The Study Of Interfacial Cu₆Sn₅Growth Process And Influence Factors During Lead-free Soldering

Rencent years, the progress of lead-free, miniaturization and densification of electronics all raise new challenges on the reliability of solder joints. The microstructure of solder joints has great influence on the reliability of solder joints, and the interfacial intermetallic compound (IMC) is always a great core of solder joints microstructure. The growth behavior ofCu6Sn5, formed between Sn-based lead-free solder and Cu substrate, including morphology, size, thickness and distribution all have great influence on the reliability of solder joints.Sn, Sn-3.5Ag, Sn-0.7Cu and Sn-3.5Ag-0.7Cu four kinds of solder ball were chosen to solder on Cu substrate at300℃for5s,10s,30s and60s, respectively. High pressure-air was used to remove liquid solder when the soldering was about to finish, in order to keep the morphology of soldering process, i.e the heat perservation stage. Compared with the samples cooled by air, the morphology of Cu6Sn5in heat preservation and cooling stages were studied. In addition, the growth behavior of Cu6Sn5in heat preservation stage at different soldering time was solely observed to study the effect of soldering temperature. And Sn/Cu and Sn-3.5Ag/Cu, Sn/Cu and Sn-0.7Cu/Cu were chosen to study the effects of Ag and Cu elements on the growth of Cu6Sn5. The soldering temperature was300℃and soldering time was60s. The conclusions are as follows.(1) In soldering heat preservation stage, Cu6Sn5was mainly in cross growth, the morphology was basically scalloped and the growth was mainly controlled by diffusion flux which was similar with the riping mechanism. In cooling process, Cu6Sn5mostly grew in longitudinal direction, the morphology was faceted, prismatic, rodlike and even the lamellose and the growth was mainly controlled by the decrease of saturated solubility of Cu caused by the reduce of temperature.(2) Cu6Sn5continued to grow in cooling process. The existence of undercooling in cooling process decreased the dissolution of Cu in the solder, Cu6Sn5precipitated at the surface of the original Cu6n5grains.(3) With the increase of soldering time and soldering temperature, the total amount of Cu6n5grains reduced, accompanied with the size increase of large grains and the size decrease of small grains and finally disappeared, which can be explained by Gibbs-Thomson effect. Smaller the grain size, larger the concentration of Cu, leading to the diffusion of Cu towards the adjacent larger grain.(4) Ag3Sn particles formed in containg Ag solders generally ranged in linear or concentric circles on the surface of Cu6Sn5grains. Along with the soldering time, the grain size of Ag3Sn had no obvious change but the amout increased, because of the increase of surface free energy with the growth of Cu6Sn5grains, providing more energy for the nucleation of Ag3Sn. Along with the increase of soldering temperature, the size of Ag3Sn particles decreased due to the rise of temperature destroyed the medium-range structure in liquid solder.(5) In soldering heat preservation stage, there were no obvious effects of Ag and Cu elements. While in cooling process, the formation of Ag3Sn in Sn-3.5Ag solder and the decrease of the dissolution rate ofCu substrate all suppressed the cross growth of Cu6Sn5and retained path for diffusion, promoting the the longitudinal growth of Cu6Sn5. That’s the reason why the Cu6Sn5grains at the Sn-3.5Ag/Cu and Sn-0.7Cu/Cu interfaces are longer and thinner than that at the Sn/Cu interface. In addition, the effect of Ag was larger than that of Cu.