Green Electronics Soldering by Application of Third Generation Lead-Free Solder Alloys: Studies on Their Wettability and Interface Microstructure Formed with Copper

Abstract Due to the regulations restricting the use of lead, the implementation of lead-free solders became a legal requirement more than 15 years ago. In addition to the well-known SAC305 solder, new alloys containing various concentrations of Bi, together with other additives improving the soldering process and reliability of joints, have appeared. The paper focuses on comparative study of wetting properties of SAC305 and commercially available third-generation lead-free solder alloys containing Bi: REL61, REL22, and HRL-1. The improved sessile drop method, was applied to measure the contact angle ( θ ) of the selected solders on a copper substrate. The highest contact angle value (36°) was observe for HRL-1 and the lowest (27°) for REL22. Initially the spreading phenomenon was very fast, below the resolution of the measurement method. In some cases of the wetting disturbance by the external factors, wetting has been inhibited and the intermetallic phases developed at the solder/substrate. The formation of a halo area at the substrate around the drop was also observed, most likely formed due to the surface diffusion of solder atoms. This phenomenon led to a further reduction of the contact angle, but the spreading of the drop was much slower. Comprehensive microstructure and chemical composition examination of the drop/substrate interfaces after wetting tests evidenced the presence of Cu 6 Sn 5 phase for all samples. Moreover, it was proved that the alloy additives (e.g., Ag and Bi) influenced the achieved contact angle and reduced unfavorable intermetallic phases formation such as Cu 3 Sn (e.g., Ni, Sb, and Ti).