Surface diffusion controlled reaction in small size microbumps

We made head-shaped Cu-Sn solder joints with diameters about 10 μm, 20 μm, and 50 μm and reflowed the samples at 240 °C, 260 °C, and 280 °C for 60 s to 600 s. In the 10 μm bumps, there is only one Cu₆Sn₅ grain after reflow, thus the classic model of scallop-type grain growth of Cu₆Sn₅ does not apply. Also, the grain growth in the small size bumps has the faster growth rate. We proposed a surface diffusion-controlled model to explain the new kinetics. According to our model, the diffusion activation energy is calculated to be Q_S = 0.18 ± 0.02 eV/atom and diffusion frequency factor to be D_S0 = 5.65 × 10⁻³ cm²/s.