A physical model examination of micro-brazing reaction with gallium and copper

Brazing processes are largely controlled by the wetting phenomenon, i.e. how well the liquid spreads on the surface of solids. Because successful integration of alloys into engineering structures often relies on joining technologies, this study investigated the pertinent factors which control the brazing reaction. A liquid flux is usually used to prevent oxidation and to remove thin oxide films from substrates. This reaction is difficult to examine due to the high melting point of solder, which is 183°C even for lead-tin solders. In this study the Ga-Cu system was used whereby the wetting action can be dynamically recorded on videotape with the experiments being carried out in a glass cup. By immersing the copper plate and a droplet of gallium in an acid solution, the action of flux can be observed. The surface tension of liquid gallium measured is greatly affected by the type of flux used and also by the method of preparation of liquid gallium. The sessile drop test to determine the contact angle showed that even if a thin oxide film is present adhesion rather than wetting is observed with contact angles larger than 90°. This contact angle depends on the specific oxide such as copper oxide, alumina, chromia or silica. Once this oxide layer is removed away by the flux, wetting occurs with contact angles less than 90°. The surface tension of liquid gallium at 32°C in 2.5% HCl was 658.6 mN/m which decreases to 400 mN/m in 15% HCl.