The effects of flux on the wetting characteristics of near-eutectic Sn-Zn-In solder on Cu substrate

Abstract

The effects of flux on the wetting characteristics of the near-eutectic Sn-Zn-In solder alloy [composition: 86Sn-9Zn-5In] on Cu substrate have been studied by using dimethylammonium chloride (DMAHCl), stearic acid (SA), lactic acid (LA) and oleic acid (OA) as fluxes. Wetting time and maximum force were estimated from the wetting experiments. According to the wetting curves obtained by wetting balance apparatus, the SA and OA are not suitable as flux for the near-eutectic Sn-Zn-In solder on Cu substrate. However, the LA and DMAHCl provide a good wetting behavior. The lowest wetting time (0.27 s) was obtained with 3.5 wt% DMAHCl as flux as-dipped at 300°C. When the dipping temperature increased from 250 to 300°C, the wetting time decreased obviously from about 0.6 to 0.4 s while the LA and DMAHCl were used as flux. When the content of LA was less than 5.0 vol% at 250°C and 2.5 vol% at 300°C, non-wetting or partial wetting was observed as determined by wetting curves. In addition, for the content of DMAHCl less than 1.5 wt% at 250 and 300°C, non-wetting or partial wetting was also observed. Quite different from the most tin-based solders for Cu substrate, intermetallic compound γ-Cu₅Zn₈ was found by the X-ray diffraction (XRD) and selected area electron diffraction (SAED) analyses at the interface of solder and substrate after etching out the unreacted solder layer. The Zn element was enriched at the interface between solder and Cu substrate as analyzed by line scanning.