A printed circuit board (PCB) made of materials that can be repeatedly recycled could help alleviate the world’s growing electronic-waste problem. The recyclable PCBs, described in a paper published 26 April in the journal Nature Sustainability, perform as well as conventional boards, but unlike conventional PCBs they can be readily repaired. Additionally, the materials used to make recyclable PCBs can be recovered and reused.
According to the United Nations, the world produced 62 million tonnes of electronic waste in 2022, and we’re on track to produce 82 million tonnes in 2030—an increase of over 30 percent. Chemicals can leach out of electronic waste and pollute water supplies; when e-waste is burned to recover valuable materials like gold and copper, it produces air pollution.
Conventional PCBs, which are the substrates that hold computer chips and their interconnections, are at the heart of this environmental problem. They are made up of glass fibers mixed with a type of plastic called a thermoset. These polymers look like a pile of cooked spaghetti. When they’re heated in the process of strengthening the board, all the points where the polymers cross one another become chemically bonded. Those chemical bonds are not reversible, and so conventional PCBs are not recyclable. They also can’t be readily repaired when they crack.
“Having a way to recycle the substrate would be desirable.” —Zhenan Bao, Stanford University
Now researchers have made recyclable PCBs from a different kind of polymer called a vitrimer. Chemical engineers have been exploring these durable, self-healing materials since the 2010s. Like conventional thermoset polymers, vitrimers chemically link up where they cross one another when heated and can be mixed with glass fibers to make durable composite materials. But vitrimers’ cross-links are reversible, and with the right treatment, vitrimers can be fully recovered and reused repeatedly with no damage.
Bichlien Nguyen, a chemist at Microsoft Research in Redmond, Wash.is using machine learning to help design these polymers. She worked with computer scientist Vikram Iyer and mechanical engineer Aniruddh Vashisth, both at the University of Washington, to make vitrimers suitable for recyclable PCBs.
When PCBs made from these vitrimers are treated with the right solvent, the polymers unlink, and the material becomes gel-like. It’s then possible to separate the components layered on the PCB, including chips and other electronic devices, copper, and the glass fibers. The fibers and the vitrimers can then be used to make new PCBs. During recycling, 98 percent of the vitrimer used to make the PCBs can be recovered, as can all of the glass fibers and 91 percent of the solvent.
The vitrimer-based PCBs have electronic and mechanical properties comparable to those of conventional ones, and they are compatible with existing manufacturing processes, says Iyer. The team placed environmental sensors on conventional and recyclable PCBs and found they have comparable performance when monitoring temperature, humidity, and pressure. Iyer says the recyclable substrates should offer a seamless replacement since they are compatible with the etching and laser micromachining processes used on today’s PCBs.
There is currently no infrastructure for recycling PCBs. “This is a new technological capability,” says Iyer, adding that by making the PCB recyclable, it adds value to what has historically been treated as trash, which could encourage circularity in the electronics supply chain.
During recycling, 98 percent of the vitrimer used to make the PCBs can be recovered.
Nguyen imagines that PCB recycling could be adopted by large companies like Apple, which already uses robotic systems to recover components from phones returned by consumers, or by Microsoft, which has systems for recovering some materials used in data center electronics. She notes that about 7 percent of the gold that has ever been mined is trapped in electronic waste in landfills.
Other researchers have previously developed recyclable PCBs that incorporated materials like paper—however, those PCBs are not compatible with existing manufacturing processes. Zhenan Bao, a chemical engineer at Stanford University who develops self-healing and biodegradable electronics, says the vitrimer work is very interesting. “Of course PCBs are already currently commercially used and the substrate accounts to a large percentage of the waste,” she wrote in an email. “Having a way to recycle the substrate would be desirable.”
The team also showed that the vitrimer-based PCBs can be repaired. When conventional PCBs crack, they often need to be thrown away. Vitrimers are self-healing materials. Heating them again readily patches up any holes. Easily repairable PCBs could also open up new ways of thinking about electronics design, making it easier to develop modular laptops and other consumer electronics that can be upgraded, instead of thrown away and replaced, says Iyer.
This story was updated 6 May to correct the amount of e-waste produced in 2022.
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Katherine Bourzac is a freelance journalist based in San Francisco, Calif. She writes about materials science, nanotechnology, energy, computing, and medicine—and about how all these fields overlap. Bourzac is a contributing editor at Technology Review and a contributor at Chemical & Engineering News; her work can also be found in Nature and Scientific American. She serves on the board of the Northern California chapter of the Society of Professional Journalists.
