Rate of consumption of Cu in soldering accompanied by ripening

Abstract

What is the rate of consumption of Cu in soldering reactions has been a critical question in electronic packaging technology. The Cu films are consumed by Cu–Sn compound formation. Because the Cu thickness is limited and the rework of a solder joint requires a layer of unreacted Cu, the loss of Cu in soldering must be under control. At the solder interface, Cu–Sn intermetallic compounds do not form layered structures. Rather, the Cu6Sn5 phase grows as scalloplike grains into the molten solder and ripening occurs between the grains. Therefore, it has been difficult to determine the compound growth rate, and in turn the Cu consumption rate. Using cross-sectional and top-polished samples, we have measured the total volume of Cu–Sn intermetallic compounds formed between eutectic SnPb alloy and Cu substrate as a function of reflow time and temperature. We have deduced that after 1 min reflow, for example, the thickness of Cu consumed was about 0.36, 0.47, and 0.69 μm at 200, 220, and 240 °C, respectively.