A new type of solder can be melted and shaped in three dimensions under the force of a weak magnetic field. Using a magnet to pull the solder up through narrow holes makes it possible to create electrical connections between stacked silicon chips, for example. These three-dimensional chips pack more computing power in a given area, but making connections between them is expensive, a problem that the new solder might address. The solder also contains no lead, and it is stronger than other lead-free solders.

"It's like the liquid metal robot from Terminator 2: you can shape it and make it flow using a magnetic field," says David Dunand, a professor of materials science and engineering at Northwestern University, who was not involved with the research.

The new solder was developed by researchers led by Ainissa Ramirez, professor of mechanical engineering at Yale University, who was named on Technology Review's TR35 list of young innovators in 2003. The solder gains both its strength and magnetic properties from iron particles suspended in the mixture.

Part of the motivation for developing the solder, says Ramirez, is regulatory. Many countries, including Japan and the members of the European Union (although not the United States), have banned imported electronics that contain lead. However, the best alternatives to tin-lead solder aren't nearly as strong, and they tend to have a much higher melting point. The heat needed to melt the solder can put delicate electronic structures on computer chips at risk. Other research groups have developed composite solders that incorporate oxide or metal particles for additional strength. "We decided to put in magnetic metal particles to not only increase strength but to also give new properties," says Ramirez.

The result is a tin-silver alloy that contains a dispersion of iron particles tens of micrometers in diameter. When a magnetic field is applied to the solders, two things happen. First, the iron particles heat up, locally melting the solder. This localized heating, which works on the same principle as inductive stoves, remains completely contained, keeping the surrounding area cool. And second, the iron particles line up with the direction of the magnetic field, squeezing and pushing the liquid in that direction. This alignment is retained when the solder solidifies, and the well-ordered particles provide mechanical reinforcement that's greater than that afforded by a regular dispersion of particles.

"It's a big deal to be able to move a liquid like this," says Dunand. "You would expect the particles to resurface, not to entrain the liquid with them."

Ramirez believes the solder may provide a better way to make electrical connections between the layers in three-dimensional chips. Today, the interconnects between stacked chips are made by chemically drilling a hole through silicon and coating its sides with copper. The surface tension on the copper encourages solder to climb up through the hole, but the process has its limitations. "They hope that the solder will wick up the copper walls," says Ramirez, but there are many opportunities for failure, and the copper coating process is expensive. In contrast, the magnetic solder can be pulled up through silicon by using a relatively weak magnet. "Our process is very cheap," she says.

Ramirez says she's been in conversation with interested chipmakers, and may commercialize the solder through Adhera Technologies, a startup based in New York.