“If you can’t stand the heat, get out of the kitchen,” says the old adage. But in a manufacturing process, when temperatures climb higher than certain components can withstand, the solution isn’t as simple as decamping for a cooler room.
Heat poses a puzzle for engineers who are working to replace the lead-based solders widely used in electronics today with more environmentally friendly materials. The problem arises during a manufacturing step called reflow, which sends an electronic assembly through an oven in order to melt the solder paste that holds components to the board. As it cools again, the solder hardens, forming permanent joints.
For many years, engineers have designed electronic components to endure reflow temperatures above 183 degrees C, the melting point of tin-lead solders. But the most promising lead-free solders melt at temperatures at least 20 degrees C higher than that, said Daryl Santos, associate professor of systems science and industrial engineering at Binghamton University’s Watson School of Engineering.
“There’s a concern about what happens with some of these expensive components on the board if they see temperature heights that they weren’t originally designed for,” Santos said. “We want to protect those components,” while still allowing the solder paste at the interface between their leads or connectors and the circuit board to reach melting point.
Cookson Electronics, a manufacturer based in Providence, R.I., has enlisted Santos and his graduate students to help create a device to protect sensitive electronic components during reflow. Alan Rae, former vice president of technology at Cookson, has proposed developing a “thermal cap” to fit over and protect each of the electronic components on a board.
“As the component goes through the reflow oven, the solder joints around the perimeter will still reach those high temperatures,” Santos said. But the thermal cap will shield the component itself from the extreme heat.
In two related projects, with combined funding of $77,250, Santos and his graduate research team are experimenting with different designs and materials that might be used to create thermal caps.
A successful cap would function like the radiator that carries heat away from a car’s engine, Santos said. And like a radiator, a thermal cap might use water to help control temperature, since materials that hold water seem to be good candidates for the caps.
“I’m not talking about a sponge,” he said, but rather a substance that behaves like plaster of Paris, which is mixed with water to make a mold and retains some of that liquid as it hardens. The trapped water “will really absorb a lot of the heat. Or, at least, that’s the idea.”
Santos and his team test the caps by using a series of thermocouples attached to a temperature recorder to gauge the temperature at different points on a board as it passes through the reflow oven. They are also testing caps of different shapes and sizes.
The work, so far, has yielded both successes and failures, Santos said.
“But overall it’s a successful idea that’s going to come to fruition and be marketable for the company.”
Companies must find ways to accommodate new solders so they can continue to compete in key markets, such as Japan and the European Union, that are requiring manufacturers to get the lead out of electronics. The work Santos is conducting for Cookson complements another of his projects that also explores the challenges that lead-free solders present. This one has gained $360,000 in support from the National Science Foundation under its Grant Opportunities for Academic Liaison with Industry (GOALI) program. The grant runs through August 2005.
Santos is collaborating on the project with Eric Cotts, professor of physics and co-director of BU’s materials science program, and Peter Borgesen, a project manager at Universal Instruments in Binghamton. The goal is to identify potential substitutes for traditional tin-lead solders.
Santos and his research team are building test assemblies, following standard manufacturing procedures but using solders that combine tin, silver and copper in different proportions. The materials that compose these solders are known as SAC alloys, for the chemical symbols that represent their component elements-Sn, Ag and Cu.
Cotts and his team subject these assemblies to accelerated aging studies, running them through extremes of heat, cold and humidity to see how they endure the stress.
Borgesen lends the project his company’s expertise and has provided products on which the BU researchers can run experiments.
Results of the tests so far are encouraging, Santos said.
“We’re showing certain types of reliability tests where the lead-free solders are not appreciably different from the lead-based solders,” he said.
“It does look like the SAC alloys are going to be working, and we’re going to be using them as replacements for tin-lead.”