A study of Ag additive methods by comparing mechanical properties between Sn57.6Bi0.4Ag and 0.4 wt% nano-Ag-doped Sn58Bi BGA solder joints

Sn–Bi solder was proposed as one of the most promising substitutes for lead solder due to its lower melting temperature, good wettability, good yield strength and cost efficiency. With Ag elements added, the mechanical properties of Sn–Bi solder were improved obviously. There are two ways that are commonly used to add the reinforced particles into the solder. The first way (Way I) is to blend the reinforced particles with solder powders together, and then followed by pressure forming, sintering, cooling, crystallization and serious machining methods under inert atmosphere to make the solder paste. Another way (Way II) is to directly add the reinforced nano or micro particles into the solder paste by sufficient mechanical-stirring. In this research we would like to get fully understanding on the effects of these two ways of Ag addition on the mechanical properties of Sn–Bi–Ag solder joints during aging. Sn57.6Bi0.4Ag solder stands for the Way I and the doped Sn58Bi + 0.4Ag solder stands for the Way II. These two kinds of joints were compared via micromorphology observation, thermal failure analyses as well as balls shear strength measurement after different aging time (under 100 °C, from 0 to 800 h). The mechanical properties of Sn57.6Bi0.4Ag and the doped Sn58Bi + 0.4Ag solder joints during aging were shown to be associated with the changes of micromorphology, the dissolution of IMCs, as well as the flatness of the joints’ interface. Before long-time aging, the doped Sn58Bi + 0.4Ag solder joints showed better mechanical performance than Sn57.6Bi0.4Ag solder joints. During aging, Sn56.7Bi0.4Ag solder joints had better performance in preventing the dissolution of Ni–Sn IMCs into the solder side, having smoother interfaces, comparing with Sn58Bi + 0.4Ag solder joints. The degenerated phenomenon of Ag nanoparticle reinforcement seriously happened in the doped Sn58Bi + 0.4Ag solder joints. After longtime aging, Sn57.6Bi0.4Ag solder joints had better mechanical properties than the doped Sn58Bi + 0.4Ag solder joints.