Staying flexible

July 12, 2007
Flexible printed circuits change shape to squeeze in where ordinary boards can't.

Mark Finstad
Applications Engineer
Minco Products Inc.
Minneapolis, Minn.

Smaller electronic footprints are the rule today. Cellular phones and personal entertainment players are perhaps the most visible examples, but the same trend can be seen in medical and aerospace products.

It is not just the electronic components that are becoming miniaturized. Circuit boards, too, are becoming more compact. In particular, printed circuits on flexible substrates provide compact, low-mass packaging that can reduce weight and space by up to 75% over normal rigid printed circuits. While flexible circuits have been around for many years, it's the demand from portable-device manufacturers for smaller and lighter devices that is pushing flexible circuits to the forefront.

However, many engineers lack enough in-depth knowledge to integrate flexible circuits into designs. Fortunately, the basics for designing with flexible circuits are fairly straightforward.

First, design engineers must understand how the various types of flexible circuits work, their design capabilities, applications, and limits. Work with a flexible-circuit manufacturer for guidance on material properties and limitations. You'll get the straight info on what does or, sometimes more importantly, what doesn't work.

The dynamic nature of flex circuits results in a multitude of design options. One method for determining the validity of a design is to physically create a flexcircuit mock-up.

When producing a mock-up, determine the system points that connect via the flex circuitry and the termination methods employed such as connectors, pins, ZIF sockets, and so forth. This lets the designer approximate the circuit footprint that routes the conductor to each termination.

Many circuit boards now use multilayer designs. Flexible circuits are no exception. An approximate layer count can come from a close look at the schematic or net list details along with special electrical needs like plane layers for shielding and isolation. Look at sample circuits with the same number of layers to see if the design is sufficiently flexible. A quick call to a flex-circuit manufacturer should secure samples if none are locally available.

Look at the mechanical requirements to ensure bend radii fall within acceptable limits for the thickness and layer count. The IPC-2223 standard outlines acceptable bend radii for different material types of varying thickness. IPC, formerly the Institute for Printed Circuits, is a global trade organization for the electronic interconnect industries.

Paper-doll patterns help check the fit of the flexible-circuit design. Modifications are easy to make until you get the desired fit. With the paper-doll cutout formed, use 0.01-in. (0.25-mm) polyester film to reconstruct the paper-doll mock-up. Again it's wise to dimensionally adjust this mock-up as needed before committing to an actual circuit design.

Before investing a lot of time and money in creating a functional flex-circuit prototype, test a mechanical sample first to ensure the flex circuit has the right form and fit. Form refers to the physical size, shape, and mass of the part. Fit refers to the environmental interfaces. A mechanical sample helps avoid installation problems or latent mechanical issues that lead to failures.

Primary cost drivers for flexible circuitry are the overall circuit size, the number of layers, and feature size such as line widths and spaces, pad sizes, and hole diameters where smaller typically means higher costs. Some areas may have no adhesive bonding between the insulating layers to improve circuit flexibility. These unbonded areas should only reside where needed.

Most components mount on substrates containing both rigid and flexible areas. The rigid areas hold the components along with conductors and plated-through holes linking different layers. The flexible areas provide connectivity. If there are no surface-mount components, or if they're only on one side, consider using stiffened flex in lieu of rigid flex. Stiffened flex adds rigidity to a flexible circuit for component mounting, but there are no conductors on the stiffeners.

A final caution is that problems arise when a flexible circuit is bent sharply. Compression can form wrinkles in the cover coat on the inside of a bend. Likewise, stretching around the outside of a bend can result in tears in the cover material and broken conductors. It's best to establish the tightest bend radius at the start. A good rule-of-thumb makes the radius 10 × the thickness of the material to assure reliable operation. A caveat here is to not specify material too thin in an effort to create sharper bend radii. Thickness in the flex area makes the circuit more robust to better withstand flexing.

Minco, (763) 571-3121,

Edge connectors, contacts, and components directly connect to printed circuits that contain rigid and flexible areas. Connectors attached directly to the flexible-circuit substrate eliminate wiring harnesses and speed assembly.

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