Understanding and preventing Cu–Sn micro joint defects through design and process control

The risk of significant Kirkendall voiding within the intermetallic bond between a conventional solder joint and an electroplated Cu pad is relatively low and directly related to the presence of certain impurities in the latter. However, while the Cu–Sn reaction has been thoroughly studied so far, there is very limited understanding of the impact of space confinement on the Cu/solder interface dynamics. While no voiding has been found in micro joints on high-purity Cu, the propensity for voiding in the Cu 3 Sn is higher in such joints than in Ball Grid Array scale (macro) joints for a given quality of electroplated Cu. Systematic trends are also somewhat different. In general, the intermetallic compound growth rate varies with interactions between solder volume to pad area ratio, solder composition, pad finish, initial assembly process parameters, subsequent heat treatments, and specifics of the further thermomechanical history. Notably, when the solder is thin enough, the Cu 3 Sn growth rate is much higher than for thicker configurations and this is what leads to a substantial increase of the risk of severe voiding in micro joints. A mechanistic understanding leads to strategies for electroplating of Cu with low voiding propensity for use in micro solder joints. Graphic abstract