Effect of deposit microstructure on the reflow discoloration of electroplating pure tin

In response to lead-free requirements and market forces, pure tin finishes have been widely used as a Pb-free option for semiconductor lead frames and electrical connectors in the microelectronics industry. Pure tin finishes could easy have a discoloration during reflow process since the reflow temperature is as high as 260°C, which could lead to a reliability issue. In this work, matte tin was electroplated onto C194 substrate. The microstructure of the deposit fabricated at different current density and different stannous concentrations was investigated with scanning electron microscope (SEM). It was found that, with increase of the current density and decrease of the stannous concentration, the grain size became smaller and the grain structure became looser, which resulted in the discoloration of pure tin finishes. Moreover, the compactness of the deposit and the current efficiency were also reduced. Copper diffusion was found to occur more easily in the deposit with a lower compactness, which may promote interfacial reaction to form intermetallic compounds (IMC) and further accelerate reflow discoloration of the pure Sn deposits.