PCB Copper Weight: How Foil Thickness Is Chosen

Copper weight is quoted in ounces, and the number means something specific: the weight of copper per square foot of area, which for one ounce corresponds to a thickness of about 35 micrometres. It is one of the few design parameters that changes the electrical, thermal and manufacturing behaviour of a board at the same time, and it is usually chosen too early to be reconsidered cheaply.

What the Numbers Mean

Standard boards use 1 oz copper on the outer layers and often half an ounce on the inner layers, since inner copper does not have to survive soldering and erosion in the same way. Below 1 oz copper the foil becomes fragile and the handling and etching losses rise. Two ounce copper appears on boards carrying significant current or needing better heat spreading, and heavy copper boards start at three ounces and extend to six ounces or more.

The thickness matters because resistance falls in proportion. A one ounce trace has half the cross section of a two ounce trace of the same width, so it has twice the resistance and dissipates twice the heat for the same current. Doubling the copper weight is one way to carry more current without widening the trace, which is useful where board area is constrained.

Where Copper Weight Is Decided

The decision belongs with the current carrying requirement and the thermal requirement, not with the signal routing. Power rails, motor drivers, heating elements and battery connections are the nets that drive it, and the rest of the board usually has no opinion. Where a design mixes heavy current and fine pitch, the usual outcome is a compromise weight with local widening rather than a uniformly heavy board.

Thermal spreading is the other driver. Copper is a good conductor of heat, so a heavier layer spreads heat from a dissipating component over a larger area and lowers its operating temperature. That effect is often worth more than the electrical benefit, particularly on boards where heat has to reach the enclosure through the board itself.

Manufacturing Consequences

Heavy copper is harder to etch. The etchant has to remove more material, and it does so unevenly, attacking the top of the trace faster than the bottom and leaving a trapezoidal cross section. That means the minimum achievable trace width and spacing both increase as the copper weight rises, and the design rules have to be relaxed accordingly.

Etching tolerance is the practical constraint. A design that achieves 0.1 mm features on one ounce copper may only reach 0.2 mm on three ounce copper, which can make a fine pitch escape pattern impossible. The fabricator should be asked for the achievable geometry at the intended weight before the layout is committed, and the etching chemistry involved is discussed in electroplating additives.

Current Capacity Calculations

The current a trace can carry is limited by the temperature rise it is allowed to reach, not by a fixed current density. The conventional allowance is ten degrees Celsius rise for an internal layer and twenty for an external one, because an internal trace is surrounded by laminate that conducts heat away slowly while an external trace can dissipate into air.

That means the same trace width carries noticeably more current on an outer layer than on an inner one, and the calculation has to be applied to the layer the trace actually occupies. Where the board runs in a hot enclosure, the ambient should be the internal temperature rather than room temperature, and the method is set out in trace width and current calculation.

Cost and Supply Implications

Heavy copper costs more for three reasons: the foil itself is more expensive, the etching takes longer and consumes more chemistry, and the yield is lower because the process window is narrower. Plating heavy copper also takes longer, since the deposited thickness is a function of current and time.

Availability matters too. Not every fabricator stocks three and four ounce laminate, and fewer still can plate to the thicknesses required by a heavy copper design. Where the design needs it, the supplier should be confirmed before the stackup is fixed, in the same way that a special laminate would be.

How Weight Interacts With the Stackup

Copper weight is not independent of the rest of the stackup. Heavier foil requires a thicker etch resist and a longer etch, both of which erode the accuracy of the layer it is applied to, so a stackup that mixes heavy copper on one layer with fine geometry on the next has to be built with the constraints of both in mind.

Prepreg flow is affected as well, because the resin has to fill the space around copper that is thicker than usual. A design with heavy copper islands next to large copper-free areas can produce resin starvation at the boundary, which shows up as voids after lamination. Balancing the copper distribution is one of the reasons heavy copper boards are more expensive to engineer as well as to build.

Thermal Vias and Heavy Copper Together

The two techniques solve different halves of the same problem. Heavy copper moves heat laterally across the board, while thermal vias move it vertically from the component side to the opposite side or into an internal plane. A high power design usually needs both, with the copper spreading heat away from the device and the vias delivering it to a plane that can carry it further.

The vias themselves have to be sized for the current they will carry if they are also part of the electrical path. A single barrel cannot carry large currents without overheating, so several are used in parallel, and they are usually filled and capped to give a flat surface for the component. The construction options are described in board outline and mounting design.

Choosing a Weight in Practice

Start with the highest current net and calculate the width needed at the intended weight on the layer it occupies. If the resulting width fits, the weight is adequate. If it does not, either increase the weight or split the current across layers with parallel conductors joined by stitching vias.

Then check the fine pitch regions of the same board. If the relaxed design rules that heavy copper imposes prevent the escape pattern for a fine pitch package, the answer is usually a mixed construction with heavy copper on the power layers and standard weight on the signal layers, which costs less than forcing one weight across the whole stackup.

FAQ

Is one ounce copper enough for a power supply board? It depends on the current and the allowable rise. Many power boards use two ounce copper as a practical compromise, with local widening or parallel layers where the current is highest.

Does heavier copper improve impedance control? Not directly, and it complicates it. The trace thickness becomes a significant fraction of the dielectric thickness, so the impedance formula has to account for it, and the etched trapezoid makes the effective width uncertain.

Can heavy copper be used with fine pitch components? It can on the same board but not on the same layer region. The usual approach is heavy copper on the power and thermal layers with standard weight on the layers that carry fine pitch escapes.