Selecting Copper Thickness for PCB Prototypes

Copper thickness is specified in ounces, a unit inherited from the weight of foil used per square foot. One ounce copper is about thirty-five microns thick, and it is the default for most boards. Raising it changes what the board can carry, how it dissipates heat and how difficult it is to etch, and each of those effects should be considered before the drawing is released.

oz copper and Current Capacity

A trace carries current through its cross sectional area, which is the width multiplied by the thickness. Two ounces of copper carries roughly twice the current of one ounce at the same width and the same permitted temperature rise.

The permitted temperature rise is the variable that makes the calculation subtle. A trace that rises twenty degrees is acceptable in a product that runs warm and unacceptable in one that must stay cool, and the same trace carries different currents in the two cases. The copper thickness decision therefore depends on a thermal limit, not on current alone.

Trace Width and the Space It Consumes

Heavy copper can be avoided by widening the trace instead, and the choice is often one of board area. Doubling the thickness and doubling the width have a similar effect on current capacity, but they cost differently.

Width consumes routing space on the layer and may force more layers or a larger board. Thickness costs in fabrication, because the plating runs longer and the etching becomes harder. Where the board has free area, widening is usually cheaper; where it is dense, the thickness is the only available lever.

Heat Spreading and Thermal Benefit

Copper carries heat as well as current. A power device with a large copper pad beneath it spreads its heat over the board, which reduces the local temperature and improves the life of the surrounding components.

This is why power designs frequently specify heavy copper even when the current could be carried by narrower traces. The metal serves as a heatsink rather than as a conductor, and the benefit is measured by the reduction in the temperature of the device rather than by any electrical parameter.

Impedance and High Frequency Effects

For controlled impedance traces, the thickness is part of the calculation. A thicker trace is wider for the same impedance on a given dielectric, because the increased cross section lowers the inductance and raises the capacitance.

At high frequency there is a second effect: current concentrates near the surface of the conductor, so the centre of a thick trace carries less than its area suggests. Heavy copper therefore offers less benefit at high frequency than its cross section implies, and the designer should not assume that a thick trace behaves like its DC equivalent.

Etching and Fabrication Limits

Etching removes copper from both the sides and the top of a trace. The lateral removal is roughly proportional to the thickness, so a heavy copper layer produces traces that are noticeably narrower than drawn unless the artwork is compensated.

That also limits how fine a feature can be produced. Heavy copper and fine pitch do not coexist easily, because the etchant cannot define a narrow gap through a thick layer without undercutting the trace beside it. Where both are needed, an additive process that plates only the pattern may be used, at higher cost.

Cost and Lead Time

Heavy copper affects the plating time, the etch time and the yield of the panel, so it is quoted higher and produced more slowly. It also constrains the suppliers who can build it, which matters when the schedule is tight.

For a prototype, specifying heavy copper on the whole board when only one area needs it wastes both money and time. A design that keeps the heavy copper local and the rest standard is cheaper and easier to build, and the difference is worth checking before the order is placed. Our electrical test notes describe how the resulting traces are verified.

One Ounce Copper as the Baseline

One ounce copper, written as 1 oz copper, is the default because it is the thickness the industry is organised around. Foils are stocked in that weight, plating lines are tuned for it and etching parameters are established, so a board at one ounce is the cheapest and fastest to produce.

Deviating from it in either direction adds process work. Half ounce copper saves a little material and makes fine features easier to etch, while two and three ounce copper require longer plating and compensation in the artwork. The designer should have a reason before moving away from the baseline in either direction.

Working With the Fabricator on Copper Weight

The finished copper thickness is not simply the foil thickness. Plating adds copper to the surface as well as to the hole walls, so the final figure depends on the process the fabricator uses.

That is why the requirement should be stated as a finished thickness with a tolerance, rather than as a foil weight. A fabricator asked for a foil weight will deliver what the process produces, which may be above or below what the design assumed. Our layer assignment notes describe how the copper weight interacts with the stackup and the impedance targets.

Spacing Rules for Heavy Copper

Minimum spacing increases with copper thickness, because the etchant undercuts the resist and the gap between two traces closes more than the artwork specified. A design that uses one ounce spacing rules on a two ounce board will produce shorts.

Compensating the artwork helps, but only up to a point. Beyond a certain thickness the fabricator must widen the gaps, which may not be possible in a dense area, and the design is returned for revision. Checking the spacing against the chosen copper weight before release avoids the loop. The same discipline applies to pad geometry, as described in our pad and hole size article.

Balancing Copper Weight Against Layer Count

Copper thickness and layer count are two answers to the same problem, and a design usually has a cheaper option between them. Where a power net needs more current, adding a layer so that the trace can be made twice as wide may cost less than doubling the copper on the whole board.

The choice depends on where the current is concentrated. A single heavy net justifies local widening or a heavier layer, while a design with many moderate nets is usually better served by an extra layer of ordinary copper. Comparing the two on total cost rather than on copper weight alone is what produces the right answer.

Documenting the Requirement

Whichever choice is made, the requirement should be stated in the fabrication drawing as a finished thickness with a tolerance, on the specific layers that need it.

A note that simply says heavy copper is not a specification, and it invites the fabricator to interpret it. Being explicit also makes the requirement visible at quotation, which is where an unexpected cost should appear rather than after the order is confirmed.

FAQ

Is two ounce copper twice as strong as one ounce? It carries roughly twice the current for the same temperature rise at the same width, but the relationship with heat spreading and with high frequency behaviour is not proportional.

Can copper thickness vary across one board? It can, where a stepped plating process is used, but it requires an extra step and is only worth it on production boards with a clear need.

Does heavy copper change the drill sizes? It does not change the hole diameter directly, but plating thicker copper reduces the finished hole slightly, which the fabricator compensates for.