Characterization of the reaction process in diffusion-soldered Cu/In-48 at.% Sn/Cu joints

This work describes a Pb-free solder alternative for the interconnection technology and its implementation in a diffusion soldering technique: In-48 at.% Sn solder (eutectic alloy), with a melting point of 120 degreesC. The system proposed has the advantages of both traditional soldering and diffusion bonding, i.e., good joint filling, high service temperature, and good mechanical properties. The diffusion reaction processes in Cu/In-48 at.% Sn/Cu joints were investigated between 180 and 400degreesC. Electron microprobe analysis revealed the presence of one or two intermetallic layers in the interconnection zone: a layer of the eta phase below 200degreesC, and layers of the eta and zeta phases above 200degreesC. The eta and zeta phases form through a solid-liquid and a solid-solid diffusion reaction, respectively. Below 200degreesC the eta phase exhibits two different morphologies: large coarse grains at the eta/(originally liquid)ln-48 at.% Sn interface and a fine-grained region at the Cu/eta interface. The thickness of the zeta layer shows a constant growth rate (linear growth) at constant temperature. The temperature dependence of the growth rate constant of the zeta layer is described by an Arrhenius relationship with an activation energy equal to 121 kJ/mol and a pre-exponential factor of about 57 m/s. (C) 2002 Published by Elsevier Science B.V.