In flexible circuits, one of the most common usages involves bending the circuit repeatedly. Just as it is in the name, the circuit has to be able to flex, sometimes tens of times, sometimes hundreds or thousands of times over the entire lifespan.
However, this can prove difficult to plan for in a design and will sometimes involve careful planning of the material selection in copper weight and thickness, base material, coverlay, and the occasional selection and usage of optional materials such as strain relief and stiffeners, as well as considering design choices that can affect bending.
Base Material
Material selection is one of the key aspects of flexible circuit design. In this, two choices are commonly presented: adhesive-based and adhesiveless constructions.
- Adhesive-based constructions are how it sounds. Adhesive is used to attach copper to either side of a polyimide core. This will decrease the cost to manufacture the core and make the material cheaper, at the cost of thickening the core and impeding bending.
- Adhesiveless elects to adhere copper to the polyimide core by essentially melting the polyimide core and placing the copper directly on the core. Given the more extensive process, this can cause an increased cost for a trade-off of better flexibility.
Copper
Copper is the next biggest factor in bending, and in some cases, can impact bending even more than the core material if it is thick enough. Copper can be divided into two primary selections: electrodeposited (ED) and rolled annealed (RA). Rolled annealed is typically the best to use for dynamic flexing applications, especially ones with high bend counts, as the copper is treated to align the grain structure in the bend axis.
Processing flexible PCB materials before dry film application and etching.
Another key aspect is considering whether a design requires multiple layers of copper or if they can be reduced. Per IPC-2223, dynamic flexes are recommended to contain only 1 to 2 layers of copper to maximize the lifespan.
Bend Radius and Circuit Layout
Material selection alone does not determine how well a flex circuit will perform in a high-cycle application. The bend radius, overall circuit thickness, and conductor layout also play an important role in reducing mechanical stress during repeated movement. Tighter bend radii place greater strain on the copper and dielectric materials, which can shorten the operating life of the circuit.
A flex circuit demonstrating a bend that is below the minimum bend radius for the design.
In dynamic flex areas, conductors should be routed as straight and consistently as possible through the bend region, while vias, pads, sharp trace transitions, and other stress concentration points should be kept outside the repeated bending area when practical. The selected materials and circuit construction should be evaluated together with the expected bend radius and number of flex cycles to help ensure long-term reliability.
Coverlay
Coverlay is the next biggest item. In order to protect a circuit from exposure, the elements, and selective finish plating, coverlay is utilized across the circuit. This coverlay, similar to the core, is done by adhering polyimide with adhesive to the top of the circuitry. However, the thickness of this cannot be as easily dictated, as with increasing copper weights, the amount of adhesive required will increase. To balance the thickness of a design and increase flexibility, copper plays a vital role in maintaining a thin design for both avoiding additional thickness and avoiding additional coverlay.
While some might see soldermask as a good alternative, given that it conforms and is overall thinner, soldermask is not suited for long-term bending applications. As such, it should be avoided in only component sections as needed.
Extra Options
Many other items may need to be selected, depending on applications and bend locations, such as:
- Strain relief, for when designs may need to bend near rigid or stiffened sections, thus allowing a gradual bend rather than a sharp one that could potentially damage the circuit.
- Stiffeners, which can be used to help denote regions for components to prevent bending and breaking solder joints or to control which areas of a design will bend.
Summary
Flexible circuits can already be very difficult to design. While a rigid circuit board can be difficult, flexible circuits add complexity by requiring a balancing act between flexibility and maintaining reliability, and accomplishing the end goals.
Beyond flexibility, maintaining an extended lifetime of numerous cycles of flexing can be even more difficult. With this, it is always best to consult your manufacturer during the design stages to ensure your design can withstand the application.
Key Takeaways
- Adhesiveless materials offer better flexibility: Adhesiveless constructions reduce overall circuit thickness, making them better suited for repeated bending and high-cycle applications.
- Rolled annealed copper is preferred for dynamic flexing: RA copper is generally more durable under repeated bending than electrodeposited copper, especially in applications requiring a high number of flex cycles.
- Fewer copper layers can improve flex life: Keeping dynamic flex areas to one or two copper layers helps reduce thickness and mechanical stress during repeated bending.
- Coverlay thickness should be carefully considered: Higher copper weights often require more coverlay adhesive, increasing overall thickness and potentially reducing flexibility. Soldermask should generally be avoided in areas exposed to repeated bending.
- Strain relief and stiffeners help control where bending occurs: Properly placed strain relief and stiffeners can prevent sharp bend transitions, protect solder joints, and direct flexing toward areas designed to withstand repeated movement.
