Study On Microstructure Evolution Of Polycrystalline Solder Joints Under Electromigration And Thermal Cycling

With the development of electronic products in the direction of miniaturization and multifunction, the packaging density is increasing. The size of the solder joints becomes smaller and smaller, which makes the current density and the working temperature of the solder joints increase, resulting in more reliability problems. In related studies, the phenomenon of electromigration and thermal fatigue failure is always the key point of the research. In this background, this research studies the microstructure evolution of polycrystalline solder joints under electromigration and thermal cycling.The technology of screen printing and reflow was used in this experiment. Sn3.0Ag0.5Cu polycrystalline solder joints with uniform size were prepared by controlling the reflow temperature. Then the electromigration and thermal cycling test of these joints were carried out. Scanning electron microscopy(SEM) and electron back scatter diffraction(EBSD) were used to analyze the microstructure and crystal structure of polycrystalline solder joints in the electromigration and thermal cycling test.The electromigration test was conducted under the condition of current 2.5A and constant temperature 150?. The current crowding effect found in the corner of the polycrystalline solder joint and the polarity effect occurred at the interface. These resulted in the formation of voids and cracks at the cathode interface, which caused the dissolution of the compound and Ni layer, and increased the thickness of the compound in the anode interface. Different from one and a few grains of the solder joint, the size of a large number of grains in a polycrystalline solder joint is small, and the crystal orientation is random. Since the solder joints exhibit isotropic properties, the diffusion of Cu atoms have no "better orientation". The growth of the compound in polycrystalline solder joint was not obvious after long time of electromigration, and the long range migration of compounds was not occurred. These indicate that the electromigration in the polycrystalline solder joints is inhibited.The experimental results show that the recrystallization of polycrystalline solder joints occurred during thermal cycling test, and formed smaller grains and large angle grain boundaries. During the thermal cycling, the growth of IMC was not only affected by temperature, but also influenced by the stress and strain, the high temperature residence time and the process of heating and cooling. These factors lead to the growth rate of IMC in the thermal cycling is slower than the IMC growth rate in aging. The shear strength decreased with the thermal cycling time and the fracture mode was changed from the initial ductile fracture to brittle fracture. In addition, the diffusion of Cu atoms leads to the decrease of the interfacial adhesion between Ni and Cu, and the interface was separated in the shear test.