Electronics Assembly and High Temperature Reliability Using Sn-3.8Ag-0.7Cu Solder Paste With Zn Additives

In this paper, we report a comparison of interfacial reactions of Sn-3.8Ag-0.7Cu (SAC 387) and SAC (0-1.5 Zn) solder pastes on Cu (organic solderability preservative finish) and Au/Ni-P/Cu [electroless Ni immersion gold (ENIG)] substrate metallizations with Ni/Sn and Cu/Sn plated component leads. Zn added to the paste in the form of surface-coated micrometer-sized particles dissolves into the solder during reflow. High-temperature aging (150 ^°C and 185 ^°C), thermal cycling experiments ( -20^°C to 175 ^°C for FR4 substrate, -40^°C to 185 ^°C for ENIG polyimide substrate), and shear testing of the solder joints were carried out. At a Cu interface, adding Zn to the solder joint improves the shear strength and suppresses Cu₃Sn and overall interfacial intermetallic compound (IMC) and Kirkendall void formation . However, above this temperature, the presence of Zn accelerates IMC growth. At a Ni interface, IMC suppression with Zn was noted at all temperatures. The amount of IMC suppression depends on the Zn concentration in the IMCs, which in turn depends on the geometry of joint as well as the original concentration of Zn in the solder.