Circuits Flex Their Muscles

The demand for more-capable devices in smaller packages has led to the increased use of flexible circuits in medical device applications.

Medical device design, development, and manufacturing are increasingly being influenced by two interrelated technology trends: the use of electronics and miniaturization. The more complex a device is, the more likely that it will incorporate complex electronic systems, and the smaller it becomes, the more likely that the electronics will shrink as well. But how are manufacturers meeting the challenge of squeezing ever-smaller electronics in ever-downsized medical devices? The answer is simple: flex circuits.

“The demand for more-capable devices in smaller packages has led to the increased use of flexible circuits in high-reliability medical device applications,” remarks John Daniel, business manager of the flexible printed circuit division at Plymouth, MA–based Tech-Etch Inc. “In order to support more-complex electronic assemblies and meet the demand for miniaturization, everything from the circuit traces to the surface-mount technology components attached to the circuit have decreased in size.”

Ballad of a Thin Circuit

Flexible circuits, such as those manufactured by Tech-Etch Inc., provide the thinnest and lightest means to create interconnects.

Flex Circuits are typically fabricated on a polyimide film substrate, Daniel comments. While, the traces are most often made of thin copper, other materials can be used for specific applications.

The circuitry is fabricated using either an additive or a subtractive process. In the subtractive process, the copper is already at the required thickness, and the trace geometry is patterned using an etching technique. In contrast, the additive process starts with extremely thin copper measuring from 2 to 4 µm, upon which additional copper is plated and etched to achieve the geometry required by the design. While both methods achieve the same end result, subtractive processing is typically the preferred method for creating more-generic circuit designs with grosser traces and allowances, while additive processing supports fine-line circuits in full 1-oz-thick copper as well as tight-pitch microvia circuits.

Either way, flexible circuits provide the thinnest and lightest means to create interconnects, according to Daniel. By facilitating assembly and eliminating errors, they also contribute to reducing overall system complexity. Another advantage of flexible circuits is their ability to conform to the available geometry. Sometimes referred to as 3-D packaging, this capability allows electronics to be connected on different planes within a finished electronic device. And because flexible circuits support higher-density trace geometries than printed circuit boards (PCBs), they are well suited for use in small electronic devices.

“The extreme thinness of flexible circuits makes them very well suited for dynamic applications in which the flex will be in constant or periodic motion,” Daniel explains. “A typical two-layer flex circuit measures approximately 0.0055 in. thick, considerably thinner than a comparable rigid PCB.”

Making Flex Circuits

Flexes produced by Metrigraphics LLC enable designers and manufacturers to replace some rigid boards to achieve thinner profiles.

A variety of processes are used to manufacture flex circuits, says Jeremy Lug, manager of the new product development group at Wilmington, MA–based Metrigraphics LLC. The first step is to decide whether the circuit will be created on a polyimide film or whether liquid polyimide will be spun on to create a variety of thicknesses. “Film material,” he adds, “is usually used to create a single layer or for fairly simple circuits incorporating a single metal layer, while spin-on material can be tailored to deposit any required thickness or multilayer circuits.”

The circuits are produced on glass carriers. In the case of multilayer circuits, the polyimide is spun onto the glass and cured, after which metal is sputtered on. Next, photoresist is applied, exposed, and developed, forming the shapes required by the application. Metal can then be electroplated onto the circuits into the areas defined by the photoresist. Once the metal plating procedure has been performed, the photoresist is stripped off, and the exposed sputtered metal is removed to ensure the creation of electrically isolated lines. Then, the next polyimide layer is spun on, exposed, and developed, resulting in the formation of openings, where either vias or contact openings are created.

“One of the most common types of devices incorporating flex circuits is the cellphone,” Lug notes. “Cellphones cram all types of components into a small package with little room for play. In such devices, designers have to figure out how to route signals from one location to another without following a straight line. Thus the route that the flex must follow might be twice the distance that the crow flies because it has to route around many different components. This is where flex circuits are advantageous.”

Flexes also enable designers and manufacturers to replace some rigid boards to achieve thinner profiles while still conducting all the metal traces and incorporating all the electronics that are necessary for the application. Because they are flexible, designers can place flex circuits where they want them, unlike rigid boards that are confined to given spaces. “As metal devices shrink, this capability is clearly beneficial for medical device applications,” Lug adds. “Medical devices are becoming smaller and as noninvasive as possible. If you’re putting a circuit into a catheter, you don’t want it to be too big. You want it to fit.”

Many Benefits, Many Challenges

Flexible printed circuit interconnects offered by Molex Inc. are often used in applications in which high signal speed, power distribution, heat, or space savings are issues.

While driving medical device miniaturization, flexible circuits also offer a range of other benefits, including high routing and packaging capabilities, temperature resistance, better thermal management than PCBs, sturdy circuitry, impedance control, minimal signal loss, and high-speed capability. Nevertheless, these undoubted benefits go hand in hand with a host of manufacturing and handling challenges.

“As medical devices miniaturize, the production process must adapt to the needs of tighter flex trace spacing and trace width,” remarks Greg Kuchuris, marketing manager, printed circuit products at Lisle, IL– based Molex Inc. “As the copper traces become thinner and the spacing tighter, manufacturing requires a more precise develop-etch-strip process in order to etch circuits without removing too much copper.”

In addition, there must be well-defined processes for product handling, according to Kuchuris. Since the product can be flexed and circuit traces are extremely small, the traces can be damaged during basic material handling. Certain handling strategies are therefore recommended to reduce the risk of damaging the circuit.

The addition of multiple layers in the flex circuit poses another challenge: diminishing flexibility. To address this issue, manufacturers can incorporate blind and buried vias into the circuit design. Blind vias, Kuchuris notes, are plated holes drilled only partially through the layers, while buried vias are drilled only on the inner layers of the flex circuit. Via holes drilled through the entire flex reduce flexibility, he adds.

“Mobile health is the future of healthcare and further evidence of the convergence between consumer electronics and medical devices,” Kuchuris says. “And consolidating electronics into handheld medical devices is a growing trend. The functionality of bulky medical equipment that used to sit on the side of a patient’s bed or on a medical cart is being reduced to the size of smartphones.” This industrywide shift has increased demand for flex circuits and flexible printed circuit–type connectors. —Bob Michaels


Bob Michaels is senior technical editor at UBM Canon.




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