Diffusion soldering using a Gallium metallic paste as solder alloy: study of the phase formation systematics

In this work preliminary results are reported on the characterization of Pb-free joints produced by using a diffusion soldering method at a process temperature of 700 °C during 20 min. The solder alloy is a metallic paste involving Ga and Al and Ni powder, and the substrates are Cu and Ni. The dissolution and diffusion-reaction processes, which take place at the interfaces of the interconnection zone, have been investigated by means of SEM and EPMA. A solid solution and intermetallic compounds (IMCs) with high melting point form as layers almost free from defect, allowing service temperatures about 500 °C higher than the process temperature. The phase stability sequence starting from the Ni to the Cu interface is the following: α′-Ni₃Ga, γ-Cu₉Ga₄, β-Cu₃Ga and (Cu) solid solution of the Ga–Cu system. The relative reaction front displacement of the layers and the implications of the present findings for the applicability of the diffusion-soldering method are also discussed.