Thermoelectric coupling effect on the interfacial IMC growth in lead-free solder joint

This paper presents the morphology evolution of the interfacial intermetallic compound (IMC) in Cu/Sn-58Bi/Cu solder joint during current stressing and loading temperatures. The IMC growth kinetics and Bi migration towards the anode side was analyzed and discussed. One dimensional semicircular sectioned solder joint was fabricated and used to conduct the electromigration (EM) experiment, effectively eliminated the thermomigration effect induced by Joule heating accumulation. Results indicated that the solder matrix was melted due to the combined effect of electromigration force and high temperature. After cutting off the circuit, huge valleys were found in the solder joints. Furthermore, partial bulk solder extended upon the Cu substrates. For further detecting the microstructural evolution of the solder matrix, the tested sample was polished to reveal the interior microstructure. It can be noted that the eutectic microstructure coarsened slightly, but remained the original uniform distribution. Most important, the IMC layer formed at the anode interface and cathode interface displayed different thickness. To be precise, the IMC layer thickness formed at the cathode interface was larger than that of formed at the anode interface, which was contradictory with the traditional theory of EM effect on IMC growth in SnAgCu or SnAg solder joints. By calculation, the cathodic scallop-type IMC thickness can reach 34.3 mu m, however, merely 9.7 mu m-thick IMC layer was found at the anode interface. It should be noticed that a uniform 7.5 mu m-thick Bi layer was aggregated between the IMC layer and the solder matrix at the anode interface. The diffusion and migration path of the Sn atoms to the anode interface was blocked because of the compact Bi layer formation, which reduced the actual Sn reacted with Cu to form IMC. Accordingly, the growth rate of the IMC layer decreased, leading to the abnormal IMC growth at the interfaces.