Microscopic observations were made of Cu/Bi–22at%In/Cu interconnections obtained as a result of diffusion soldering. The choice of the material as well as technological process allowed getting thermally stable joints dedicated to the electronic equipment thanks to controllable growth of intermetallic phase(s). The θ[Cu11In9] phase was present in a Cu/In–22Bi/Cu joint manufactured at 85 to 200C. Two sub-layers of θ were identified; the first one, adjacent to copper substrate, contained (except for In and Cu) up to 6at%Bi, the second one appeared as the so-called scallops growing into liquid solder. The θ phase enriched in Bi transformed into η[Cu2In] phase. The η also showed 2 morphologies. The homogeneous layer of η grew at the Cu/In–Bi solder interface at 300–325C. On the other hand, islands surrounded by unreacted solder were present after soldering at 350C. The third intermetallic phase δ[Cu7In3] coexisted with η at 350C. Moreover, the solid solution of In and Bi in Cu was formed at the δ/copper substrate. All 3 phases belonged to the binary Cu–In equilibrium phase diagram, while Bi appeared in the form of separate areas within the θ or η phase.

Formation and Growth of Intermetallic Phases in Diffusion Soldered Cu/In–Bi/Cu Interconnections. J.Wojewoda-Budka, P.Zięba: Journal of Alloys and Compounds, 2009, 476[1-2], 164-71