Cu Pillar and μ-bump Electromigration Reliability and Comparison with High Pb, SnPb, and SnAg bumps

Failures due to Electromigration (EM) in flip-chip bumps have emerged as a major reliability concern due to potential elimination of Pb from flip-chip bumps and a continuous drive to increased IO density resulting in a reduction of bump pitch and size. Additionally, the rapid development and implementation of 3D IC structures introducing new interconnects (mu-bumps, RDL, microvias, and TSVs) at much finer geometries, raises concerns about electromigration and current carrying capacity of these interconnects. This paper presents the results of multiple EM studies on Cu Pillar, High Pb, SnAg, eutectic SnPb Flip Chip bumps and mu-bumps. A special test vehicle was designed to get a head-to-head comparison of Cu Pillar EM with that of solder bumps. Tests are being conducted using three current levels and three temperatures to estimate Black's Equation parameters. A separate test vehicle is also being tested using 5 combinations of current and temperature to estimate the current carrying capacity of Cu-SnAg-Cu mu-bumps of 25um diameter. More than 8000 hours of testing is completed on flip chip solder bump and Cu Pillar, showing Cu Pillars as having the best reliability amongst the four bump metallurgies. The worst reliability was observed for High Pb bumps followed by eutectic SnPb eut and SnAg bumps. The Cu-SnAg-Cu mu-bump structure has been tested for 5500+ hours without any failures. The paper provides the detailed test matrix, failure data, failure analysis, and an estimation of Black's Equation parameters for some of the above configurations on test.