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Abstract:
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. Their findings appear in the March 1 Early Edition of the Proceedings of the National Academy of Sciences.

New Magnetic Solders Are a Leap Towards Green Alternatives

New Haven, CT | Posted on March 2nd, 2010

Solders are low-melting-point metal alloys that act as a glue for bonding 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.

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 again solidifies.

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."

Other authors of the paper include Joshua Calabro, Xu Huang and Brian Lewis, all of Yale University.

This research was funded by the National Science Foundation and the Yale Institute for Nanoscience and Quantum Engineering (YINQE).

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About Yale University
Yale University comprises three major academic components: Yale College (the undergraduate program), the Graduate School of Arts and Sciences, and the professional schools. In addition, Yale encompasses a wide array of centers and programs, libraries, museums, and administrative support offices. Approximately 11,250 students attend Yale.

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