Engineers at OEMs work hard building products with conflicting design requirements. Take electronic devices, for instance. Users expect them to be smaller and lighter, yet still pack advanced capabilities and features. They also want the innovation pace to pick up and expect no compromise on reliability. Some designs, such as implantables, may have to operate for years on a battery. That drives the need to minimize power consumption and maximize space for the power source. And sometimes upgraded devices must be manufactured at a lower cost than the last ones. No wonder designers get headaches.

Fortunately, they can find some relief with flexible or bendable circuits. They make the most of shrinking real estate. Flex technology also solves many critical challenges of the medical-device market. Flex circuits can already be found in cardiac rhythm management, neurostimulators, infusion pumps, hearing aids, and hearing implants.

Flex circuits handle the increasing complexity and capability expected in modern electronics. Single and double-sided flex circuits make reliable connections between electro-mechanical devices, and thin dielectrics provide good electrical isolation, low mass, and a flexible package. Medical-battery packs, for example, use flex circuits that also serve as space-saving interconnects.

A variation on flex circuits — the rigid flex — can combine mother and daughter circuits with various electro-mechanical devices, and without using any connectors. Eliminating connectors cuts cost, saves space, and simplifies assembly. Joining boards and devices this way also increases reliability. Many devices use two rigid circuit boards with components on the front and back, then folded back upon themselves to increase the vertical component density. Small connecting arms often emerge from circuit edges and connect to power sources, communication coils, and external headers. Hearing, neuro, and cardiac devices take advantage of rigid-flex circuits.

The compliant nature of flex-circuit substrates works to a designer's advantage when placing small surface mount components on high-density-interconnect circuits. Compared to FR4 (a fiberglass board), flex circuits reduce solder-joint stress, thus improving thermal cycling. Flex circuits also withstand global shock and vibration better than FR4.

A few guidelines

Flex circuits are good choices for many medical devices, but they have limits. Here are a few dos and don'ts for putting them in your next design.

Get the vendor involved

Medical-device manufacturers are often fiercely protective of their intellectual property. Even with nondisclosure agreements, they may work independently in early design phases. In fact, a package's mechanical layout is often complete before a circuit manufacturer gets involved. That leads to problems. For example, we've been asked to shave circuit thickness by a mil or two to make it easier to laser weld a package only because the OEM crammed a lot into a small space without asking for input.

Flex manufacturers are glad to pass on their experience to designers in the form of good engineering practices, manufacturer skills, limits of materials, even low-volume production tips. This latter item suggests more capability (higher power or smaller vias) can be built into circuits if OEMs accept higher manufacturing costs. We recommend brainstorming ideas in conceptual phases and then regular exchanges with OEMs on feature sizes, capabilities, materials, and electrical performance.

Bending and forming

Most circuits in medical applications are categorized as “bend to install” — which means bend at assembly and only straighten to disassemble for rework. The IPC (a standards body for printed-circuit makers) provides guidelines for minimum bend radii based on circuit thickness. However, circuits optimized for bending can tolerate tighter radii than garden-variety flex circuits. A few tips can improve circuit's bending reliability.

• Rolled, annealed (RA) copper is more ductile than electrodeposited (ED) copper for forming, particularly when the grain of the copper is parallel to the tightest bend.

• Minimize circuit thickness. A double-sided circuit can be reduced to a single layer in the bend area by etching the copper from one side of an adhesive-free substrate and applying the cover to the opposite side in the bend area — a cutback cover. Similarly, a four-layer circuit can have only two layers in the bend area using cutback covers.

• Eliminate conductor plating at or near the bend. Plating finish is not as ductile as RA copper. Reserving external layers for pads eliminates plating on conductors. Alternatively, plating can be applied only to pad areas by masking the rest of the circuit during plating.

Bends over 90° place the greatest stress on formed areas. The outside radius stretches severely while the inside radius compresses and wrinkles. Straightening circuits formed this way induces further stress by compressing already stretched copper and pulling at wrinkled copper and cover materials. Either action may tear the copper or the cover, propagating the tear and producing an intermittent open. If a circuit is over-formed, past 90°, hold it to that shape during all following processes.

