Research Of High-Temperature Pb-Free Solders Based On Bi-Ag Alloy

According to the classification of package temperature, soft solders are divided into low-temperature solders, middle-temperature temperature solders and high-temperature solders.The traditional Sn-Pb solder is the most widely used as connected material in micro-electronic assemble and package industry. However, because of the bad effect of Pb on human health and the environment, the implementation of the EU RoHS and the "Electronic Information Products Pollution Control and Management" promulgated in China have regulated that the use of Sn-37Pb solder (melting point is 183℃) was prohibited. Typical middle- temperature lead-free solders being used in industry are Sn-Cu (the melting point of eutectic composition is 227℃),Sn-Ag (the melting point of eutectic composition is 221℃) and so on. The high melting point Sn-Pb solders in the RoHS directive (such as 90Pb-10Sn melting temperature 268-301℃), in which the mass percentage of lead is more than 85%, have been allowed to use, because the suitable alternative materials have not been found. People have to develop a new type of high-temperature lead-free solder to replace the high-lead solder in impendence in the electronic industry due to the harmfulness of Pb as well as its negative impact.The eutectic melting point of Bi-Ag alloy (Bi-2.5%Ag) is 262.5℃. It is more suitable and its hardness is closed to the Pb-5Sn alloy, the price can be acceptable too. In this paper, trace elements Ni, Cu, Ge are added to Bi-Ag alloy to prepare for BiAgNiCuGe solder alloy, and the effects of adding different contents of Ag on melting characteristics, the microstructure of solder alloy and solder joint, shearing strength are studied.The results showed that microstructure of the Bi-2Ag-0.4Ni-0.2Cu-0.1Ge hypoeutectic alloy (<2.5%Ag) was composed of primary Bi and eutectic microstructure of the Ag-rich particles / NiBi3 particles / Bi eutectic surrounding primary Bi. The hypereutectic alloy microstructure of Bi-(5, 8, 11, 14) Ag-0.4Ni-0.2Cu-0.1Ge was composed of tree-like primary Ag and the Ag-rich particles / NiBi3 particles / Bi eutectic. The additional content of Ag had little effect on the melting point of BiAgNiCuGe solder alloy, which change in the scope of 259-261℃. The wetting area of solder alloy on the Cu substrate decreased with the increase of the Ag content.The interfaces of the soldering joints connected from 5 different composition solders and the Cu substrate are rugged, and there is no intermetallic compounds in the joints. The microstructure at soldering joint interface solder side has no obvious difference before and after aging. Shearing strength of BiAgNiCuGe/Cu soldering joints increase in varying degrees with the increase of the Ag content both before and after aging. The reason is primary Ag can impede crack growth. Through watching the shear fracture surfaces of BiAgNiCuGe/Cu, it can be found that the connection between Cu substrate and solder is mainly that the liquid solder atomic diffusion to the Cu grain boundary, then semi-mechanical and semi-metallurgy connection can be formed. The extent of the diffusion of Bi in the solder to the Cu grain boundary is higher than that of Ag. The interface between Cu substrate and solder is the weak zone of soldering joints, which is the starting point of shear fracture.