Microstructure Control of Sn-based Alloys for Lead-free Solder as Microelectronic Interconnect Application

Abstract

Use of leaded solder was an issue in human health a few years back, while lead-free solders developed recently possesses higher melting temperature than conventional leaded solders. Thus, further research to develop more Sn-based solders with low-eutectic temperatures as compared to existing alloys are required. In this study, the effects of Gallium addition to five Sn-based alloys (Sn, SAC, Sn100C, Sn100CV, Sn-0.7Cu) at three different concentrations (0wt%Ga, 0.1wt%Ga and 2.0wt%Ga) was investigated. The microstructures of these alloys and the nature of their interactions with substrates including Electroless Nickel Immersion Gold (ENIG) and Organic Solderability Preservative (OSP) finishes is discussed.Thermal Analysis was done to investigate the resulting nucleation temperature of each alloy. The Sn-based alloys with 2.0wt% Ga additions had a reduction of 2℃-7℃ in growth temperatures when compared to their solidus temperatures. This matches the prediction from Sn-Ga phase diagram from previous studies. The intermetallic layers that formed between the solder and substrate showed that 0.1wt% Gallium resulted in a reaction with both Copper OSP and Nickel ENIG substrates, but higher concentrations of Ga (2.0wt%) did not result in a reaction at the interface. The microstructure analysis also showed that cracks formed in the cast specimens containing Ga and the solder balls appeared to have poorer wettability with increased Ga content. Both results suggested that there is segregation of Ga due to its low melting point, inducing low melting phase during solidification.In conclusion, the results indicate there is a critical composition of Ga which produces a homogeneous reaction with the alloys to assist in the fabrication of solder balls. Further studies on the properties of Ga containing alloys are required to fully understand the commercial potential.