrate preparation were considered: (1) untreated copper, (2) copper subjected to recrystallization annealing at 700 °C for 5 hours, and (3) copper processed using Surface Mechanical Attrition Treatment (SMAT), which introduces a nanocrystalline surface structure by high-energy mechanical impacts. The SMAT process is known to produce a high density of lattice defects, increase grain boundary area, and thus alter diffusion behavior during intermetallic phase formation.

To simulate soldering conditions, tin layers were applied to each type of copper substrate, and the resulting diffusion couples were subjected to isothermal annealing at 230 °C for 48 hours in an argon atmosphere. The morphology and phase composition of the diffusion zones were characterized using scanning electron microscopy (SEM). To quantify porosity in the Cu₃Sn phase, SEM images were analyzed using Adobe Photoshop. The Cu₃Sn regions and pores were manually highlighted using the Polygonal Lasso Tool, and the pixel area of each was measured to compute relative porosity.

The results demonstrate a clear dependence of Cu₃Sn porosity on the initial microstructure of the copper substrate. The SMAT-treated samples exhibited the highest average porosity in the Cu₃Sn layer (2.77 %), compared to the annealed (0.23 %) and untreated (0.35 %) samples. These findings indicate that SMAT treatment significantly enhances porosity formation, which can be attributed to accelerated copper diffusion and increased defect density facilitating Kirkendall void formation. This observation aligns with existing diffusion models that predict enhanced vacancy flux and void nucleation in systems with asymmetric interdiffusion and fine-grained microstructures.

The increased porosity in SMAT-treated substrates, while indicative of intensified diffusion activity, poses challenges for the mechanical and electrical performance of solder joints. Therefore, careful control over substrate pre-treatment is essential for optimizing interfacial integrity in electronic packaging.

This study provides a detailed quantitative comparison of porosity levels under controlled experimental conditions and highlights the critical role of substrate surface engineering in reactive diffusion processes. The insights gained here can be applied to improve manufacturing technologies for micro- and nanoelectronic devices, particularly in the context of flip-chip and BGA solder joint formation.