Optimization of Ag Composition in Cu Pillar Bumps with Sn-xAg Solders

Abstract

In order to minimize the large plate growth of Ag3Sn intermetallic compounds (IMCs) adversely affecting the mechanical behavior and reducing the reliability of solder joints, the Ag content of less than 3wt% has been suggested in ball grid array (BGA) Sn-Ag solders. The suggestion (< 3wt% Ag in the solders) is invalid in small solder bumps, and moreover there is no available guideline of the Ag content in the small Sn-Ag solders. In this study, the optimum level of Ag content in small Sn-Ag Cu pillar bumps (CPBs) of less than 50um in diameter was investigated to suppress the large plate growth of Ag3Sn. Since the undercooling of Sn-Ag solders increases with a decrease of solder ball size, the Ag3Sn IMCs are more prone to become largely plate-like in small solders. Actually, in small Sn-Ag CPBs, the large plate growth of Ag3Sn was observed on the top surface of solder bumps even though the Ag content was less than 3.0wt%. For suppression of Ag3Sn plates in Sn-Ag CPBs, two ways were suggested; increasing the cooling rate of the reflow profile and reducing the Ag content in the solders. Reducing the Ag content was effective on reducing the large plate growth of Ag3Sn, while a high cooling rate was not effective. Through the thermal analysis and thermodynamic calculations, the optimized Ag content in Sn-Ag CPBs of less than 50um was suggested, and the large plate growth of Ag3Sn was effectively reduced within the suggested Ag content.