Medical Device Link.

Flex Circuitry

Designing Compact Medical Devices with Flex Circuitry

Sonny Dorren

The vice president of engineering, attending the weekly managers meeting, is getting an earful from the marketing director. The company's brand-new product is too big, too slow, and too heavy. The engineers have to redesign the product, making it smaller, faster, more lightweight. And oh, by the way, those new features they were going to add to the next generation? They have to be incorporated now, before the competition can get a jump. Within four weeks. Smaller, lighter weight, with added capability: does this sound familiar?

The medical device industry faces dynamic packaging challenges these days. In this age of microprocessor control and miniaturization, medical manufacturers confront the dilemma of how to supply products enhanced with more and more features while at the same time reducing the size and weight of product packages. Many manufacturers are looking to flex circuitry as the solution to creating compact medical equipment for use in the ambulance, the operating room, and the up-to-date doctor's office.

Flexible circuits are not a new concept. Flex technology was utilized as early as the 1950s in military and avionic applications where space and weight restrictions have long been standard. But it is within the past 15 years that flex has become commonplace in commercial products. The use of flex as an interconnect alternative has made possible many of the breakthroughs in modern electronics. Products such as notebook computers, cellular phones, antilock brakes, high-speed processors, and automated teller machines all take advantage of flex technology. Flex circuits are also central to many leading-edge medical advancements, including state-of-the-art pacemakers, defibrillators, hearing aids, surgical instruments, and operating-room visualization equipment.

Advantages of Flex Technology

So how does flex circuitry help designers address challenging space, weight, and capability problems? To understand this, compare the nature of flex to that of standard rigid printed circuit boards (PCBs) utilizing wire cabling and connectors.

Rigid flex can be used as layers for additional circuitry.

Space. Flex is able to occupy three dimensions. It can be bent around corners and over itself in order to fit into a much smaller device enclosure. This contrasts with the clutter and bulkiness often seen in rigid interconnects.

Weight. Because flex is significantly thinner and lighter than standard rigid boards, products incorporating it will naturally be lighter and thus more portable.

Double-sided flex provides access to copper on both sides.

Enhanced Capability. Flex lends itself to use in multilayer designs as well as in designs in which the flex circuit serves as an active layer in rigid boards. Such rigid-flex applications can actually remove the need for cabling and connectors; designers thus can use the flex layers for additional circuitry and component placement.

Designing Flex Applications

A product designer considering the development of new applications using flex technology must first recognize and take into account the environments and conditions that define how and where a flex circuit would be used.

Static Application. In a static mechanical environment, the flexible circuit is installed between two or more stationary parts. The circuit can be installed flat, or, in a flex-to-install application, it can be bent or molded into position for installation in the package. Once bent to install, the flex will not be bent again.

Dynamic Application. A dynamic environment is one in which the flex circuit is subjected to numerous bends. The circuit is connected to one or more moving parts, as, for example, in a printer arm that goes back and forth continually.

Double-sided flex consists of prelaminated copper-clad material.

Controlled Impedance. When an application calls for controlled impedance, the base material and adhesives can be adjusted to achieve the specified value. Typical target impedance ranges between 50 and 100 .

Suitable Circumstances for Designing with Flex. The flex alternative can be considered under a variety of conditions, such as when

• Reduction in package size is required.
• Reduction in package weight is sought.
• Shortening of assembly time is sought.
• Lower assembly costs are desired.
• Reduction in assembly scrap is necessary.
• System reliability must be improved.
• More than 25 point-to-point wires are required.
• Dynamic flex of circuitry is a design requirement.
• Lower-inductance cabling is needed.
• Heat dissipation must be maximized.
• Airflow within the system must be improved.
• Three-dimensional packaging schemes are sought.
• A compliant substrate for surface mounting is required.
• Product appearance could benefit from enhancement.

Determining whether flex is a viable design substrate typically involves defining end-product requirements for package size and weight, analyzing the temperature and relative humidity of prospective operating environments, and deciding what the electrical requirements are, specifically, dielectric withstand (hipot), dielectric constant (permittivity), and current-carrying capacity. And, of course, flex circuits are relatively costly, although the price has come down considerably over the years. Flex materials are expensive, and the higher cost of manufacturing with them relates to issues of their handling and processing.

Substrates. There are essentially two types of substrate materials to consider. The first is polyester based. Because it is less expensive, this material is often used in disposable or low-cost applications. It can be employed when no through-hole technology is required and when high electronic reliability and challenging environmental conditions of temperature and humidity are not considerations. The second material is a polyimide-based substrate. This much-higher-priced material has many of the characteristics found in its rigid counterpart. It is most often used when through-hole or multilayer technology is required and high reliability is essential.

