|Production yields for a transcutaneous electrical nerve stimulation (TENS) device designed for pain management were improved by preventing deformation of a relay during the epoxy cure stage.|
Building on its experience with electropressure regeneration therapy devices for the sports and lifestyle markets, EPRT Technologies Inc. (Simi Valley, CA) decided to apply its technology to a transcutaneous electrical nerve stimulation (TENS) device for healthcare applications. Designed for chronic pain management, the noninvasive BodiHealth TENS system is engineered to deliver ultralow currents through the body that block pain signals traveling to the brain and ultimately provide pain relief.
Upon finalizing the design of the TENS device, EPRT transitioned to volume manufacture; however, it was only obtaining 80 to 85% yield. "When the modules were assembled, they would go through a test, they would pass, and then they would go to the epoxy step where they're encapsulated. But after the encapsulation, they were retested and failing the self-test," explains David Estes, director of engineering at EPRT. "With the modules buried in epoxy, it made it difficult to troubleshoot, so deductive reasoning had to be applied."
To address this problem, EPRT approached contract electronics manufacturing service provider Digicom Electronics (Richmond, CA) for assistance. Together, the companies examined the problem and deduced that the failure stemmed from the deformation of a miniature relay during the epoxy cure stage. "When epoxy cures, you have thermal issues and heat is produced," Estes says. "You have to control the process carefully so you don't have too much heat that damages components; you also have shrinkage. We conjectured that this relay was maybe being damaged by the shrinkage and, indeed, our further investigation confirmed that."
Shrinkage of the epoxy as it hardened resulted in a clearance issue for the relay. Consequently, the necessary switching function was delayed, affecting overall device performance, according to Mo Ohady, general manager at Digicom. The engineers found that covering the relay with a small box prevented deformation and improved performance, however.
In addition to the initial yield problem, manufacture of the BodiHealth TENS system proved to be challenging because of the extreme cleanliness and process control required. For example, Digicom had to consider such factors as humidity in order to obtain good yields, Ohady recalls. "The circuit also has to be very clean," he says. "With a very-low-current and high-impedance situation, any contaminants could cause malfunction or device failure." To prevent such a problem, Digicom developed custom cleaning techniques to ensure that even ultralow levels of contaminants that were byproducts of the soldering process were removed.
This process control and validation, along with Digicom's engineering expertise, contributed to the development of a better product, Estes notes. "You have to listen to the customer, understand what the problem is, propose a strategy, work it out, and see the yield. If you're getting results, you're that much closer to the end product. If you're not, you go back to the bench and try something else," Ohady adds. "This isn't a guessing game. These are calculated steps to get the yield to a point that's acceptable and can lend itself to volume production."