SBIR Phase I: Stable Undercooled Metal Materials for Soldering Heat-Sensitive Components and Substrates

This SBIR Phase I project will enable the use of flexible and printed electronic technologies for heat-sensitive materials such as plastics, papers, and organic electronics that cannot employ traditional soldering or conductive adhesives because of temperature, cost, and/or performance limitations. The foundation for the proposed research is a no-heat-added solder paste made from liquid-metal particles that act like filled balloons, releasing a liquid metal when popped and then turning solid. Printed electronics offer potential for major manufacturing growth for the United States, because the device structure lends itself to being manufactured domestically, offering transportation advantages and overall lower costs of ownership. This technology may have applications in commercial flexible displays, wearable biomedical sensors, lightweight electronics for military equipment, and radio frequency identification tags for supply-chain connectivity. The development of the products enabled by this technology is expected to make U.S. citizens, institutions, military, and government agencies more efficient and effective with better inventory/waste tracking, biometrics collection, weight reduction, and object networking. Enabling new U.S. technologies creates market opportunities for the Internet of Things and telemedicine, as well as manufacturing opportunities to make electronics in the U.S., creating significant jobs, businesses, and tax revenue. This project focuses on developing unique undercooled metal microparticles for use in no-heat-added soldering of printed/flexible electronics. The research will decouple the heating from the joining of the metal?significantly lowering the processing temperature and thus preserving function for heat-sensitive electronics and substrates. The key to the innovation?an oxide encapsulating shell?prevents heterogeneous nucleation with a thin oxide shell and undercooling's enhancement on the micro- and nanoscale. It can be mechanically sheared or removed by chemical flux action to enable the liquid metal inside to first flow and coalesce, and to then solidify. The metal alloys can remain as liquids at room temperature for months. Research will increase the production yield and reliability of bismuth alloys that melt at 139°C to less brittle tin/silver/copper SnAgCu solder that melts at 217°C. Objectives are to 1) develop a material that can join/coat materials with solid metal without adding heat, 2) demonstrate the properties of the joint compared to conductive adhesives and traditional soldering, and 3) demonstrate a Wheatstone bridge and flexible Micro Electro Mechanical Systems (MEMS) prototype device. Undercooled solders will be applied using flux strategies to make mechanically and electrically robust joints in an electronic device at room temperature.