Put away that hot iron, and stow the solder: you can now forge a connection between two bits of metal at room temperature.
Soldering is a mainstay of the microelectronics industry, which uses hot molten metal to connect components on printed circuit boards. But conventional hot soldering techniques can sometimes damage the materials used in flexible electronic devices, or the ever-smaller parts on today’s microchips.
Martin Thuo at Iowa State University in Ames has now developed a clever way of soldering that requires no heat, using an alloy of bismuth, indium and tin called Field’s metal. At room temperature, this mixture is normally a solid — it melts at 62 °C. But if tiny droplets of the molten metal are encapsulated in a protective shell, they remain liquid when the shell cools and solidifies.
This effect, known as undercooling or supercooling, arises because the liquid metal is prevented from coming into contact with anything that triggers solidification — a speck of dust, say. The effect has been widely studied, but until now there was no way to make stable undercooled metal particles in a readily usable form. “They’re almost like water balloons of liquid metal,” explains Michael Dickey, a materials scientist at North Carolina State University in Raleigh.
Crushing the particles releases the liquid metal, which quickly solidifies to form a neat, conductive solder joint. “I haven’t seen this before, I think it’s quite unique,” says Johan Liu, a materials scientist at Chalmers University of Technology in Gothenburg, Sweden, who develops techniques for electronics manufacturing.
In one demonstration, Thuo’s team put the liquid-metal particles on a thin film of gold, and placed a thin gold wire on top. They rolled a glass rod over the assembly to squish the particles, and within seconds the wire was firmly stuck to the film (see video, Heat-free soldering). The particles could also repair a hole in a thin film of silver, or stick foils of gold and aluminium together. For precision work, the particles can be punctured with the tungsten probe of a scanning electron microscope (see video, Puncturing a liquid metal 'balloon' using a microprobe), or a beam of ions. The work is published in Scientific Reports1.
Thuo’s team made the particles by adding molten Field’s metal to a solution of acetic acid in a common solvent called diethylene glycol, and whizzing it up with a power tool running at 17,000 r.p.m. to break the metal into tiny droplets. (“A heated soup maker also gives really good particles,” notes Thuo.)
The droplets first react with air to form a thin oxide shell that is less than a nanometre thick, and then further react with acetic acid to form a second nanolayer. This adds flexibility so that the shell doesn’t crumble once it cools. Changing the details of the recipe, such as the viscosity of the liquid or the blending speed, can produce particles ranging in size from 4 nanometres to 5 micrometres in diameter. Once the particles have been filtered from solution, they can be stored for months without breaking down. Thuo has recently set up a company called Safi-Tech to commercialize the technology, and hopes to sell the particles to the microelectronics industry.
Liu reckons that the manufacturing method looks scalable, and that the research offers a useful proof of concept. But he adds that the microelectronics industry would certainly want to know more about the reliability of the joint, and whether it has any long-term corrosion issues.
Thuo is now testing other alloys that have higher melting points, which could be used as solders in microelectronic applications that must withstand temperatures above 62 °C (when the Field’s metal solder would melt). His team has already found that a mixture of bismuth and tin, which normally melts at 139 °C, can be encapsulated in liquid form at room temperature just like Field’s metal.
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