High-Temperature Thermal–Electrical Coupling Damage Mechanisms of SnAgCu/Cu Solder Joints

In this study, the thermal-electrical coupling damage behavior of the SnAgCu/Cu solder joints at a temperature close to the melting point of the SnAgCu solder was investigated. Plastic deformation and/or local melting in the solder joints were observed. Plastic deformation occurs in the solid solder, and the degree of deformation is related to the solder grain orientation, while the solder softens and the slip bands are not obvious. When some solder not adjacent to the Cu pad melts at higher temperature, solidification looseness and surface oxidation appear at the melting zone. For these two conditions, electromigration of Cu obviously increases the content of the Cu₆Sn₅ in the solder, sometimes at the anode side, and the density of the Cu₆Sn₅ in each solder grain is related with the solder grain orientation, while the interfacial Cu₆Sn₅ layer at the cathode side is very thin. If the solder adjacent to the Cu pads melts, the Cu pad will be rapidly dissolved, especially at the cathode side, makes the Cu₆Sn₅ in the melting zone increases significantly, and the pattern of the Cu₆Sn₅ is a solidification dendrite. In this condition, electromigration of Cu is much more severe and the solder joint can fail more easily. It is proved that soften, local melting of solder and significant electromigration occur at the high temperature, and the mechanical property and corrosion resistance of the solder joint decrease sharply.