Soldering is still an incredibly common and useful process for repairing electronics, but it could be about to get a little cooler. Quite literally, because researchers have developed a new way to solder without heat. Researchers from Iowa State University have developed a way to encapsulate liquid droplets of metal in a thin shell so that they remain in a liquid state even when their surroundings are cooler than their melting temperature. Then, when the shells of the spheres are broken, the liquid metal floods out, cools and solidifies. It's possible to bond two pieces of metal together using the resulting mess, effectively providing a means of cold soldering.
The approach uses a process known as undercooling. What the team does is entirely encapsulate the liquid metal in a smooth shell that contains no imperfections known as nucleation sites, which are required for solidification to occur. If you've ever seen an ice crystal form from a single spot, you've seen this in action: A small imperfection provides a low-energy spot at which solidification can start. Without any nucleation sites, liquids can be cooled without setting solid.
The droplets themselves were created by whipping liquid metal, above its melting point, into a froth. Oxygen in the froth causes the very outsides of droplets to form a thin metal oxide shell, further strengthened through a reaction with acetic acid to form a tougher oxide-acetate shell. Gentle cooling allows their cores to stay liquid. Then, the spheres can be spread onto a surface and pressure applied to split them right open. The team has used the techniques to solder a gold wire to a sheet of gold film, as well as repairing holes in silver film and bolding sheets of foil together. The research is published in Scientific Reports.
It might not quite be time to throw out your soldering iron, but the technique could certainly prove useful for repairs where high temperatures may prove problematic.
Image by Iowa State University