Formation and growth of intermetallic phases in diffusion soldered Cu/In-Bi/Cu interconnections

The paper presents microscopy observations of Cu/Bi-22 at.%In/Cu interconnections obtained as a result of diffusion soldering process. 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 the Cu/In-22Bi/Cu joint manufactured in the temperature range of 85-200 °C. Two sublayers of θ were identified; the first one, adjacent to copper substrate, contained (except for In and Cu) up to 6 at.% 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 showed also two morphologies. The homogeneous layer of η grew at the Cu/In-Bi solder interface at 300-325 °C. On the other hand, islands surrounded by unreacted solder were present after soldering at 350 °C. The third intermetallic phase δ[Cu7In3] coexisted with η at 350 °C. Moreover, the solid solution of In and Bi in Cu is formed at the δ/copper substrate. All the three phases belong to the binary Cu-In equilibrium phase diagram, while Bi appears in the form of separated areas within the θ or η phase.