Theoretical study of thermomigration effect on the pancake void propagation at the current crowding zone of solder joints

As the trend develops towards miniaturization to meet the requirement of performance improvement in portable consumer electronics, Joule heating has become a key reliability issue in the future electric packages, especially in the 3D integrated chip, in which the micro-bump is approaching 10μm. Heat flux must be dissipated away by temperature gradient which could reach 1000K/cm under which condition thermomigration will occur and induce the mass migration from the cold to the hot end of solders. The mass migration will result in the initiation and propagation of voids in the solder joint and, eventually, the whole failure of the device. In this paper, a numerical thermal-electrical finite element simulation is conducted to analyze the Joule heating at the current crowding zone and it is found that the joule heating is extremely high when the current density is above 10³ A/cm. An obvious temperature gradient is observed so that the thermomigration effect cannot be ignored when taking into account the void evolution problems in the solder joints. Based on the mass diffusion model, a pancake void propagation model considering the thermomigration force is proposed firstly and then the analytical void velocity is obtained which gives some insights into the reliability of the future interconnect under the condition of high Joule heating.