Liquid-metal particles can be used for heat-free soldering

Thuo believes Iowa State is just the place for a new development in soldering technology. Back in 1996, a research team led by Iver Anderson of the Ames Laboratory and Iowa State's department of materials science and engineering patented lead-free solder. That patent expired in 2013. But at its peak, the technology was licensed by more than 50 companies in 13 countries.

Thuo is hoping his heat-free soldering technology is just as useful. To try to help make that happen, he's worked with Tevis to launch SAFI-Tech. Thuo said the company plans to locate to the Iowa State Economic Development StartUp Factory when it opens in the ISU Research Park later this year.

The project started as a search for a way to stop liquid metal from returning to a solid - even below the metal's melting point. That's something called undercooling and it has been widely studied for insights into metal structure and metal processing. But it had been a challenge to produce large and stable quantities of undercooled metals.

Thuo's research team thought if tiny droplets of liquid metal could be covered with a thin, uniform coating, they could form stable particles of undercooled liquid metal. The engineers experimented with a new technique that uses a high-speed rotary tool to sheer liquid metal into droplets within an acidic liquid.

And then nature lends a hand: The particles are exposed to oxygen and then an oxidation layer is allowed to cover the particles, essentially creating a capsule containing the liquid metal. That layer is then polished until it is thin and smooth.

Thuo's research group proved the concept by creating liquid-metal particles containing Field's metal (an alloy of bismuth, indium and tin) and particles containing an alloy of bismuth and tin. The particles are 10 micrometers in diameter, about the size of a red blood cell.

"We wanted to make sure the metals don't turn into solids," Thuo said. "And so we engineered the surface of the particles so there is no pathway for liquid metal to turn to a solid. We've trapped it in a state it doesn't want to be in."

Those liquid metal particles could have significant implications for manufacturing. "We demonstrated healing of damaged surfaces and soldering/joining of metals at room temperature without requiring high-tech instrumentation, complex material preparation or a high-temperature process," the engineers wrote in their paper.

Thuo and the Iowa State Research Foundation Inc. have filed for a patent on the technology. Thuo supported the project with faculty startup funds from Iowa State and funds from a Black and Veatch faculty fellowship. The project also included imaging work at the Center for Nanoscale Systems at Harvard University in Cambridge, Massachusetts.

Tevis, of the SAFI-Tech startup, said the company is still testing the liquid-metal technology for electrical conductivity and mechanical reliability. He said the company is also developing the technology for product demonstrations.

Thuo said the project is a good example of his frugal approach to science: it should be practical, sustainable, inexpensive and all about innovating and solving problems.