Typically, when stiffeners and adhesives are thought of, they are only considered for flexible PCBs rather than rigid-flex PCBs. In reality, stiffeners can be applied to rigid-flex designs just as they are in flexible circuits.
Adhesives can be used beyond rigid-flex circuit board designs in both the flexible and rigid portions of the rigid-flex PCB stack-up. When properly applied, these additions widen the range of mechanical, thermal, and assembly solutions that a rigid-flex design can achieve.
Where Stiffeners Fit in Rigid-Flex PCBs
Many engineers overlook the idea that stiffeners can be used in rigid-flex PCBs. The assumption is that rigid sections already supply enough support, so additional stiffening should not be necessary. However, real-world assemblies often require localized thickness, improved mechanical stability, or reinforcement in areas that the rigid layers do not or cannot naturally cover.
A stiffener is often added to a flexible tail or a component area when the region must mimic the rigidity of a thicker circuit board but cannot match the full stack-up thickness of the rigid section. This is common when the design requires controlled thickness for a mating connector or a well-defined mechanical interface. In these cases, a polyimide or FR4 stiffener provides the needed stability without altering the entire rigid-flex construction.
Front side view of rigid-flex PCB with stiffeners.
Stiffeners also play an important role when Zero Insertion Force (ZIF) connectors are used on the flex portion. ZIF connectors require a very specific and consistent thickness for the flex tail to mate properly with the connector housing. Polyimide stiffeners are commonly used to create this exact thickness and ensure that the ZIF termination remains reliable throughout the product life cycle.
Beyond standard FR4 or polyimide stiffeners, there are special-purpose stiffeners designed for more demanding applications. Aluminum stiffeners can be introduced when localized heat dissipation is needed, especially near high-power components that transfer heat into the flexible area. Stainless steel is another option when a design requires significant mechanical rigidity in a compact footprint. Stainless steel stiffeners can meet rigidity requirements and improve durability in designs that experience pulling or repeated handling.
There are, however, certain design situations where stiffeners may not be recommended due to structural complexity, manufacturing constraints, or concerns about stress concentration at rigid-to-flex interfaces. For these cases, it is best to consult with Epec early in the design phase so that the stack-up and mechanical layout can be optimized before final drawings are released.
Adhesives in Rigid-Flex Applications
Similar to stiffeners, adhesives are often associated exclusively with traditional flex circuits. Yet adhesives are commonly used in rigid-flex PCBs as well. They can appear within the flexible dielectric layers, between sub-laminations, or as bonding materials for final assembly.
In flexible regions, adhesives are used to bond copper to polyimide films, secure coverlays, or attach stiffeners. These adhesives are typically acrylic or epoxy-based, selected for their flexibility, temperature resistance, and compatibility with the chosen materials. The performance of the adhesive is critical because the bond must endure bending, flex fatigue, vibration, and thermal expansion without cracking or lifting.
Back side view of rigid-flex PCB with stiffeners and adhesives.
Adhesives can also be used in the rigid sections of a rigid-flex PCB design. Specialized adhesive systems allow rigid sections to be bonded into housings, enclosures, frames, or mechanical interfaces when fasteners are not viable. For example, a rigid-flex circuit board assembly may need to be adhered to a plastic enclosure wall to prevent rattling, enhance structural stability, or create a sealed interface. In these cases, epoxy pads or specially formulated adhesives can be applied depending on the mechanical or thermal demands of the application.
In some instances, assembly processing may require adhesives that can withstand solder reflow or other high-temperature operations. High-performance adhesive films are available for these environments and can maintain bond integrity even when exposed to lead-free solder temperatures or elevated thermal profiles typical of dense assemblies.
The versatility of adhesives expands the use cases for rigid-flex PCBs far beyond simple interconnects. With the right adhesive selection, rigid-flex designs can be included in curved housings, sealed assemblies, shock-resistant enclosures, or areas where fasteners or hardware cannot be used due to space limitations.
Selecting the Correct Materials and Approaches
Choosing the right combination of stiffeners and adhesives depends on both mechanical and electrical design constraints. Engineers must consider bend radius, layer transitions, thermal requirements, and assembly stresses. For example, a design that must withstand repeated bending will need careful placements of bends to not interfering with adhesive areas. Likewise, placing a thick FR4 stiffener too close to a bend can cause cracking or damage during assembly.
Material compatibility is essential both during fabrication and in the final application. Stiffeners must bond cleanly to the flex layers, and adhesives must not interfere with soldering, impedance, or lamination steps. The adhesive thickness must also be considered because it adds to the total stack-up and bend radius.
Working closely with a rigid-flex manufacturer like Epec ensures that the material choices are appropriate for the required reliability level. Manufacturers can also identify cost or labor-saving options, such as replacing a metal stiffener with a thinner FR4 stiffener or reducing adhesive coverage if it is not needed across the entire region.
Summary
Many do not consider stiffeners and adhesives in rigid-flex PCBs. With the addition of both, many use cases of rigid-flex PCBs can be expanded, with some uses being added, like the ability to have heat dissipation or to adhere a PCB to an assembly using an adhesive.
In many cases, the best option is to discuss with the manufacturer and get advice on options, since designing a rigid-flex PCB can be trickier than both rigid and flex designs.
Key Takeaways
- Stiffeners can be used in rigid-flex designs to create localized rigidity, support connectors, or add thickness only where it is needed.
- Adhesives play a role in both flexible and rigid regions and can support bonding, lamination, thermal management, or final assembly attachment.
- Materials such as polyimide, FR4, aluminum, and stainless steel each offer different benefits depending on mechanical, thermal, or assembly requirements.
- Proper placement of stiffeners and adhesives is critical to avoid issues at bend areas and to ensure the flex portions of the design perform as intended.
- Early collaboration with a rigid flex manufacturer helps ensure that materials, thicknesses, and mechanical strategies align with the overall reliability goals of the project.
