Study On Thermomigration In Cu/Sn-xZn/Cu Solder Joints During Soldering Reaction

In electronic packaging technologies, soldering interfacial reaction is a key issue to achieve reliable interconnect of solder joint. During soldering, if there is a temperature difference between the two interfaces ofa solder joint, i.e., there exists a temperature gradient, directional migration of metal atoms—thermomigration (TM) may occur. Hence, the dissolution of substrate and the growth and transformation of interfacial intermetallic compound (IMC) will be affected under TM. However, TM in micro solder joints during soldering is still a new topic in the field of electronic packaging technology. Related research is lacking, and in-depth research should be carried out urgently. In this work, Cu/Sn-xZn/Cu solder joint was taken to investigate the effects of TM on soldering interfacial reaction, migration of atoms, and phase and morphology evolutions of interfacial IMC. Moreover, the key parameter migration heat (Q*) of Zn atoms was calculated.The following conclutions are drawn in the present work:(1) For Cu/Sn/Cu solder joints, same IMCs formed at both cold and hot ends, i.e., the thick Cu6Sn5at close to solder and the thin Cu3Sn close to Cu substrate, during TM. TM caused a significant difference in the thickness of interfacial IMC layers at both cold and hot ends. In addition, the TM effect was promoted at a higher soldering temperature, and the migration of Cu atoms towards the cold end was more obvious. Due to the huge number of Sn atoms in the solder, the dissolved Cu atoms from substrate were the main migration element. During the soldering, the net stream of Cu atoms from the hot end to the cold end formed by the temperature gradient. Thus, the IMC growth at the cold end was much faster than that at the hot end, leading to the thicker IMC layer at the cold end. The thickness difference between the IMC layers at the two interfaces increased with the soldering time. Furthermore, the thickness of the IMC layer at the cold end varied linearly with soldering time, indicating a reaction controlled growth process; while that at the hot end varied in a parabola form, indicating a diffusion controlled growth process.(2) For the TM in Cu/Sn-lZn/Cu solder joints at250℃, Cu5Zn8formed at both cold and hot ends at the beginning. Then the interfacial Cu5Zn8gradually transformed into Cu6(Sn,Zn)5, which happened much earlier at the cold end. During TM, the conversion of dominant diffusion element occurred:before TM for60min, Zn was the dominant diffusion element and migrated to the hot end, thus the Cu5Zn8layer at the hot end was thicker than that at the cold end; after TM for60min, Cu migrated to the cold end as the dominant diffusion element since the depletion of Zn, thus the Cu6(Sn,Zn)5layer at the cold end grew rapidly and was much thicker than that at the hot end. Additionally, the heat of transport of Zn was calculated to be+9.053kJ/mol. The evolution of the interfacial IMCs during TM at280℃was the same with that at250℃. As the temperature raised, however, the rate and extent of the interface reaction increased, resulting in a earlier conversion of dominant diffusion element at45min.(3) For the TM in Cu/Sn-5Zn/Cu and Cu/Sn-9Zn/Cu solder joints at230℃, the transformation of CuZn5to Cu5Zn8occurred at both ends and it was later at the hot end, resulting in a single Cu5Zn8layer at each interface. During TM, Zn was the dominant diffusion element and migrated to the hot end, leading to the thicker IMC layer at the hot end. The thickness difference between the IMC layer at the two ends continued to increase with the soldering time. Due to the increase of Zn content, types of interfacial IMCs in Cu/Sn-5Zn/Cu and Cu/Sn-9Zn/Cu solder joints were different from those in Cu/Sn-lZn/Cu solder joint and the conversion of dominant diffusion element did not occurred.