FPC Copper Thickness And Line Width: Working Limits

On a flexible printed circuit, or FPC, the copper is doing three jobs at once. It carries the current, it provides the mechanical strength that lets the part be handled and bent, and it has to survive repeated flexing without cracking. Copper thickness and line width are the two variables that set all three, and they are also the two that a designer is most likely to specify without checking what the fabricator can actually build.

This article looks at how the two parameters interact on a flexible printed circuit, what the working limits are, and how the choice differs between a static installation and a part that flexes in use.

The flexible circuit industry has its own conventions, and a design that ignores them will either be built at a premium or returned for changes.

How Thickness Is Specified

Copper thickness on a flexible circuit is normally quoted in ounces, in the same way as on a rigid board, but the thicknesses used are generally lighter. A third of an ounce, roughly twelve micrometres, is common for fine line work, while one ounce or two ounces appears where current or stiffness is needed. The finished thickness is not simply the starting foil, because plating adds copper on the traces and the surface finish adds a further small amount.

The type of copper matters as well. Rolled annealed foil has a grain structure that is elongated in the plane of the sheet, which makes it far more tolerant of bending than electrodeposited foil, whose grains are columnar. Where the circuit will be flexed repeatedly, rolled copper is the usual choice, and the additional cost is justified by the difference in flex life.

Line Width, Spacing And What Sets Them

The minimum line width and spacing that can be produced depends on the copper thickness, because thick copper etches differently. A thin foil can be patterned with fine lines and tight gaps, while a heavy foil needs wider lines and larger spaces to etch reliably, since the etchant has to remove more material and the sidewalls become more sloped in the process. This is the trade off that drives most of the design discussion on a flexible circuit.

The spacing rule exists for two reasons. Electrically, closely spaced conductors are easier to short during handling and are more susceptible to the growth of conductive anodic filaments in a humid environment. Mechanically, a narrow gap between thick conductors is a place where the adhesive and the coverlay must still be applied cleanly. Both push the minimum spacing wider as the copper gets heavier, and a fabricator’s capability table is the place to confirm the numbers rather than a rule of thumb.

Current Capacity And Heat

A trace carries current in proportion to its cross sectional area, which is the product of the width and the copper thickness. Doubling the thickness has the same effect on current capacity as doubling the width, which is why a heavy copper layer is often used where a wide trace will not fit. The thermal situation on a flexible circuit is less favourable than on a rigid board, because the thin dielectric and the absence of a large plane mean heat has less material to spread into, and the temperature rise for a given current is correspondingly higher.

In practice the copper is also part of the mechanical design. Adding copper in the areas that will not bend stiffens the circuit where stiffness is wanted, and leaving it out of the bend region keeps the flex life high. Some flexible designs use a copper pour for that reason as much as for electrical performance.

Where The Lines Are Drawn

Some guidance applies across most flexible designs. A line in an open area, with no neighbouring copper, should be wider than the same line inside a dense region, because the etching is less controlled and the local heat has nowhere to spread. Coil patterns, which are effectively long spiral conductors, are usually specified with generous width and spacing for the same reason. Clearance from the conductor to the board edge, and the space between adjacent pads, are also larger than on a rigid board, since the flexible material is cut rather than routed.

For a circuit that will be bent in service, the copper should be kept thin in the bend region and the traces should run perpendicular to the bend line rather than along it. The bend radius and the number of cycles determine how much of the flex life is consumed, and the manufacturability of the whole design depends on those constraints being respected from the start.

Measuring And Verifying

Because both parameters have a direct effect on performance and on yield, they are measured rather than assumed. Copper thickness is checked with a gauge or by cross section, and line width and spacing are measured optically on the finished part. Where a design is close to the capability limit of the process, agreeing the measurement method and the sampling plan with the fabricator avoids a dispute about whether a marginal batch is acceptable.

The results also feed back into the design. If a batch measures consistently at the narrow end of the tolerance, the current calculation has to be revisited, and a trace that was acceptable at nominal may not be at the limit. Treating copper thickness as a controlled parameter rather than a nominal figure is what makes that comparison possible.

Two other process points are worth knowing because they affect what can be ordered. The coverlay, which protects the traces and defines the openings for the pads, has to be registered to the copper pattern, and its own adhesive flows during lamination. Where the copper is heavy and the gaps are tight, the adhesive has less room to move and the potential for a void or for squeeze out onto a pad rises. The second point is panel handling: a flexible circuit is supplied on a carrier or with stiffeners so that it can pass through assembly equipment, and those features have to be designed in rather than added later.

Choosing The Combination

The right combination follows from the job the circuit has to do. A static flex connector with moderate current can use a thin foil, fine lines and a wide process margin. A dynamic flex cable that moves thousands of times needs rolled copper, thin copper in the bend and a layout that respects the bend line. A high current link needs heavy copper and generous width, and may need to accept a coarser pitch as a consequence.

gopcb builds flexible and rigid flex circuits and can advise on the copper and geometry that suit a particular bend requirement, so that the specification matches what the process can deliver rather than what a rigid board datasheet suggests.

FAQ

Is heavier copper always better for current? It increases current capacity, but it also limits how fine the lines can be and how tightly the part can bend, so it is a trade rather than an improvement.

Why is rolled copper preferred for flexing parts? Its grain structure is elongated along the sheet, so it tolerates repeated bending without cracking far better than electrodeposited foil.

Do flexible circuits follow the same tolerances as rigid boards? Not exactly. The material is thinner and more compliant, and the acceptable tolerances vary with the market and with the copper thickness.