Electromigration and Thermomigration Behavior of Cu Microbumps in Stacked Die Assembly

(23:47 + Q&A) Prof. Shubhra Bansal, Purdue University
From the 2024 IEEE Symposium on Reliability for Electronics and Photonics Packaging
Summary: Advanced packaging technologies, particularly 2.5D and 3D integration, have gained traction due to their cost benefits and ability to support heterogeneous integration of multiple dies and require the development of high density interconnects and advanced substrates. As the demand for high performance computing increases, interconnect and die attach materials are needed to meet the requirements of high I/O and power density. Cu microbumps (or pillar) hybrid bonding technology has shown significant progress for die-to-die, die-to-wafer, and wafer-to-wafer bonding; however, solder-based thermocompression bonding is critical for fine pitch die-to-substrate bonding. Sn-based solder caps have been used for Cu-microbump assembly; however, reliability assessment of these material systems is unknown for fine pitch interconnects. In the presence of an applied current, Cu atoms are expected to flow in the direction of electrons, leading to higher vacancy concentration at the cathode, and formation of thicker intermetallic compounds (IMC) at anode. Here, we present electromigration and thermomigration studies of Cu-microbump assemblies with Sn-based solder caps as a function of temperature and applied current density. To corroborate the experimental results, a finite element model has been built using COMSOL multiphysics software to evaluate the effect of current density and temperature on Sn diffusion, IMC thickness, and void formation for correlations to electrical and mechanical failures.
Bio: Shubhra Bansal is an Associate Professor at Purdue University with a joint appointment in School of Mechanical Engineering and School of Materials Engineering. She holds B.Tech. degree in Metallurgical and Materials Engineering from the Indian Institute of Technology, Roorkee. She received her M.S. and Ph.D. in Materials Science and Engineering from Georgia Institute of Technology, focusing on development of nanocrystalline Cu-pillar chip-to-package interconnects at the pioneering GT-PRC with Prof. Rao Tummala. Her professional career began at GE Global Research following which, she served as a Senior Technical Advisor for President Obama’s SunShot Initiative at the Department of Energy. Prior to joining Purdue in 2023, Dr. Bansal was an Associate Professor in Mechanical Engineering at University of Nevada Las Vegas. Among others, Dr. Bansal received 2010 GE Outstanding Individual Achievement Award, 2014 U.S. Department of Energy Technical Excellence Award, 2020 UNLV Faculty Excellence Award, 2021 NSF CAREER Award, 2021 DOE Faculty Fellowship, and 2022 NASA Glenn Faculty Fellowship. Her research interests include novel materials, reliability and sustainability for renewable energy and semiconductor packaging applications. She is a senior member of IEEE, and currently serves as the Associate Editor of IEEE-Journal of Photovoltaics and Elsevier Solar Energy Journal. At Purdue, she also leads the Heterogeneous Integration and Advanced Packaging technical vertical for SCALE — a DoD-funded workforce development program.