A Study of Low-Cost Sequential Electroplating Bumping Process and its Metallurgical Behavior

Studies were conducted to validate from a metallurgical point of view whether a lower cost sequential plating approach to SnAg and SnAgCu (SAC) solder bumping is a suitable alternative to the conventional alloy plating process. A range of Ag content corresponding to typical bump applications was explored with respect to Ag diffusion and intermetallic compound (IMC) formation. Variables that can affect such IMC formation were further explored as a function of underbump metallization (UBM) structure and cooling rate during bump solidification. By comparing the results to those previously reported on SnAg-based alloys, it is demonstrated that the proposed sequential plating process produces very similar microstructures and Ag ₃ Sn IMC morphologies, due to the rapid diffusion and distribution of Ag through the liquid Sn. Known means to mitigate the less desirable large Ag ₃ Sn platelets, that is by changing the top UBM layer from Cu to Ni or by employing an ultrarapid cooling rate, are shown to be equally effective for sequential plating. These observations, in conjunction with the simplicity and flexibility of plating multiple single metals, propose adoption of the sequential plating process as a cost-effective and robust Pb-free bumping solution for fine pitch flip-chip packaging.