Nucleation and growth of intermetallics and gold clusters on thick tin layers in electroplating process

Nucleation and growth mechanism of sequential electroplating of gold over thick tin layer have been studied by SEM, EDX and X-ray diffraction method. It is found that the growth mechanism of Au layer on top of Sn layer is diffusion-controlled process and the primal Au-Sn intermetallic compound and Au phases are AuSn4(2 1 1) and Au(1 1 1), respectively. The plated gold atoms nucleate and form clusters on the ridges of tin grains. The average size of Au spheres forming the clusters depends on plating time: 0.4 μm for 1 min, 1 μm for 7 min, and 3 μm for 13 min. The equivalent Au layer thickness is 0.2 μm, 1.0 μm, and 2.0 μm, respectively. As plating process continues, Au atoms are deposited over the Au spheres already formed rather than generate more nucleation sites on the tin surface. As a result, the Au spheres grow in size and merge each other to form larger clusters. The fundamental understanding of the nucleation and growth mechanism is important in guiding the design of plating processes to produce Au-Sn composites for lead-free soldering based on AuSn alloys without using flux.