Authored By:
Yifan Wu, Ph.D., Radhika Jadhav, Ian Tevis, Ph.D.
Indium Corporation
NY, USA
Summary
Metastable solder pastes present an alternative solution to low-temperature soldering, allowing the ability to create full metal interconnects at much lower temperatures than traditional solders of the same alloy compositions. Lower temperature interconnects enable next generation electronics where material, geometry, and performance would degrade at higher processing temperatures, such as in additive printed electronics.
This paper presents progress to date using supercooled solder pastes of core-shell bismuth-tin (BiSn) particles to accomplish solder interconnects below the melting point of the alloy. Reflow of the supercooled BiSn paste is achieved in an inline reflow oven with a peak temperature of 125°C, which is 40°C lower than what is found in a typical reflow profile of the alloy. Cross-sectional images demonstrate sufficient intermetallic compound growth having taken place at 125°C. Comparisons between supercooled BiSn solder pastes and traditional BiSn pastes are discussed, and progress towards process development and application guidelines are presented.
Conclusions
This work represents progress towards a drop-in replacement BiSn-based solder paste that can be used at an LTS temperature of 125°C, which is 40°C lower than the industry -standard 165°C peak processing temperature of the BiSn alloy. The supercooled liquid microcapsule technology avoids thermal damage to components and materials, or quality issues caused by CTE mismatch. This work has shown progress on the original proof of concept for using supercooled liquid metal microcapsules in production equipment with a commercial flux. This solder joint has the three hallmarks of a solder joint by demonstrating wetting, continuous structure, and IMC formation. This is the first demonstration of all three using the BiSn alloy in a supercooled liquid state.
Initially Published in the SMTA Proceedings
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