Fabrication of 30 µm Sn Microbumps by Electroplating and Investigation of IMC Characteristics on Shear Strength

Abstract

In this paper, we prepared a pure Sn microbump with a diameter of 30 µm using an electroplating method for a solder cap on a Cu post/Ni barrier and then determined its IMC characteristics and shear strength according to reflow recovery. In order to secure uniformity of the bump height, it was optimized through WID and WIW evaluation methods. Using an optimized plating system, bumps with a diameter of 30 µm, a height of 40 µm, and a space of 50 µm were formed on a 4-inch wafer. Shear strength was measured according to the number of reflows. IMC was evaluated through cross-sectional and plane analyses of bumps. Its correlation with shear strength according to the number of reflows was derived. The Sn plating system optimized each process condition according to the Sn concentration, current density, and temperature. The shape, surface condition, and height uniformity of the bump were quantified by a 3D profiler and FIB analysis. Height uniformity (WID) according to the concentration was confirmed to be approximately 2% when the Sn concentration was 60 g/L. WID according to the additive was confirmed to be 2% when the Sn concentration was 60 mL/L. WID according to the plating temperature was excellent in the shape of the bump at 30 °C, and a value of 2% was confirmed. The WIW for the Sn plating thickness on a 4-inch wafer was confirmed to have a value of ±3.88%. A shear test between the Cu and Sn junction was conducted to verify the shear strength of the manufactured bump. At this time, reflow was performed 1, 3, 5, 7, and 10 times for each sample. It was confirmed that as the number of flows increased, shear strength first increased. It then decreased sharply. It was confirmed that as the number of reflows increased, the thickness and cross-section area of the IMC first increased. They then gradually became saturated. The IMC between Cu and Sn was created in island form at the beginning of the reflow, resulting in increased roughness and shear strength. However, as the number of reflows increased, the roughness decreased since the IMC generated by the island was combined. The shear strength also decreased sharply.