Size and geometry effects on the electromigration behavior of flip-chip Sn3.5Ag solder joints

With the increasing miniaturization of electronic devices and systems, the dimension of solder joints and pitches has been continuously scaling down, while the current density carried by solder joints increasing significantly, consequently a critical issue, electromigration (EM), has become a key reliability concern. The EM behavior in the solder joint is mainly dependent on the magnitude and distribution of the current density, and may be influenced by the temperature distribution induced by Joule heating effects in the solder. In this study, three-dimensional thermo-electrical finite element analysis is employed to characterize the current density and temperature distributions, current crowding effects as well as thermal gradients in micro-scale Sn3.5Ag solder joints with different sizes and geometries. Results show that, both the maximum and average current densities in the solder increase dramatically by power functions with the scaling down of the solder size. Accordingly, as the solder size is reduced, the serious Joule heating effect takes place and both temperatures and thermal gradients of solder joints increase significantly. Moreover, with increasing standoff height, the maximum current density increases, while the average current density decreases, which results in the increase of the crowdedness of current density defined by the ratio of the maximum current density to average current density (i.e., the crowding ratio). In addition, the thermal gradient in the solder decreases with increasing standoff height. However, compared with the increase of standoff height, the increase of contact angle has quite opposite effect on the maximum and average current densities, crowding ratios as well as thermal gradients in solder joints, respectively. Further, both the standoff height and contact angle have very limited influence on the temperature of solder joints.