Copper Thickness and Current Capacity
What Copper Thickness Means
Copper thickness is quoted in ounces per square foot, where one ounce refers to a layer of copper weighing one ounce spread over a square foot, which corresponds to a thickness of roughly 35 micrometres. Standard boards use half an ounce or one ounce, and heavier builds use two, three or more. The thickness is what carries the current, so the choice of copper weight is a thermal and electrical decision as much as a manufacturing one. A board that is designed for one ounce and then found to run hot cannot usually be fixed by changing the finish; the copper has to change.
Sizing a Trace
A trace is sized by the current it must carry and the temperature rise that is acceptable, since a conductor that is too small heats up and its resistance rises with it. The relationship is roughly inverse: doubling the cross sectional area halves the temperature rise for the same current. The cross sectional area is the trace width multiplied by the copper thickness, so a wider trace and a thicker layer are two ways to reach the same area, and the choice between them depends on the space available and on the manufacturing preference. Charts and calculators give a starting point, but they assume a condition, so the assumptions have to be checked.
Temperature Rise and the Environment
The acceptable temperature rise depends on the application. An internal layer is surrounded by laminate, which conducts heat away poorly compared with the surface, so a trace on an inner layer runs hotter than the same trace on the outer layer. A trace in still air is hotter than one with a plane nearby to conduct into. A board inside an enclosure is hotter than one in free air. Those factors are usually captured by a derating factor, and the derating should reflect the real installation rather than the reference case, since a trace that is marginal on the bench may be well outside its limit in the product.
Voltage Drop and Impedance
Current capacity is not the only criterion. A long thin trace has a resistance that produces a voltage drop, and for a power rail that drop may matter more than the temperature rise, particularly where the load current varies and the rail feeds a sensitive circuit. On a high speed net the geometry also sets the impedance, which constrains the width and the spacing far more tightly than the current does. The useful approach is to size the trace for the dominant constraint, then check the other, rather than to reach a width that satisfies only one of them.
Heavy Copper and Its Consequences
Heavy copper, generally two ounces and above, is used for high current power supplies, motor drives, LED boards and thermal spreading. It brings its own manufacturing consequences: etching a thick layer requires wider minimum features, because the etchant undercuts the resist, so the achievable line width and spacing widen as the copper thickens. Plated barrels need a longer plating time, and the registration tolerance may be larger. The result is that a design for heavy copper has to be laid out with the process in mind, and the minimum trace and gap must be taken from the fabricator rather than from the general design rules.
Planes, Thermal Reliefs and Vias
A plane carries current over a wide area, so its capacity is usually not the constraint, but the connection to the plane is. A thermal relief exists to slow the heat loss during soldering, and it also limits the current that can flow between the pad and the plane, so on a high current net the relief has to be either widened or replaced with a solid connection. Vias in the path add resistance and, in a thick board, a limited plating thickness through the barrel, so a high current path should use several vias in parallel rather than one. Where a via is used purely for thermal conduction, the same logic applies.
Checking the Design
The design should be verified by calculation, by simulation where the geometry is complex, and by measurement on a prototype. A thermal image of the board under load shows where the current is concentrated and whether any trace or via has become the hot spot, and it is usually faster than a calculation once the hardware exists. Where a copper change is required, it is worth knowing that the lead time and the cost rise with the copper weight, so the earlier the decision is made the cheaper it is to make. A design that keeps the high current paths short and direct gives itself more margin than one that relies on a wide trace in a bad location.
Copper Balance and Warpage
Copper is also a mechanical material. A layer that is mostly copper and its neighbour that is mostly laminate will expand and cure differently, and the board will bow or twist during lamination and again during reflow. The usual rule is to keep the copper distribution balanced across the stackup, to add thieving or a ground pour to a sparse layer, and to keep the outer layers similar in coverage. A board that warps is hard to place, hard to print on and hard to assemble, so the copper design is a manufacturing input and not only an electrical one.
FAQ
What does one ounce copper mean? A layer whose weight is one ounce per square foot, approximately 35 micrometres thick.
How do I size a trace for current? By the required current and the acceptable temperature rise, with the cross sectional area as the controlling variable.
Why do inner layer traces run hotter? Because the surrounding laminate conducts heat away more poorly than air adjacent to an outer layer.
When is heavy copper needed? For high current power, motor drives, LED boards and thermal spreading, and it widens the minimum feature sizes.
What limits a connection to a plane? The thermal relief or the number of vias, which can become the weak point of an otherwise adequate plane.
Conclusion
Copper thickness and trace width together decide how much current a path can carry and how much it will heat, so size the path for the real environment and check it on hardware. Conductor geometry belongs to PCB design and layout, the build that produces the layers is PCB manufacturing, and the thermal verification sits with PCBA testing. The limits the factory can hold are described under PCB capabilities in 2026.
