FPC Copper Thickness: Choosing Ounces for Flex and Current

Copper thickness is the single parameter that decides what a flexible circuit can do. It sets the current the traces can carry, the heat they can move, the tightest radius the part will bend to and the number of flex cycles it survives. Choose it too thin and the design cannot deliver the current; choose it too thick and the circuit becomes a stiff board that cracks at the first fold.

How Flexible Copper Thickness Is Measured

Copper thickness is quoted in ounces, which is the weight of copper per square foot of area rather than a direct measurement. One ounce corresponds to roughly 35 microns or 1.37 mil, so a third of an ounce is 12 microns and half an ounce is 18 microns. Above one ounce the numbers grow quickly: two ounce is about 70 microns and three ounce about 105 microns.

Designers often think in microns and fabricators in ounces, which is where misunderstandings start. A general purpose flexible circuit uses 18 or 35 microns; a high current assembly may use 70 or more. Anything from two ounce upward is normally described as heavy copper, and it behaves more like a thin rigid board than like a conventional flex circuit.

What Copper Thickness Controls

Current capacity rises roughly in proportion to thickness, but the relationship is not the whole story, because a thin trace gets hot and its resistance rises further. Voltage drop across a long flexible tail is usually the practical limit rather than the thermal limit, and the calculation follows the same method used on any conductor, as set out in trace width and current.

Bend performance moves the other way. Thin copper develops less stress on the outside of a bend, so a thinner foil survives a tighter bend radius and more cycles before a fatigue crack forms. For a dynamic application the rule is simple: use the thinnest copper that still carries the current, and never specify thickness for its own sake.

Thickness Bands and What They Suit

One third of an ounce is used for the thinnest assemblies, where the circuit has to disappear inside a display stack or wrap a tight hinge with almost no stiffness. Bend performance is excellent and current capability is limited, so it belongs in low power signal routing rather than in anything that carries supply current to a load.

Half an ounce is the common choice for compact consumer products. It balances a small bend radius against a usable current capacity and, with a rolled foil, gives a long flex life. Wearable bands, camera modules and thin battery leads are typical. It is the default when the assembly has to be thin and the currents are modest.

One ounce is the industry standard. It handles the supply current of most electronic products, holds a moderate bend radius and laminates and etches reliably, which keeps yield high and cost predictable. Where nothing in the design is extreme, this is the value that should be challenged before any other.

Heavy Copper Flexible Circuits

Heavy copper flex starts at two ounces and can go much further, with industrial power assemblies using five, ten or even twenty ounces. The attraction is straightforward: a thick conductor carries a large current in a narrow width and spreads heat along the trace instead of letting it concentrate under a component. Battery management boards, power modules and LED arrays are the classic applications.

The penalties arrive together. Etching thick copper produces more undercut, so the achievable line width and spacing widen and the designer loses routing density. Lamination stresses rise, which shows up as blistering or warpage. Bending becomes difficult, because the neutral axis is far from the copper and the strain on the outer surface is high, and the copper itself is more prone to cracking at a radius it was not designed for.

Matching Thickness to the Application

Static circuits can use whichever thickness the electrical requirement dictates, because the copper is formed once and then left alone. Dynamic circuits have the opposite priority: the flex life dominates, so the thin end of the range is used together with a rolled annealed foil, and the layout keeps the bend line away from vias and plated features.

High frequency circuits add a third constraint. Conductor loss depends on the surface roughness of the foil as well as its cross section, and thickness affects the impedance of a given trace width. Where controlled impedance is required, the thickness and the dielectric height are chosen together, then the geometry is checked against the bend requirement, as described for microstrip and stripline.

Where a single flexible assembly has to serve both purposes, the usual answer is to vary the thickness by layer or by region rather than to compromise. A thin dynamic hinge can coexist with a heavier static power section in the same part, at the cost of a more complex lamination sequence that has to be planned before the layout begins.

Manufacturing Consequences

Heavy copper changes the process. Etch compensation has to be applied to hold the finished line width, because the sides of a thick feature are attacked during the etch and the resulting profile is trapezoidal rather than rectangular. Plating and lamination cycles take longer and the process windows narrow, so fewer fabricators offer the combination of thick copper and fine geometry with good yield.

Heat is also managed differently. A thick copper plane distributes heat laterally and can reduce the need for thermal vias, but it also conducts heat into components that would rather stay cool, such as a connector body or an adhesive layer. Where a large area of copper is used, copper flooding practice determines whether the plane helps or creates a warpage problem.

Specifying Copper Thickness

A workable specification states the thickness in ounces and the foil type, then gives the current per trace, the ambient temperature, the maximum voltage drop and the bend requirement. Those five numbers determine the answer far more reliably than a preference for a particular value, and they let the fabricator flag a conflict before the panel is built.

Where the design is dynamic, the bend radius and expected cycle count belong on the drawing so that the copper thickness can be reviewed against them. A flexible circuit that meets its current requirement and fails after a thousand cycles has been specified on the wrong parameter, and that mistake is usually visible at the specification stage rather than in the field.

FAQ

What is the most common copper thickness for a flexible circuit? One ounce, or about 35 microns, is the industry default. It balances current capacity, flexibility, manufacturability and cost, and a design should have a specific reason before moving away from it.

Is thicker copper always better for current? It carries more current but reduces flex life, raises the minimum bend radius, widens the achievable line width and increases cost. Thickness should be the minimum that satisfies the current and voltage drop requirements.

Which copper thickness suits a dynamic bend? The thinnest value that meets the electrical requirement, usually half an ounce or less, and always with a rolled annealed foil. Static circuits can use whatever thickness the current demands.

Why does heavy copper cost so much more? Because etching, plating and lamination all become slower and less predictable, and yield falls as the copper gets thicker. The material premium is real but the process premium is usually larger.