Flexible PCB Material: Types, Properties and Selection
Flexible circuits are built from a small number of materials, and almost every decision in a flex design comes down to choosing between them. The substrate determines how hot the circuit can get and how many times it can bend, the copper determines how it behaves when it does bend, and the coverlay determines whether the finished part survives handling and moisture. Treating the flexible PCB material as a single choice rather than three separate ones is the most common reason a flex design performs worse in the field than it did on the drawing.
Substrate Films
Polyimide is the default for any circuit that will be soldered or that must survive repeated bending. It tolerates the temperatures of reflow, retains its mechanical properties across a wide range, and has good dielectric strength even when thin. It is also available in a range of thicknesses, which matters because the total stack thickness sets the minimum bend radius.
Polyester is the cheaper alternative and it is used in very high volume products where the circuit is never soldered and the temperature stays low. It cannot be soldered directly, it degrades at temperatures that polyimide ignores, and its mechanical performance is inferior, but for a membrane switch or a simple sensor tail in a disposable product it is entirely adequate and costs a fraction as much. Between them sit the liquid crystal polymer films, which offer excellent electrical performance at high frequency and very low moisture absorption, at a price that limits them to specialised radio frequency work.
Copper Types and Why They Matter
Two copper types dominate. Electrodeposited copper is grown on a drum and has a columnar grain structure that makes it strong in the plane but brittle when it must stretch. Rolled and annealed copper is rolled to thickness and then annealed, which produces an equiaxed grain structure that deforms far more gracefully. For a static bend the difference is modest; for a circuit that will be flexed thousands of times it is the difference between a product and a field failure.
Copper thickness is the second variable. Thicker copper carries more current and also resists bending more strongly, so a design that needs both current and flexibility is forced to choose: either accept a larger bend radius or split the conductor into several thinner traces. Thinner copper is also more susceptible to handling damage before the coverlay is applied, which is a manufacturing consideration rather than a design one, but it influences the yield and therefore the price. Our flexible circuit materials overview describes how those choices combine in practice.
Adhesive and Adhesiveless Construction
The traditional way to bond a copper foil to a polyimide film is with an acrylic or epoxy adhesive, and the adhesive layer is the weakest part of the stack. It is the first layer to fail in thermal cycling, it absorbs moisture, and it limits the temperature the finished part can withstand because it degrades before the polyimide does.
Adhesiveless constructions deposit the copper directly onto the polyimide or bond it with a very thin tie layer. The result is thinner, which improves flexibility, and it is more resistant to heat and moisture, which improves reliability. The cost is higher and the material is supplied in narrower widths and fewer combinations, so it is normally specified where the flex life or the thermal requirement justifies it rather than as a default.
Dielectric Constant and Electrical Behaviour
For a static or low speed circuit the electrical properties of the film hardly matter, but once the flex carries a high speed signal they matter as much as they would on a rigid board. The dielectric constant of polyimide is around 3.4 and varies with moisture content, which is a real consideration in a humid environment, and the loss is higher than the dedicated high frequency laminates. Adhesive layers have a different dielectric constant from the film, so a stack with thick adhesive is electrically inhomogeneous and the impedance calculation has to account for both materials.
Where impedance must be controlled, the designer specifies the trace width for the actual stack including the adhesive, and the fabricator confirms it with a coupon. Using a rigid board impedance calculation with the film thickness substituted will produce a number that is comfortably wrong.
Coverlay, Stiffeners and Finishes
The coverlay is the protective layer applied over the finished circuit, and it is usually a polyimide film with an adhesive, punched or laser cut to expose the pads. It serves the same purpose as solder mask on a rigid board but it also contributes mechanical strength, and it is the layer that determines how the finished part tolerates handling. Photo-imageable coverlay allows finer openings than a punched film and is used where pad pitch is tight.
Solder mask is sometimes used instead on the areas that will be soldered, because it is thinner and does not need an opening cut for every pad. It provides less mechanical protection, so the usual arrangement is a coverlay over the flexible areas and solder mask over the rigid, populated areas. Stiffeners of FR-4, aluminium or stainless steel complete the assembly where components are mounted or where the circuit must be pressed into a connector.
Selecting Materials for a Real Design
The selection sequence is straightforward if it is done in the right order. First determine whether the circuit must be soldered, which decides between polyimide and polyester. Then determine whether it will be flexed repeatedly, which decides between rolled and annealed copper and electrodeposited copper, and whether an adhesiveless construction is justified. Then determine the electrical requirement, which sets the copper thickness and whether a specialty film is needed at all.
Only after those three questions are answered does it make sense to talk about price, because a circuit that fails in the field costs far more than the difference between material grades. Our design release checklist places those decisions in the review sequence, and our thermal management notes describe how the material choice affects the heat that the board can carry away.
Moisture, Storage and Long Term Behaviour
Polyimide absorbs moisture from the air, and that absorption changes both the dielectric constant and the mechanical properties of the film. A flex circuit that has been stored in a humid room will behave differently during reflow than one that has been dried, which is why a bake before assembly is specified for flex just as it is for moisture sensitive packages. For circuits that will spend their life in a humid environment, the absorptivity of the film and the moisture resistance of the adhesive matter more than any electrical parameter.
Adhesiveless constructions are superior here as well, because they remove the layer that absorbs the most water. Where a standard construction must be used, a coverlay on both sides seals the film and greatly slows the ingress, and the circuit should be designed so that any moisture that does penetrate cannot collect in a region that will be bent.
FAQ
Can flexible circuits use FR-4 as the substrate? Not for the flexible portion. FR-4 is rigid and will crack when bent, which is why it is used only as a stiffener bonded to a genuine flex material.
Is adhesiveless material always better? It is better thermally and mechanically, and thinner for the same dielectric strength. It is not better for cost, and for a circuit that is bent once during assembly and never again, the standard adhesive construction is usually sufficient.
Why does my flex circuit crack at the pads? Pads are rigid islands in a flexible material, so stress concentrates at their edges. Keeping pads out of the bending region and rounding the transition from the trace into the pad removes most of the problem.
