Electromigration study of 50 µm pitch micro solder bumps using four-point Kelvin structure

Electromigration (EM) of micro bumps of 50 mum pitch was studied using four-point Kelvin structure. Two kinds of bumps, i. e., SnAg solder bump and Cu post with SnAg solder were tested. These bumps with thick Cu under bump metallization (UBM) were bonded with electroless Ni/Au (ENIG) pads. The results showed different EM features comparing with larger flip chip joints. Under various test temperatures from 100 to 140degC, the increasing of electrical resistance under current stressing was mainly due to the formation of the high temperature intermetallic compounds (IMCs). The resistance increase-rate in solder bump interconnects was faster than that of Cu post with SnAg bump joints since there was more low temperature solder and under current stressing, more IMCs would be formed. When Cu post with SnAg bumps were tested at 140degC with the current density of 4.08times10⁴ A/cm², after certain stressing time the resistances would reach a plateau region, where the diffusion between different materials, i. e., Cu, Ni and Sn reached equilibrium, and IMCs became stable. Large number of Kirkendall voids and a number of cracks were found in the Cu post interconnects which was caused by the electron wind since less voids and cracks were found in the adjacent bump interconnects. When Cu post with SnAg bumps were tested at 140degC with the current density of 2.04times10⁴ A/cm² for 1000 h, the resistance did not reach steady state. The electron flow direction also has an effect on the diffusion of materials. The degradation of resistance increased faster when electrons flow from Cu UBM to ENIG.