Heavy Copper Traces: Sizing for Current and Thermal Rise

Heavy copper is specified when a board has to carry tens of amperes rather than a few hundred milliamps, and the decision changes the fabrication process as much as the layout. Thicker copper etches differently, laminates differently and plated differently, so the weight of the foil has to be chosen with the fabricator rather than simply written onto a title block.

Why Heavy Copper Is a System Choice

The copper weight sets the resistance of every conductor on the board, and it also sets the minimum trace width that the etcher can produce. A three ounce inner layer cannot hold the same fine geometry as a half ounce layer, so heavy copper and high routing density generally cannot share the same layer.

It follows that the layer stack, the routing and the copper weight are one decision. A design that needs heavy copper for a motor drive stage and fine geometry for a controller usually ends up with a hybrid stack, where the power layers are thick and the control layers are conventional.

Current Capacity and Where the Heat Goes

Current capacity is not a property of the trace alone. A conductor of a given cross section reaches a temperature rise that depends on how quickly heat leaves it, and a trace on an outer layer with air above it runs cooler than an internal trace buried between two layers of laminate.

That is why published tables of current against width carry a condition, usually a defined temperature rise such as ten or twenty degrees on an outer layer. Applying an outer layer figure to an inner layer trace is one of the most common ways of underestimating the copper needed.

Trace Width, Length and Voltage Drop

Resistance scales with length, so a long thin trace can carry the current happily and still deliver the wrong voltage at the far end. Where a rail feeds a device several centimetres away, the voltage drop matters more than the temperature rise, and the two requirements can point in different directions.

The usual answer for a distribution rail is a plane rather than a trace. A solid copper area has a cross section measured in millimetres rather than microns, the drop across it is small, and the return path is directly beneath the forward path, which keeps the loop area and the inductance low.

Heavy Copper Fabrication Limits

Etching thick copper is a subtractive process with an aspect ratio problem. The etchant attacks sideways as well as downwards, so a three ounce layer produces a trapezoidal trace with a narrower top than bottom, and the minimum space between traces grows along with the weight.

Lamination, drilling and plating all become harder as well. Resin has to fill a deeper space between heavy traces, drilled holes need more energy, and plating a barrel through a thick copper layer takes longer. Our plating thickness notes describe how the barrel is built.

Thermal Rise and the Adiabatic Assumption

Worst case calculations often assume that no heat leaves the trace at all, which produces a very conservative width. In practice the copper is connected to planes, vias and component pads that all act as heat sinks, and the real thermal rise is lower than the isolated calculation suggests.

The safe approach is to use the conservative figure for the initial sizing and then to check it against the thermal environment of the finished assembly. A trace inside a sealed enclosure runs hotter than the same trace on an open board, and a copper pour nearby changes the answer again.

Planes, Bus Bars and Multiple Layers

Where a single layer cannot carry the current, the usual solutions are to parallel layers with via arrays, to use a copper bus bar soldered or bolted to the board, or to move the current path off the board entirely. Each has a different cost and a different assembly implication.

Parallel layers only work if the current is shared evenly, which requires the layers to be stitched with enough vias at both ends. A via array carries current in the same way as a trace, and a handful of small vias will add resistance and a hot spot at the point where the current transfers.

Vias and Current Path Bottlenecks

A via is a short conductor with a small cross section, and it is often the weakest link in a heavy current path. The copper in a plated barrel is typically twenty five microns thick, so a via that looks generous on the drawing may carry far less current than the trace it joins.

The remedy is a spread of vias rather than one large hole, placed along the current path so that each carries a share. Our notes on how to handle one hundred amp designs describe the via and copper arrangements that make that possible.

Measuring and Verifying the Result

The finished copper weight is verified by cross section and by weighing, and the trace width is measured on a coupon rather than on the board itself. Both checks matter because the etch factor changes the width that the artwork specified, especially on thick copper.

On a working prototype the useful measurement is temperature rise under load, taken with a thermal camera or a thermocouple at the hottest point of the conductor. That figure either confirms the sizing or shows how much margin remains, and it is the only check that reflects the real thermal environment.

Specifying Copper Weight on the Drawing

The drawing should give the finished copper weight per layer, not only the base foil, because plating adds to the total. It should also state the minimum line width and space that the design requires, so that the shop can confirm that the weight and the geometry are compatible before the panels are started.

Where the copper is unbalanced between layers, the note should say so, because a heavy layer on one side of the stack can cause warpage during pressing. Our copper balance notes explain how thieving and balancing are used to control that.

Process Control and Verification

On a design of this kind, trace width is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

How much current can one ounce copper carry? There is no single answer. A one ounce outer layer trace around three millimetres wide will carry roughly ten amps at a twenty degree rise, and the same trace on an inner layer will carry less.

Is heavy copper always the cheapest solution? No. Where the current is very high, a bus bar or a thicker cable is often cheaper than adding copper weight to every layer of the board, and it removes heat from the assembly at the same time.

How does gopcb approach heavy copper work? We confirm the weight and the minimum geometry with the designer, check the stack for balance, control the etch and plating process against a coupon, and report the measured copper thickness with the lot.