Bring all bend areas to the circuit maker's attention, usually by identifying bend lines on the drawing. Problems arise without proper communications. For example, after a manufacturer reported puzzling failures in flex-circuit prototypes, analysis detected cracks in conductors plated with electrolytic nickel and gold where they were sharply deformed. The problem could have been avoided, but the original drawings showed no bend area, and none was communicated by other methods.

Etching aspect ratio

Ever-smaller packages mean higher trace densities even though many applications need low-resistance traces. To maintain the same cross-sectional area of copper and accommodate these two conflicting requirements, designers are specifying thicker copper with narrower traces. Manufacturers of flex circuits use RA copper for its ductility and adaptability to etching traces. Chemical etching removes copper from traces, but it also works laterally, to some degree. Consequently, lateral etching gives conductors trapezoidal profiles, with tops slightly smaller than bottoms. The effect grows more pronounced with thicker copper or narrower traces. The etching aspect ratio helps define the condition. Find the etching aspect ratio with:

R_etching = w/t

where R_etching = etching aspect ratio, w = trace width, in., and t = trace thickness, in. Lowering the etching-aspect ratio diminishes control of the cross section. Small ratios also mean less copper is bonded to the circuit. Copper resists forming, and etching circuits with small aspect ratios tends to separate conductors from substrates when bent. Some designers incorrectly specify circuits with uncovered fingers using 0.003-in. wide conductors and 0.003-in. spacing. This conductor will likely break off the substrate when formed.

Generally, 5:1 is the preferred etching aspect ratio. This means a copper conductor of 0.0028-in. thick (also called 2 oz. copper) should be designed for a minimum etch width of (0.0028 in.×5 =) 0.014 in. for best reliability. Smaller ratios are acceptable with thinner copper. Designers get good performance at 4:1 ratio for 0.0014-in. thick copper and even at 3:1 for 0.0007-in. thick copper.

Hole aspect ratio

Designers keep demanding smaller holes that push the limits of circuit making. Small-diameter holes present two challenges: Getting the holes into the circuit economically, and making layer interconnections through small holes. The aspect ratio for holes is:

R_hole = d/D

where R_hole = hole aspect ratio, d = hole depth, in., and D = hole diameter, in. A thicker circuit or smaller hole both raise the aspect ratio. Most flex circuit makers prefer to use mechanical drills to make holes. The practical limit for mechanically drilled holes is on the order of 0.006 to 0.008 in., depending on circuit thickness. For smaller holes, manufacturers use micro-via laser systems. These can form holes down to about 0.001-in. diameter. Lasers have problems with beam focus through thicker circuits.

Layer interconnections are added after forming the traces. Interconnections usually rely on copper plating to create conductive paths. The challenge of high-aspect ratio holes is ensuring that fresh plating chemistry continually circulates in the hole for consistent copper deposition. If fresh solution doesn't circulate, copper ends up near the hole entry point, leaving the center of the hole with voids or thin copper prone to fracturing during thermal cycles. Either condition results in electrical failure. Check with flex-circuit makers for hole limits.

Tolerances, CAD, and other rules

There are other manufacturing issues. Tolerances, for one. Flex circuits can't be toleranced like rigid circuits. Dimensions can be held tightly in clusters (small areas) but tolerances across flexible regions should be looser. Within clusters, tolerances can be held to ±0.001 in. Because the substrate is plastic, flex circuits stretch and shrink. So larger clusters must have looser tolerances, about 1 to 2 mils/in. of cluster distance. Laser imaging provides scaling that lets designers optimize each circuit image across the panel.

Design software is another consideration. Most designers use 3D modeling software based on sheet-metal-fabrication techniques. It should be no surprise that flex circuits behave differently than sheet metal. For test fits, we recommend generating outlines on Mylar sheets to simulate flex characteristics. Designers can get physical mockups using actual circuit stack-up materials to verify fit before ordering prototypes. The circuit maker can provide this service to save the expense of prototypes, when dimensional adjustments may be needed.

Lastly, software that checks design rules usually finds a number of violations. This checking is not an industry standard but proprietary software run during data check-in. It is set to find violations that would prevent manufacturers from meeting design requirements for finished parts.