Improvement of thermomigration resistance in lead-free Sn3.5Ag alloys by Ag interlayer

Abstract

A significant improvement of thermomigration (TM) resistance in Pb-free Sn3.5Ag alloys was achieved by a Ag interlayer. Ag interlayer was used to form Ag3Sn intermetallic compound (IMC) during reflowing process, which can serve as a barrier to suppress thermomigration-induced Cu flux under a temperature gradient. Two different kinds of sandwich samples were designed and compared, Cu/Ag/Sn3.5Ag/Cu and Cu/Sn3.5Ag/Cu. A heat sink and a heat source were employed to establish a temperature gradient of 7000 °C/cm. The growth rate of IMCs at cold end in Cu/Ag/Sn3.5Ag/Cu was slower than that in Cu/Sn3.5Ag/Cu. Thermomigration-induced Cu fluxes were also effectively inhibited in Cu/Ag/Sn3.5Ag/Cu specimens, and the calculated value of JCu,TM was about one-third times as that in Cu/Sn3.5Ag/Cu specimen. In addition, the dissolution of IMCs and under bump metallization (UBM) at hot end was barely observed in Cu/Ag/Sn3.5Ag/Cu specimen. These results suggest that Ag3Sn layer can be a diffusion barrier to effectively inhibit the Cu flux from hot end to cold end under a temperature gradient. Furthermore, crystallographic orientation of Sn grains was affected by the Ag interlayer, and it leads to the c-axis of Sn grain being more vertical to the direction of JCu,TM for the samples with Ag interlayer deposited, which further enhances the effect of suppression on JCu,TM.