Electromigration failure mechanisms for SnAg3.5 solder bumps on Ti/Cr-Cu/Cu and Ni(P)/Au metallization pads

The electromigration behavior of SnAg3.5 solder bumps is investigated under the current densities of 1x10(4) A/cm(2) and 5x10(3) A/cm(2) at 150degreesC. Different failure modes were observed for the two stressing conditions. When stressed at 1x10(4) A/cm(2), damage occurred in both the anode/chip side and the cathode/chip side. However, failure happened only in the cathode/chip side under the stressing of 5x10(3)A/cm(2). A three-dimensional simulation of the current-density distribution was performed to provide a better understanding of the current-crowding behavior in the solder bump. The current-crowding effect was found to account for the failure in the cathode/chip side. In addition, both the temperature increase and the thermal gradients were measured during the two stressing conditions. The measured temperature increase due to Joule heating was as high as 54.5degreesC, and the thermal gradient reached 365degreesC/cm when stressed by 1x10(4)A/cm(2). This induced thermal gradient may cause atoms to migrate from the chip side to the substrate side, contributing to the failure in the anode/chip side. Moreover, the formation of intermetallic compounds in the anode/chip side may also be responsible for the failure in the anode/chip side. (C) 2004 American Institute of Physics.