Design tricks for tight bends.
Since you are designing flex layers into a circuit, it’s a safe bet that circuit is intended to bend. The key question is, will it survive being bent, twisted or otherwise contorted? When considering flexing, focus on a few key variables: the flex thickness, bend radius, bend angle and number of bend cycles over a lifetime. Some flexes operate in dynamic applications that experience continued flexing. Examples include the hinge in a laptop, a printer head, a drawer-type movement or a rotating mechanism. Other applications involve a limited number of bend cycles, where the circuit is bent into position and may move again during product servicing. Finally, some circuits bend into place and never move, experiencing only minor vibration. Let’s start with the static applications. Generally, if the part is bending less than 60°, it is almost impossible to cause damage. The IPC rule of thumb for static bending is simple. For one or two copper layer designs, the bend radius should be 10X the thickness of the flex. For three or more copper layers bonded together, the rule is 20X the thickness of the flex.
Figure 1. Determine the degree and number of bends during design.
These rules of thumb are intentionally conservative. I like to say that if you follow them, you and your program manager can sleep well at night. In many instances, however, the circuit may need to bend more tightly. Don’t despair; it is possible, but it involves some homework.
For any bend, the concern is that stretching the material beyond plastic deformation will result in a crack. If the degree of bend and the radius is known, the amount of stretch in cross-section of the circuit can be approximated, and determined if it will be excessive. Simply calculate the arc lengths of each layer from inner radius to outer radius of the cross-section. The delta in the lengths is the worst-case elongation.
When the bend is going to be tight, a few design tricks can be employed. First, make the traces as wide as possible. Place the narrowest signal lines either on the neutral bend axis or on the inside radius of the bend. For a two-layer section, put the plane layer on the outside radius and the signal layer on the inside radius; traces hold up better to compression than tension. Keep signal traces in the middle of the circuit, not out near the part edges, where they may see some torsion along with the strain if the part is not bent perfectly. Wide ground traces as guard bands along the two edges are a good option. If it is a bonded flex of three or more layers, make the top and bottom flex layers planes and bury the trace layers. It is virtually impossible to break the signal layer without breaking a plane layer first.
When installing the circuit, ensure the bending process is well-controlled. Use a mandrel of a defined radius to ensure the part is not bent tighter than necessary. Also, a fixture can define the location, radius and degree of the bend. This type of preforming can be quite useful. While the part will likely relax a bit, it will follow the forming tool dimensions at final installation.
Finally, once bent, don’t reopen or flatten it again. That is only asking for trouble.
Now for the dynamic flex situations. The range of motion on dynamic designs varies tremendously. Some move a little up and down, while others have more dramatic bends of 90° or more, and in some cases, +90° and back to -90°. Some designs coil up and wind and unwind. These dynamic flexes are hard to model or predict. In these cases, usually just one or two copper layers are seen. The circuit is made as thin as possible to maximize flexibility and to keep the copper as close to the neutral bend axis. A single copper layer construction with the copper perfectly in the middle is preferred. IPC guidelines suggest bend radii of 88X the thickness of the flex. This is a rough rule of thumb. The concern here is gradual work hardening of the copper. If the bend is in one direction only, the life expectancy is much higher than if it goes back and forth in two directions, which accelerates the work hardening.
Consider how the flex will sit in its “resting” state when installed. Can the degree of bending be reduced by mounting it at an angle? Can you design the part so that it only bends in only one direction?
The bend region will require close attention at design. Maximize conductor width; wider conductors equal more bend cycles. Keep conductors straight as they go through the bend area. Ensure that no stress risers are present in the materials, part profile or artwork. Any changes can be a source of early failure. Make sure the mounting method permits the flex to move freely and no stress or shear force on the part is induced.
A qualification test is recommended for these dynamic designs, either on a part or on a flex coupon, to confirm that the number of bends possible exceeds the expected number of bends in the product lifetime. This is almost always a custom test, as no two designs are the same. Your flex manufacturer will probably suggest a third-party test lab, or you may perform one in-house if you have the facilities.
Don’t be afraid to ask your supplier for their opinion on whether a part can handle a bend. They have seen many examples and can offer advice specific to the design.
Nick Koop is director of flex technology at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee; This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Mark Finstad (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your suggestions. They will speak at PCB West in September.
