Effect of Underfill and Bump Density on Electromigration and Thermomigration Failures in Thermo-Compression Bonded Cu Microbumps for 3D-HI

(33:28 + Q&A) Prof. Shubhra Bansal, Purdue University — The relentless pursuit of miniaturization and enhanced performance in microelectronics has driven the transition from planar integrated circuits (ICs) to three-dimensional heterogeneous integration (3D-HI), where multiple device types are vertically interconnected. This paradigm shift sustains Moore’s Law beyond the limits of transistor scaling by enabling high transistor density, reduced interconnect length, and superior system performance. Central to this architecture are through silicon vias (TSVs) and microbumps, which serve as the vertical interconnects between stacked dies. Unlike conventional flip-chip solder joints, thermo-compression bonded (TCB) microbumps can be scaled down to ?10um diameters, with solder volumes reduced by orders of magnitude. This miniaturization enables thousands of interconnects per chip, essential for high-bandwidth data transfer in applications such as artificial intelligence accelerators and high-performance computing. However, the extreme scaling introduces unprecedented reliability challenges, particularly under high current densities and thermal gradients, where electromigration and thermomigration emerge as dominant failure mechanisms.
Most existing studies examine electromigration and thermomigration in isolation, overlooking their coupled effects in TCB microbump systems. The influence of underfill materials remains inadequately explored, particularly regarding their impact on localized thermal dissipation pathways, stress evolution from coefficient of thermal expansion (CTE) mismatches, and atomic diffusion kinetics in confined geometries. Furthermore, while extreme current densities (in the order of 104 – 105 Acm-2) in microbumps are well-documented, the resulting Joule heating effects and their role in amplifying thermal gradients (?T), accelerating thermomigration, and triggering solid-state phase transformations have not been systematically decoupled from pure electromigration phenomena. Existing predictive models fail to capture these synergistic electro-thermomigration interactions or incorporate the effect of underfill on thermo-electro-mechanical reliability. Consequently, the long-term reliability implications of Joule heating hotspots and underfill interactions under combined electro-thermomigration stress lack a holistic framework, hindering the design of robust TCB microbump interconnects for 3D-HI applications. In this talk, coupled electro-thermo-mechanical effects will be discussed for Cu microbumps with Sn-Ag solder caps. The effect of pitch, underfill, temperature, and current density will be discussed.
Bio: Shubhra Bansal is an Associate Professor at Purdue University with a joint appointment in the School of Mechanical Engineering and the School of Materials Engineering. She holds a 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. At Purdue, her research group (Clean Energy and Advanced Packaging Lab) focuses on novel materials, reliability and sustainability for energy conversion and semiconductor packaging applications. At Purdue, she also leads the Heterogeneous Integration and Advanced Packaging technical vertical for SCALE — a DoD-funded workforce development program.

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