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Designing with Flexible Circuits: Focus on Bending

Wed, 02/07/2007 - 11:26am

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With patients demanding medical devices that are convenient, yet discreet, electronics manufacturers are tasked with making internal components as small and compact as possible. This article presents tips for design engineers who want to take advantage of the benefits of using flexible circuits in the manufacture of new medical devices.

By Mark Finstad
The medical device industry is advancing in leaps and bounds. This new wave of medical devices brings increased functionality in a smaller electronic footprint—largely due to the use of flexible circuits.

Flexible circuits can reduce space and weight
by up to 75% and their ability to bend
and flex allows for increased functionality without
jeopardizing reliability.
While flexible circuits have been around for years, demands from medical device manufacturers for smaller and lighter applications are bringing flexible circuits to the forefront as a viable way of meeting these goals.

Flexible circuits can reduce space and weight by up to 75% and their ability to bend and flex allows for increased functionality without jeopardizing reliability. Using flexible materials in the manufacturing of a printed circuit does not in itself guarantee that the circuit will function reliably when bent or flexed. There are many factors that contribute to the reliability of a printed flex circuit and all of these factors must be taken into account during the design process to ensure that the finished circuit will function reliably.

Following are several factors one must take into account when bending flexible circuits.
Bend Ratio=
Bend Radius:Circuit Thickness
The bend radius is typically measured on the inside of the bend.
Measure Bend Radius
The bend radius is the distance from the center of an imaginary circle on which the arc of a formed bend falls to a point on that arc. Laminated stacks will have a single bend radius wherever formed. Some circuits are designed with unbonded layers to improve flexibility at the bend. In that case, there is a secondary bend radius for each unbonded segment. Examine a secondary bend radius for puckering when bent. In most cases, a mechanical sample can verify that puckering is not limiting the bend radius. Look at how tightly the circuit is being bent, and how much distance there is between the rigid areas. A typical bend radius is measured at the actual bend itself, without a secondary bend radius concern.
Concerns with Too Sharp a Bend
The circuit may be "flexible" but the design engineer must still pay careful attention to how much the circuit will be forced to bend in its end application.

Several problems can arise when a circuit is bent sharply. Compression can cause wrinkles in the cover coat on the inside of the bend. Compression can also cause rippled conductors. Cover wrinkles often result in delamination, and rippled conductors can lead to cracks. Stretching can result in tears in the cover material and/or broken conductors on the outside of the bend. If the outer conductor is stretched and a hairline crack is formed, this would be very difficult to detect during a visual inspection and would probably even pass a continuity test. The result would be a defective circuit that could very likely end up installed in the medical device where handling and/or vibration would almost surely cause the conductor to open.

The circuit must be designed to withstand the stretching and compressing without exhibiting any of the aforementioned problems.
Thinner Circuits Are More Tolerant
A thicker circuit is going to be less tolerant to multiple bending operations than a thinner circuit. With a single layer circuit, bend reliability is a function of the bend radius, bend angle, and how many cycles it will be flexed. Typically with multiple layers, the IPC recommends that once it is formed, it isn't moved. It should stay in that configuration and it is a good idea to constrain the circuit after it is formed so any additional handling around the assembly does not exercise that bend at all. For a single or two layer circuit, typically, it can be moved a little bit without having a problem. The exception to this is in the case of an extremely tight bend radius that measures a 3:1 or 4:1 bend ratio (bend ratio = bend radius:circuit thickness).
IPC Guidelines Are Flexible
Some applications allow a safe bend radius lower than the IPC guidelines, but it really depends on the application. The best thing to do in this situation is to contact a manufacturer since every application is different. Single sided circuits that are built properly can go down to about a 3:1 bend ratio without having any problems as long as it is constrained after it is formed. Occasionally, even multi-layer circuits can go down to around a 5:1 bend ratio, provided there are no discontinuities in the bend area. Having everything very uniform will allow the circuit to be bent considerably sharper than the IPC guidelines recommend, without creating any problems. When the IPC guidelines were being written they had to be intentionally conservative because there are so many different variations of flex circuitry. It was important that the guidelines be laid out so they would work for any application.
Flex Circuits Can Be Twisted
It is possible to twist a flexible circuit 45° following the Z axis, but it also depends on the application. The thicker the circuit, the more the twist will have to be elongated. A circuit cannot be twisted from two points only a quarter inch apart—the circuit would be sheered. However, as a general rule, if the twist can be accomplished by a person using only their fingers, it probably will not hurt the circuit. Conversely, if tooling is being utilized where the circuit is being manipulated with the use of a vise or similar device and twisted, that technique can certainly do some damage.

Following these general rules when manipulating flexible circuits can help to ensure a successful product. However, it is always best to consult with an experienced professional to achieve optimal results.
ONLINE
For additional information on the technologies and products discussed in this article, visit Minco at www.minco.com.

Mark Finstad is Principal Applications Engineer for the Flex Circuit Division of Minco. He has 24 years experience in design, process development, and production of flexible printed circuits. Finstad can be reached at 763-586-2826 or mark.finstad@minco.com.
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