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: a¢-Ni3Ga, c-Cu9Ga4, b-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 diffusionsoldering method are also discussed.