Flexible circuitry is intended for applications that require a circuit to bend, flex, or conform to an application's usually limited volume. Manufacturing a printed circuit using flexible materials does not guarantee the circuit will function reliably when bent or flexed. Many factors contribute to their reliability and all of them must be taken into account when designing a product.
Bending or flexing a circuit stretches the outside layers of the bend and compresses the inside layers. The tighter the bend or flex, the more concentrated these forces. The centermost layer in a uniform construction forms a neutral bend axis in which the material is not appreciably stretched or compressed.
Several problems arise when sharply bending a circuit. Compression can cause wrinkles in the cover coat on the inside of the bend. Compression can also ripple conductors. Cover wrinkles often result in delamination, and rippled conductors can lead to cracks.
Stretching can tear the cover material, or break conductors on the outside of the bend, or both. Stretching an outer conductor may form a hairline crack that can be difficult to detect during in a visual inspection. It might also pass a continuity test. But the result would still be a defective circuit that could likely end up in a finished assembly where handling, or vibration, or both would almost surely open the conductor and cause the product to fail.
Flexible circuits must withstand stretching and compressing without exhibiting the aforementioned problems. Here are a few of the most commonly asked questions about bending flexible circuits.
Where is the bend radius measured at the neutral axis? And what is bend ratio?
The bend radius is typically measured on the inside of the bend. Bend ratio is a measure of the bend radius to circuit thickness, or
Rbend = r/t
where Rbend = bend ratio; r = inside bend radius; and t = circuit thickness.
How many times can a circuit be bent without cracking?
Assuming first of all that this is a “flex-to-install” or non-dynamic use, the answer depends a lot on the particular application. It should be obvious that a thicker circuit will be less tolerant to multiple bends than a thinner circuit. With a single layer circuit, the number of bends to break is a function of the bend radius, bend angle, and the number of cycles it will be flexed. Single and two-layer circuits can be flexed up to 50 times without a problem unless it is an extremely tight bend radius (3:1 or 4:1 bend ratio) or it is being flexed more than 90°. Multilayer circuits should not be moved after forming the bend. The circuit should stay in that configuration. It's also beneficial to constrain a multi-layer circuit after it is formed so additional handling around the assembly does not exercise the bend.
Can some flex circuit applications allow a safe bend radius lower than IPC guidelines?
Yes. The best thing to do in this situation is to contact a flex-circuit manufacturer because every application is different. Properly built, single-sided circuits can go down to about a 3:1 bend ratio without problems as long as it is constrained after it is formed. Many multi-layer circuits can even bend to about a 5:1 bend ratio for a single bend provided there are no discontinuities in the bend area. Having everything uniform will let the circuit bend considerably sharper than the IPC guidelines (from the former Institute of Interconnecting and Packaging Electronic Circuits) recommend without problems. The IPC had to be conservative when writing the guidelines because of so many different variations of flex circuitry. It was important the guidelines work for any application.
How close to the bend area can one place a through-via? Also, if a design includes a line of vias, should they be staggered across the width?
Vias not in a bend area can be placed anywhere. Lining up vias causes problems when trying to form a bend close to the vias because they generate discontinuities and may actually pull the bend to them and away from the required bend location. A general recommendation: keep vias 100 to 150 mils (0.1 to 0.15 in.) from a bend end.
In a rigid-flex circuit, how close can a rigid-PCB portion be to the beginning of the bend?
In a rigid-flex circuit, it is beneficial to put some kind of strain relief along the rigid-flex interface. A strain relief, usually a bead of epoxy, will consume about 100 mils (0.10 in.) between the bead and the manufacturer's application tolerance. Don't place a bend immediately adjacent to the strain relief. Add at least another 100 mils (0.1 in.) In total, a good reference would be about 200 mils minimum from the rigid/flex interface.
Can flex-circuit manufacturers pre-form a flex bend to alleviate issues?
Forming is routinely done for OEMs that do not feel comfortable doing it themselves, so yes, preforming can be done by the flex-circuit manufacturer.
Manufacturers also provide forming tools that let customers do the form themselves but with tools made just for the circuits they are forming.
Of course, the application determines the best forming method. For instance, if the circuit is larger and the finished-shape volume ends up being close to a cubic foot, preforming at the manufacturer may not be an option. Preforming a circuit depends on its size before and after it is formed. An option for smaller circuits would becustom trays that constrain the preformed circuit during shipping and storage.
Does preforming a circuit to a minimum-bend radii apply to flex cycling?
No, especially when referring to a dynamic-flex application that exercises the circuit on a regular basis. The bend ratio for a dynamic application would have to be much larger than what is described above. If the circuit is to be routinely bent or flexed, it needs a greater bend radius.
There are different levels of flexing. Static, for example, is a circuit bent once and not moved. A semi-static circuit can be exercised a few cycles, less than 20. And a dynamic circuit is moved on a regular basis, thousands or millions of cycles. In dynamic applications, in addition to a generous bend radius, the overall stack of materials must be extremely balanced. The conductor layer must be perfectly centered and must have exactly the same type and material thickness top and bottom so the neutral bend axis falls at the center of the copper conductors. What's more, the circuit should be limited to a single layer.
If a flex circuit with more than one conductive layer is used in a dynamic application, the copper will begin work hardening after much flexing and its conductors will break. There are a few applications where a two layer circuit may perform reliably. Such a circuit calls for the manufacturer's involvement in the design stage to ensure all parameters are within the operating window.
Can a flex circuit also be twisted 45° following the Z axis?
Yes, flex circuits can be twisted, but it depends on the application. The thicker the circuit, the more elongated the twist must be. It would not be possible to grab the circuit in two places a quarter inch apart and twist because that would sheer the circuit. Thicker circuits need longer areas to form the twist without circuit damage. A good reference is that if the circuit can be flexed by hand, it will probably not hurt the circuit. But expect to damage the circuit if you twist it with a tool, such as a vise grip or pliers.
If stitching vias are pulled out of bend areas, will EMI problems arise there?
There is little advantage to stitching vias because flexible dielectrics are so thin. Typically, if there are stitching vias around the perimeter, there must also be a guard trace running around the perimeter. The purpose of a guard trace is to push or relocate noisy or quiet conductors inboard enough so no EMI goes in or out regardless of whether there are stitching vias. Placing stitching vias inboard is a small cost adder. However, we recommend removing stitching vias from the bend area because they represent a discontinuity which could cause problems, such as cover coat cracks.
Where is the bend radius measured for an unbounded flex circuit?
The bend radius is measured at the center of the bend. A secondary bend radius might also be measured where the circuit puckers. Puckering should not drop the bend radius below an acceptable ratio and become a limiter. The distance between bonded regions and the bend angle will determine the amount of puckering in the circuit. To determine a bend ratio with puckering, measure the bend radius at the bend, and measure the associated puckering to determine if it causes a secondary bend-radius concern.