Flexible PCB Construction and Process

A flexible circuit is a different product from a rigid board with a thin base. Its materials, its fabrication steps and its design rules all differ, and the reason for using one is usually that the product must fold, must fit a curved space or must survive repeated movement. Understanding the construction is what makes those requirements achievable.

The Base Film

The insulating base of a flexible circuit is a polymer film, most commonly polyimide. It is thin, typically a fraction of the thickness of a rigid laminate, and it remains stable across a wide temperature range.

Polyimide is chosen because it survives the temperatures of soldering without deforming, and because it is mechanically tough at very small thicknesses. The film thickness is a design variable: thinner film bends more easily, and thicker film offers more dimensional stability. Our rigid PCB structure article describes the contrasting choice.

The Copper

Two kinds of copper foil are used. Rolled annealed copper is produced by rolling, which aligns its grain structure and gives it excellent fatigue resistance under repeated bending. Electrodeposited copper is produced by plating onto a drum and is cheaper, but its grain structure is less tolerant of flexing.

Where a circuit will be bent once during assembly, either is adequate. Where it will be flexed many times in service, rolled copper in a thin gauge is the right choice, because the fatigue life depends on the strain in the copper and thin copper reduces that strain. Our copper thickness notes describe the trade-offs on the rigid side.

Coverlay and Adhesive

The copper is covered by a layer of polyimide with an adhesive, called a coverlay, which protects the conductors and defines where the solder pads are exposed. It performs the same role that solder mask performs on a rigid board, but it is a separate film rather than a printed coating.

The adhesive thickness matters mechanically as well as electrically. A thick adhesive layer reduces flexibility, so circuits that must bend repeatedly use thin or adhesiveless constructions. The openings in the coverlay have their own tolerance, and their registration to the pads is one of the design rules that governs how close a component can be placed to the edge of a bend.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1700208791461916-1.webp" alt="coverlay and stiffener detail on a flexible circuit” />

Stiffeners

A flexible circuit cannot support a connector or a heavy component by itself. A stiffener, usually a piece of rigid laminate or a metal plate bonded to the flexible film, provides local rigidity where components are mounted or where a connector must be inserted repeatedly.

The stiffener is placed in the design rather than added later, because it changes the thickness of the assembly. Where the flexible circuit must also match a connector height, the stiffener thickness is chosen with that in mind. Our layout notes describe the placement discipline that applies.

Bend Radius and Design Rules

The bend radius is the single most important design rule on a flexible circuit. The strain in the copper depends on the radius and on the thickness of the copper plus the film, and the required radius is usually expressed as a multiple of the total thickness.

Conductors should run perpendicular to the bend line where possible, because a trace running along the bend sees the highest strain. Where a trace must cross the bend area, it should be as wide as the layout allows, since a wide trace distributes strain better than a narrow one and is less likely to crack. Plated through holes should be kept away from the bend area entirely.

Fabrication Steps

Flexible circuits are made by laminating the copper to the film, imaging and etching the circuit, applying the coverlay with openings for the pads, and then finishing the exposed copper with a surface finish. Stiffeners and connectors are added before or after, depending on the construction.

Because the base is thin and compliant, handling is a larger part of the process than it is on a rigid line. Panels are carried on carriers, and each transition between processes is an opportunity to crease or stretch the material. That handling discipline is why flexible circuits cost more per unit area than rigid boards even when the process steps look similar. Our fabrication article describes the equivalent rigid steps.

Assembly on a Flexible Circuit

Components are assembled onto flexible circuits with the same processes used for rigid boards, with two differences. The first is that the circuit must be supported flat through printing, placement and reflow, which usually means a carrier or a pallet.

The second is that the finished assembly must still flex where it was designed to. Reflow temperatures and the mechanical handling of assembly can affect the film, and the bend areas must not be creased during handling. Our quality control notes describe the checks that catch handling damage before it ships.

When to Choose Flexible Construction

The decision is usually driven by space. Where a product must fit a shape that a rigid board cannot, or where two sub-assemblies must move relative to each other, a flexible circuit removes connectors and cables from the design.

That removal is often the real benefit: fewer connectors means fewer failure points and less assembly labour. Where the product needs neither, a rigid board remains cheaper and easier to assemble, and the flexibility is not worth paying for. Our PCB types article compares the options.

Dynamic and Static Flexing

The requirement to bend falls into two categories, and they are designed differently. A static bend is made once, during assembly, and stays in that position for the life of the product. A dynamic bend is flexed repeatedly during use, sometimes millions of times.

A dynamic application needs thin rolled copper, a bend area free of plated holes, and a radius large enough that the strain stays inside the fatigue limit. It also needs the bend to be constrained mechanically so that the flexing happens where it was designed to rather than at a stress concentration near a stiffener. A circuit designed for a static fold will fail quickly if it is used dynamically, and the difference is a design decision rather than a material property.

Testing and Qualification

A flexible circuit should be tested in the way it will be used. A bend test that folds the sample once says nothing about fatigue life, and a visual inspection of a small radius bend will not reveal the microcracks that appear after a few thousand cycles.

Qualification therefore means cycling the finished assembly through the intended motion and inspecting the conductors afterwards, which usually requires a test coupon built into the same panel. Our electrical test coverage article describes how continuity is verified on the production panels themselves.

FAQ

How tight a bend radius is possible? It depends on the total thickness and on whether the bend is static or dynamic. A dynamic bend needs a much larger radius than a one time fold.

Can a flexible circuit have multiple layers? Yes, and multilayer flexible circuits are common. Layer count reduces flexibility, so the construction is chosen with the bend requirement in mind.

Is a flexible circuit more reliable than a rigid board with a cable? In many products it is, because it removes connectors. The reliability claim depends on the design respecting the bend radius rather than on the material alone.

Why is a stiffener needed? To support components and connectors that a thin film cannot hold. Without it, insertion force would deform the circuit.

Can flexible circuits be reworked? They can, with more care than a rigid board. The film is easily damaged by heat and by mechanical force, so the process is slower and needs better tooling.