Flexible PCB Coverlay: Material, Thickness and Openings

The Layer Nobody Discusses Until It Fails

When a flexible circuit is designed, attention goes to the base film, the copper weight and the bend radius. The coverlay, the protective layer laminated over the finished conductors, is usually specified last and discussed least. Yet it is the layer that determines whether the circuit survives its environment: it insulates the traces, protects them from abrasion and moisture, and holds the whole construction together through repeated bending.

A flexible circuit without a properly specified and applied coverlay is vulnerable in ways that do not appear in functional testing. Delamination, moisture ingress and cracks in the conductor all begin at the protection layer, which is why it deserves the same engineering attention as the rest of the stack.

What a Coverlay Is

A coverlay is a two part construction. The first part is a polyimide film, chosen for its high temperature capability, flexibility and dielectric properties. High end materials in this class are used in aerospace and medical products where the operating conditions are demanding. The second part is the adhesive, typically acrylic or epoxy based, which bonds the film to the copper pattern. The adhesive formulation is not a detail: it determines the flexibility of the finished assembly, the maximum temperature it can tolerate and its resistance to chemicals.

Together they provide three functions. Electrical insulation prevents adjacent conductors from shorting and protects exposed copper. Mechanical protection resists the abrasion, bending and vibration that a flexible circuit experiences in a real product. Environmental protection keeps out moisture, dust and chemical contamination, and resists the temperature extremes the product will see. Our notes on flex PCB assembly describe how the protected circuit is then populated and handled.

Coverlay or Solder Mask

The two are often confused because both protect the circuit, but they are different materials with different behaviour.

• Material. A coverlay is a polyimide film with an adhesive layer. A solder mask is a photoimageable liquid polymer that is printed and cured.
• Flexibility. Coverlay is highly flexible, which is the point. Solder mask is comparatively rigid, and it cracks when the circuit is flexed repeatedly.
• Thickness. Coverlay runs from about 25 to 50 micrometres. Solder mask is thinner, around 10 to 20 micrometres.
• Application. Coverlay is applied by thermal lamination under heat and pressure. Solder mask is printed and cured with ultraviolet light.

The rule that follows is straightforward: for a circuit that will bend, especially one that bends repeatedly, a coverlay is the only appropriate choice. Solder mask is used on flexible circuits only in areas that will remain flat, and even then the mismatch in mechanical behaviour between the two materials has to be considered.

Thickness Selection

Three thicknesses cover most designs. A 12.5 micrometre film, equivalent to half a mil, gives maximum flexibility and is used in areas that bend tightly or repeatedly. A 25 micrometre film, one mil, is the standard choice that balances flexibility against protection, and it suits most applications. A 50 micrometre film, two mils, is used where additional protection is needed, typically in static areas and at connector ends where the circuit is handled.

The choice follows the bend radius, the operating environment and the mechanical stress the region will see. A useful pattern is to vary the thickness across the part: thin film in the dynamic bend zones where flexibility matters, thicker film in the static areas and at terminations where durability matters. Because the coverlay adds to the stack thickness, its thickness also affects the bend radius the finished part can achieve, which means the mechanical design and the coverlay specification have to be developed together. The same principle governs the overall construction described in our PCB manufacturing notes.

How It Is Applied

Application is a controlled thermal process with several steps, each of which can go wrong.

• Cleaning. The circuit surface is cleaned thoroughly to remove dust and contamination, because anything trapped under the film becomes a defect.
• Alignment. The polyimide film is positioned precisely over the circuit pattern, since every opening in the film has to land on its pad.
• Lamination. The film is bonded under heat and pressure in a vacuum laminator or hydraulic press. The vacuum matters because trapped air becomes a bubble.
• Post cure. The adhesive is cured to complete crosslinking, which is what produces the final adhesion and the temperature and chemical resistance.
• Opening. Pad openings and windows are cut by laser or by mechanical drilling, either before or after lamination depending on the process.

Temperature, pressure and time are the three parameters that decide the result, and they have to be matched to the adhesive system. A profile that suits one material will not necessarily suit another.

Design Guidance

Three design practices prevent most coverlay problems. Make the openings generous enough that normal registration tolerance does not cause the film to intrude on a pad, since an opening that is too small produces a partial pad and an unreliable joint. Match the thickness to the function of each region, keeping dynamic bends thin and static areas better protected. And avoid stress concentration by using smooth transitions in the conductor routing, because a sharp corner under a coverlay is where a crack begins. Openings should also be kept away from the bend axis where possible, since an opening reduces the local stiffness and creates a stress riser.

Defects and How to Prevent Them

Three defects account for most coverlay failures. Wrinkles and bubbles come from insufficient lamination pressure or from contamination trapped under the film. Delamination results from an adhesive that is incompatible with the process or the environment, or from surface contamination that weakens the bond. And misregistered openings, where the film does not align correctly with the copper pattern, produce partial or obstructed pads.

Prevention is procedural rather than corrective: operate in a clean environment, use materials that are compatible with each other and with the intended service conditions, and use lamination and cutting equipment accurate enough for the feature sizes involved. These controls are part of the same quality system that governs any flexible circuit programme, as described in our quality management notes.

Cost

As a 2026 reference, coverlay material and application costs fall between about 0.10 and 0.50 US dollars per square inch, depending on the material grade and the design complexity. Three factors move the figure. The material cost, since high performance polyimide films and their adhesives are more expensive than standard grades. The production complexity, because more openings and tighter tolerances mean more processing. And the order quantity, since volume amortises the setup. Our overview of PCB capabilities sets out how the coverlay specification fits the wider flex construction.

Choosing a Manufacturer

Four capabilities distinguish a supplier who can deliver a reliable coverlay. Experience with flexible circuits, because the process is materially different from rigid board work. Material traceability and the certifications the target market requires, so the film and adhesive can be accounted for. An inspection system capable of catching the defects that matter, including the ones hidden under the film. And responsive engineering support, because a coverlay problem discovered after assembly is far more expensive than one caught at lamination.

Frequently Asked Questions

What is a coverlay made of? A polyimide film with an acrylic or epoxy adhesive layer, laminated over the circuit under heat and pressure.

What is the difference between coverlay and solder mask? Coverlay is a flexible film applied by lamination; solder mask is a rigid printed polymer cured with ultraviolet light. For circuits that bend, coverlay is required.

Which thickness should be used? 12.5 micrometres for tight dynamic bends, 25 for general use, and 50 for static areas and connector ends that need more protection.

Why do bubbles form? Usually from insufficient lamination pressure or contamination trapped under the film, which is why vacuum lamination and clean handling matter.

Does the coverlay affect the bend radius? Yes. It adds to the stack thickness, so it changes the achievable bend radius and has to be considered in the mechanical design.

Conclusion

The coverlay is the part of a flexible circuit that makes the difference between a design that works on the bench and a product that survives its environment. Choose a film and adhesive combination suited to the temperature and chemical exposure, select the thickness by bend radius and mechanical duty, laminate it under proper vacuum and pressure control, and design the openings large enough to be manufacturable. It is a layer that adds cost and receives little attention, and it is the one that decides how long a flexible circuit lasts.