A rigid flex construction consists of several layers.

Styles of Flex Circuit Construction. Flexible circuits manufactured today are configured in a variety of popular styles.

Single-sided flex circuits feature one construction layer, with a polyimide coverlay laminated to the copper, and allow access from one side only (Figure 1). The dual-access flex has basically the same construction as a single-sided flex; however, openings in the base polyimide layer and the top coverlay are prerouted, allowing access from the top and the bottom.

The double-sided flex construction consists of prelaminated double-sided copper-clad material with top and bottom coverlays for accessing copper from both sides via plated through-holes and surface-mount technology (Figure 2).

In multilayer flex construction, multiple layers of single- and double-sided material are laminated together. There can be from 3 to 16 layers.

A rigid-flex construction consists of single or multiple layers of flex with single- and double-sided FR-4 or polyimide laminated to the outer layers of flex. Layers can range in number from 1 to 16 in the flex area and up to 20 in the rigid areas (Figure 3).

Obviously, in multilayer and rigid-flex applications, the more layers that are laminated together, the less bendability there will be in the flex areas.

Key Design Guidelines and Options

Although rigid PCB and flex circuit design guidelines show some similarities, several design practices are essential to creating a flex circuit that is manufacturable and guaranteed to exhibit high performance and reliability. The following guidelines should be helpful to the first-time flex designer.

Figure 1. A typical single-sided flex circuit viewed in cross section.

Mechanical Mock-Ups. Employing a paper or Mylar mock-up can be useful in determining adequacy of flex lengths, minimum bend radius, and service loops. Lending physicality to the three-dimensional design, the mock-up can help to pinpoint problem areas early in the design process.

Figure 2. A typical double-sided flex circuit viewed in cross section.

Trace Routing. Flex circuits require curved trace routing to relieve stress along curves and bends. This is especially essential in dynamic applications. Copper usage should be maximized whenever possible, and traces and annular rings should be made large. This will create a more robust circuit, minimizing the chance of copper cracking and enhancing material stability.

Filleted Pads/Tear Stops/Tie-Downs. Radiused corners, with the addition of copper tear stops, are useful to reduce ripping and tearing of the flex circuit.

Crosshatched Shields and Planes. Diagonal crosshatching of shields and ground planes is a technique used to gain flexibility.

Copper Grain and Trace Direction. The copper used in flex circuits is a rolled annealed variety and is more flexible in the direction of the grain. The direction in which the majority of traces run should match that of the copper grain. This can increase the life of the flex.

Coverlay. A coverlay is a single piece of polyimide with adhesive on one side. Openings are predrilled and routed out. The coverlay is laminated to the copper after the etching process. This layer protects the traces, leaving openings for through-hole and surface-mount devices. It serves the same purpose as solder mask on a rigid board.

Coverlay Openings. With single-sided flex circuits, coverlay openings are typically 0.010 in. smaller than the pad size; that is, a 0.060-in. pad would have a 0.050-in.-diam coverlay opening. For double-sided circuits, this opening can be the same as the pad size. The barrel of the plated through-hole helps to keep the pad down: however, it is always a good idea to have a 0.005-in. encroachment whenever possible. Surface-mount devices need a 0.005-in. encroachment of coverlay. For fine-pitch parts (0.005 in. or less), the opening is gang-routed.

Stiffeners. Typical stiffener material is polyimide or FR-4. Stiffeners can be installed at the top or bottom of the circuit to provide specific areas of rigidity as required. An example would be an area where particular components or connectors require enhanced rigidity for assembly or installation. Holes in stiffeners usually are 0.015–0.020 in. larger than the flex circuit hole size. This aids in registration of the stiffeners and allows clearance for soldering and inspection.

These are only guidelines, not "everything a designer needs to know." Someone undertaking a first endeavor in the design of a flex circuit is advised to seek outside assistance with design and application questions. Many design houses are familiar with the idiosyncrasies of flex design. In addition, a reliable, experienced manufacturer of flex circuitry can be an invaluable source of application guidance.

Conclusion

The advantages of flex circuitry grow greater as the size of electronic products offered in the marketplace continues to shrink. Nowhere is this trend more evident than in the medical device industry. Every kind of medical equipment in use today is as large as it is ever going to be. Successor versions will almost surely be smaller. And with today's emphasis on micropackaging likely to intensify, the benefits of designing with flex circuitry become more appealing than ever.

Sonny Dorren is vice president of sales and marketing for Lenthor Engineering (Milpitas, CA).

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