Corrosion behavior of 64Sn-35Bi-1Ag solder doped with Zn in NaCl solution and its electrochemical migration characteristics in high humid thermal condition for electronic packaging

Due to lead-free pressure, Sn-Bi-Ag solder (SBA) is regarded as an another potential substitute [1]. However, low mechanical strength restricts its wide application. Metal doping can improve its mechanical characteristics and soldering strength. In this thesis, Zn doping in 64Sn-35Bi-lAg solder is discussed. Effect of Zn doping on corrosion behavior of lead-free SBA solder in 3 wt.% NaCl solution is investigated by potentiodynamic polarization. The electrochemical migration (ECM) causing to nano-crystal growth is employed by the designed experiment under high temperature (80°C) and high humidity (85%RH) conditions in DC=3V electric field. The surface morphology and elemental composition of corrosive products and nano-crystals in SBA or the doped solder are determined by SEM, ED AX, XRD technologies. Results show the nano-crystal growth can be accelerated after Zn doping comparing to 64Sn-35Bi-lAg solder. The shapes of crystals are completely different from each other, the former looked like as small petals, while for the latter, there are two kinds of nano-crystals, one looks like open flowers with Zn-rich, one looks like shives with Sn-rich. The "bridge time" between 3mm fine pitch in DC=3 V electric field is 240hrs without doping, while 100hrs with Zn doping. EDAX results show that the content on nano-crystals of 64Sn-35Bi-lAg solder is mainly Sn, only little other elements, while the main contents of SBA solder with Zn doping were Zn and Sn. Corrosive experiment shows that the corrosive current density (Icorr) increased with Zn percent increasing when Zn weight percent is less than 1%, and is up to maximum when Zn% is equal to lwt.%, then Icorr rate decreases as a function of Zn percent. From the above points of ECM, it is unbenefit to fine-pitch electronic packaging with Zn doping although there are some advantages after doping. Thus provides a good technical support to develop a new lead-free solder substituting for Sn-37Pb.