New type of solder material eliminates usage of toxic lead

Yale University scientists have developed a magnetic solder that can be manipulated in three dimensions and selectively heated, and offers a more environmentally friendly alternative to today’s lead-based solders. It resembles the liquid metal robot from movie Terminator 2 and you can shape it and make it flow by using a magnetic field. Once the material solidifies it stays fixed.

Solders are low-melting-point metal alloys that act as glue used to bond microchips and other electronic devices, such as transistors and resistors, and can be found in everything from computers to cell phones to MP3 players. Until recently, virtually all solder was made from a tin-lead alloy. But because lead is a toxic substance, there is a lot of interest in trying to find a greener alternative. Recent legislation in Japan and the European Union bans the import of electronics with lead solders.

“We took this as an opportunity to improve solder for the environment, but we also took it as an opportunity to reexamine how to enhance solder in general,” said Ainissa Ramirez, associate professor at the Yale School of Engineering & Applied Science and lead author of the study. Other authors of the paper include Joshua Calabro, Xu Huang and Brian Lewis, all of Yale University.

Until now, scientists had difficulty coming up with a suitable alternative for lead-based solders that are just as strong and have a similarly low melting point. Now Ramirez and her team have developed a non-toxic solder made of tin-silver containing iron particles. Not only is using a tin-silver alloy an environmental advantage, the addition of iron particles has other benefits.

First, the iron makes the alloy much stronger than it would ordinarily be. When an external magnetic field is applied to the molten solder, these particles align themselves within the solder, making it even stronger once it solidifies again.

Second, the iron overcomes the problem of tin-silver having a higher melting point than traditional lead-based solders. By subjecting the solder to an alternating magnetic field, the solder can be selectively heated. This keeps surrounding materials at safe temperatures while melting only the solder itself.

Third, an external magnetic field can be used to remotely manipulate the solder, so it can be moved into hard-to-reach places, such as narrow vertical channels. This means that broken connections within devices can be “self-healed” by applying a magnetic field to melt the solder and attach the ends together.

“There is a whole range of possibilities for this new kind of solder,” Ramirez said. “In addition to helping make the fabrication of microelectronics more environmentally responsible, these new solders have the potential to solve technological challenges.”