Study On Kinetics And Mechanism Of Sn/Cu Soldering Interfacial Reaction Using Real-time Imaging Technology

The growth behavior and evolution rule of intermetallic compounds (IMCs) at a soldering interface were in situ observed using synchrotron radiation real-time imaging technology. Based on the observations, the mechanism of Sn/Cu soldering interfacial reaction was studied thoroughly and systematically. It was found that the mechanisms of interfacial IMC growth in heat preservation and cooling stages were different. Annexation model of interfacial IMC in heat preservation stage and morphology evolution model of interfacial IMC in cooling stage were established, which well revealed the whole interfacial reaction process in Sn/Cu solder joint. In addition, relevant factors that affect the Sn/Cu interfacial reaction were also investigated in depth. The main conclusions are as follows:1. Synchrotron radiation real-time imaging technology was introduced to in situ observe the liquid/solid interfacial reaction behavior during Sn/Cu soldering process for the first time. By using the synchrotron radiation real-time imaging technology, the dissolution of the Cu substrate and the growth behavior of the interfacial IMC during the whole soldering process can be obtained and continuously observed. The findings provide direct and no hypothetical experimental data for perfecting the theory of soldering interfacial reaction.2. It was found that the mechanisms of interfacial IMC growth in heat preservation and cooling stages were different. Annexation model of interfacial IMC in heat preservation stage and morphology evolution model of interfacial IMC in cooling stage were established. These two models constitute the interfacial reaction mechanism of the whole soldering process. The annexation model revealed the specific pattern of the annexation between interfacial IMC grains in heat preservation stage, which confirmed that the dissolution of Cu substrate and the growth of interfacial IMC were controlled by the combination of grain boundary diffusion and IMC annexation. The morphology evolution model revealed the specific pattern of the morphology transformation of the interfacial IMC in cooling stage, which confirmed both faceted and prismatic Cu6Sn5IMCs at the interface were controlled by two-dimensional nucleation mechanism.3. Base on the above findings, the effects of four relevant factors affacting the interconnect reliability on Sn/Cu soldering interfacial reaction were studied separately. The formation of Ag3Sn in Ag containing solders affected the annexation behavior of adjacent in heat preservation stage. The Ag3Sn particles were absorbed on Cu6Sn5surface in cooling stage, and thereby changed the size and morphology of the interfacial Cu6Sn5grains. By changing the distribution of surrounding solder, bubbles at the liquid/solid interface affected the dissolution of Cu substrate and the growth of interfacial IMC markedly. Liquid/solid electromigration (EM) accelerated the dissolution of interfacial IMC at cathode and promoted the growth of that at anode. Meanwhile, EM also obviously speeded up the dissolution of cathode Cu and suppressed that of anode Cu. All these phenomena associated with the distribution of current density, i.e., the effect was more obsvious where the current density was higher. Liquid/solid thermomigration at the soldering interface changed the diffusion flux of Cu and caused the directional diffusion of Cu atoms from hot end to cold end, which restrained the dissolution of Cu substrate at the cold end. In adition, thermomigration also caused the rapid dissolution of IMC at the hot end and the rapid growth of IMC at the cold end, causing the asymmetries in growth, morphology and composition of the interfacial IMC at the cold and hot ends.