Medical Equipments: Flexible Circuits in Hearing Aids

Estimated reading time: 3 minutes

By Thomas H. Stearns, Brander International Consultants

Hearing aids were one of the first modern medical electronic devices worn on the patient. These externally mounted devices make use of flex and rigid circuits in highly miniaturized assemblies. Starkey Labs and Sonic Innovations weigh in on this report about flexible PCBs enabling medical advances.

At a lower level on the FDA's risk scale than implantable electronics, because failures don't place human life at immediate risk, are externally mounted devices. Of these, perhaps the oldest form of medical FPC-outside-the-skin is the hearing aid. According to John Dzarnoski, Ph.D., director of microelectronic packaging at Starkey Labs, the typical Starkey hearing-aid FPC is a four-layer circuit 0.25 × 0.50" in outline, polyimide insulated and having 0.003" lines and spaces assembled with tin/lead or lead-free solder and protected by a conformal coating. The circuit is SMT-mounted with components then formed into a 3D shape to maximize circuit function in the least possible volume.

The technology of hearing aids has advanced considerably in the past few years. Modern circuitry comprises a flip-chip mounted ASIC with EEPROM, a microphone, controls, a battery, and a receiver, all contained within a housing that may be carried in- or behind-ear. Expected lifetime is five years although changes in ear canal size, the patient's hearing loss, or technology may force earlier replacement. Newer devices incorporate some wireless capability, i.e. communicate directly with cell phones, iPods, or each other.

Because the circuitry is small and passive components down to 0201 are used, assembly requires a level of precision and control not normally found in an assembly job-shop. Starkey runs an in-house assembly operation for this reason. FPC is delivered to the assembly group in small panels, 5" square with the FPC outline pre-cut leaving a few small "tugs" to hold the circuit in-panel.

A flex-rigid panel used in Siemens hearing aids. Photo courtesy of Dyconex Inc.

FPC has been used in hearing aids for about 10 years, according to Darin Hale, R&D engineering manager of Sonic Innovations, another hearing aid manufacturer. Sonic's devices are similar to Starkey Labs in packaging concept and use of FPC; they range from a single ASIC and 45 discretes to a multi-chip with a dozen or more passives. The company has four-layer flex PCB technology as thin as 0.002". Like the Starkey circuits, Sonic also uses conformal coating to protect the components and FPC from the near-body environment of high humidity and acidic fluids. Transducers and receivers can fail in severe environments and hearing aids, because manufactured in small volume, are quite costly. These are designed for easy repair or rework. Sonic's packaging places the folded FPC/component subassembly inside a "chassis" which is in turn inside a molded shell. The chassis contains and protects the folded FPC and components while leaving attachment points for replaceables accessible. Older electronic assembly techniques such as PWBs and ceramic substrates, even point-to-point wire, are used for hearing aids depending on production volume. But when more than a handful of these tiny electronic aids is needed, FPC is necessary. FPC carries a high startup cost with plenty of engineering input, but once designed and tooled it delivers high-density interconnections with smooth, low-cost manufacturing and repeatable performance.

Thomas H. Stearns, president, Brander International Consultants, previously designed and developed connectors and flexible printed circuit products for IBM, Raytheon Company, JPL, GE and Sandia Labs, among others. A noted engineer and inventor in the field of flexible and rigid-flex circuitry, he holds 22 patents and has written many technical articles. Contact him at thomasstearns@comcast.net. You can read more about flex circuits in Stearns' article "Medical Use of Flexible Printed Circuitry" in the March/April 2009 issue of SMT.