Size distribution and morphology of Cu6Sn5 scallops in wetting reaction between molten solder and copper

During the reaction between molten solder and copper, ripening and growth of Cu6Sn5 scallops take place at the solder/metal interface. An experimental study on the morphology, size distribution and growth rate of Cu6Sn5 scallops was conducted. The measured size distributions of Cu6Sn5 as a function of time from top-view and cross-sectional view scanning electron microscopy images were in good agreement with the flux-driven ripening (FDR) theory. The FDR theory assumes a non-conservative ripening under a constant total interfacial area between the scallops and the solder, while the total volume of scallops increases with reaction time. The measured average radius of the scallops was proportional to the cube root of time. Comparing the experimental results and the theoretical model, the width of the liquid channel between scallops was calculated to be ∼2.5 nm. Morphology of the scallop-type Cu6Sn5 was dependent to the composition of the solder. The scallop morphology became more faceted when the composition was further away from the eutectic composition. The Cu6Sn5 scallops with a shape close to hemispheric gave better agreement with FDR theory. The small difference between the experimental data and theory was explained by taking the noise factor into account. The modified FDR model showed even better agreement with the experimental data.