PSAs and carefully chosen connectors can reduce the shock of intense conditions.

A reader has a flexible circuit application that experiences significant shock and vibration in service. Are there special rules or guidelines to follow for such applications?

Flexible circuits are inherently robust in high shock or vibration environments due to their ultra-low mass. Their flexibility comes from the use of very thin materials, resulting in a lightweight final product. Beyond the natural tolerance for shock and vibration, however, additional features can be incorporated to enhance their performance in demanding applications.

Flexible circuits can endure various levels of shock, ranging from light vibration (less than 100G force), common in the cab of an off-road vehicle, to ultra-high shock (20,000+ G force) experienced in guided munitions. The level of ruggedizing depends on whether the application is on the high side, low side or somewhere in between.

Figure 1. For ultra-high G force applications, even the low mass of flex can be amplified by the hundreds or thousands.

Low G vibration and shock. For lower-level vibration and shock, abrasion typically becomes the primary issue. Large service loops in the flex circuit can flop around and contact other features in the system. To minimize damage from repeated contact, limit the length of service loops to the minimum necessary. If a large service loop is required, consider adding an additional layer of polyimide (up to 0.005") only in the area contacting other objects. Keep in mind that anything added to the flex will increase mass, so strive for a balance between protection and minimizing weight.

Another trick is to add patches of pressure sensitive adhesive (PSA) with a release liner to select areas of the circuit. Once the circuit is installed, remove the release liner to expose the PSA. This can then be used to bond the service loops to the case or other secure surfaces to stabilize the flex. If the design has mechanical stiffeners, choose the thinnest FR-4 that will do the trick. In these cases, consider using a 0.005" polyimide film stiffener instead of the usual FR-4. While providing strain relief at the stiffener-to-flex interfaces may seem unnecessary for static “flex-to-install” applications, it becomes crucial in dynamic situations. Finally, look for and mitigate any stress-concentrating features.

High G vibration and shock. For applications that experience very high G loads, the same principles apply, but they become critical. When experiencing G forces in the thousands, anchor everything. Although flex circuits are very low mass, that mass can be amplified hundreds or thousands of times in ultra-high shock situations. Evaluate materials for opportunities to reduce mass – for example, consider using 0.5oz. copper instead of 1oz.

For soldered components like connectors or larger electrical parts, apply a ruggedizing epoxy over them or use a bead of strain relief epoxy around the perimeter to bond them to the flex circuit.

Avoid situations where solder joints or solder pads bear the full load of ultra-high G forces. Instead, use epoxy encapsulation or strain relief beads to distribute most of the load to the connector or device body, which can better handle the stress than the solder pads or joints. A connector could potentially have more mass than the entire flex; where applicable, use a connector set up for mounting hardware.

Every application is unique. Factors such as the direction of high G load forces, magnitude, frequency of vibration and more can significantly affect the performance of the flex circuit. Combine this with the vast number of potential flex constructions, and it’s clear that predicting the interactions and outcomes can be challenging.

When designing a flex circuit for a hostile environment, it is a good idea to involve the flex supplier early. While the flex circuit may not have seen the exact application, they have probably seen something similar and can help guide you to a successful final product.

Mark Finstad is director of engineering at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Nick Koop (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your suggestions. They will speak at PCB West at the Santa Clara Convention Center in October.