Microstructure And Soldered Interface Of Sn-Bi-In-Zn-Ag Lead-Free Solders

Sn-2.5Bi-1Zn-0.3Ag alloy with superior properties was successfully developed in our research group in response to the worldwide wave of―lead-free‖solder and―green manufacturing‖. However, compared to traditional Sn-Pb alloy system, Sn-2.5Bi-1Zn-0.3Ag solder exhibited relatively higher melting point and larger brittleness as a result of Bi segregation. In order to offset these shortages, In was added to Sn-2.5Bi-1Zn-0.3Ag alloy as the fifth component. In this paper, the microstructure evolution and hardening mechanism of the slowly-cooled Sn-2.5Bi-xIn-1Zn-0.3Ag solder was explored and then, effects of In addition on microstructural formation and the soldered interfaces of Sn-2.5Bi-xIn-1Zn-0.3Ag was investigated. At last, the evolution of intermetallic compound (IMC) layer after high temperature aging, which was used to simulate the working conditions, were explained.Under slow-cooling condition, the microstructures of Sn-2.5Bi-xIn-1Zn-0.3Ag (x=0, 0.7, 1.4, 2.1 and 2.8) were composed of AgZn3 (Ag(Zn3-3x,Inx/3)) phase, Bi segregation phase and Zn-rich phase embedded in theβ-Sn matrix. Addition of In affected the microstructure and properties of the solder by increasing the solubility of Bi and decreasing the solubility of Zn.Stratification was observed at interfaces between all Sn-2.5Bi-xIn-1Zn-0.3Ag solders and Cu substrate after soldering at 230℃for 1 min. Of the two separate layers, the one adjacent to the Cu substrate was Cu6Sn5 IMC, and the other was CuZn layer at Sn-2.5Bi-1Zn-0.3Ag/Cu interface. When In was added to the system, the latter transformed to Cu5Zn8 due to the effect of In on Zn solubility. The formation of the interface experienced three stages: the formation of Zn-rich layer, the formation and growth of Cu5Zn8 layer and the growth of Cu6Sn5 layer.The aging process provided enough time and temperature for atomic diffusion, leading to constant change of concentration and atomic ratioin local area followed by microstructural changes. During the aging process, the interface transformed from Cu6Sn5+Cu5Zn8 two layers to Cu6Sn5 layer and then to Cu6Sn5+Cu3Sn two layers. At the same time, within the soldered dot, phase transformation from Ag3(Sn,In) to Ag(Zn3-3x,Inx/3), and from Cu6Sn5 compound via Cu5Zn8 to CuZn also progressed gradually.