Copper Weight: Choosing the Right Thickness
What Copper Weight Means
Copper weight is the thickness of the copper foil on a layer, expressed as the weight of one square foot of the foil. One ounce per square foot is about 35 micrometres, half an ounce about 18 micrometres, and two ounces about 70 micrometres. The term is a legacy of the foil industry, but the physical consequence is straightforward: heavier copper means a larger cross section, which means more current capacity, lower resistance and a lower thermal resistance along the trace. It also means the etching step has more material to remove, which changes what features can be produced.
Current Capacity
The current a trace can carry is set by the cross section of the copper and by the temperature rise that is acceptable. A trace on one-ounce copper that is 1 mm wide carries several amps comfortably, and the same trace on half-ounce copper carries about half as much for the same temperature rise. Where a design has a high current path, the options are to widen the trace, to increase the copper weight, to use multiple layers in parallel, or to use a bus bar or a cable. Widening the trace is usually the cheapest of these, but it costs board area, and where the area is not available the copper weight becomes the answer. The calculation should be done with the actual ambient temperature and the actual allowable rise rather than a rule of thumb, because the difference between a 10 and a 30 degree rise is a factor of nearly two in current.
Etch Limits and Fine Features
Etching removes copper in every direction, so a thicker layer produces a more pronounced trapezoid and a greater undercut for the same lateral dimension. The practical consequence is that the minimum line width and spacing rise as the copper weight rises. Fine features at 0.1 mm are routine on half-ounce copper and difficult on two-ounce, and heavy copper of three ounces or more is generally limited to coarse patterns. A design that needs both a heavy current path and a fine logic pattern therefore has to split the two onto different layers, which is one of the most common reasons a board has more layers than the signal count suggests.
Impedance and Copper Thickness
Copper thickness affects the impedance of a line, because it changes the effective cross section and, on a thin line, the proportion of current that flows near the surface. A 0.1 mm line on one-ounce copper has an impedance that differs measurably from the same line on half-ounce, and the difference grows as the line becomes thinner relative to the copper. A stackup document that specifies an impedance without specifying the copper weight is incomplete, and an impedance model built on a nominal thickness will be wrong on a board built with a different one. This is one of the reasons the stackup has to be agreed with the fabricator before the widths are finalised.
Thermal Effects
Copper is also the thermal conductor on the board, so heavier copper spreads heat more effectively and reduces the temperature rise of a hot component. This is a second, often overlooked benefit of increasing the copper weight, and it is the reason many power boards use heavy copper on the layers adjacent to the heat sources even where the current alone would not require it. The effect is significant: a layer of two-ounce copper spreads heat several times better than a layer of half-ounce, which can be the difference between a design that needs a heatsink and one that does not. On a metal-core board, the same principle applies with the metal providing the spreading.
Layer-by-Layer Specification
Copper weight is a property of a layer, not of a board. A typical power design has heavy copper on the outer layers or on a dedicated power layer and thin copper on the fine-line signal layers, and the stackup document should record the weight of each layer separately. Specifying a single weight for the whole board either wastes money on the signal layers or produces a board that cannot carry the current, and in the second case the problem is discovered only after the design is populated. Where the design is asymmetric in copper, the balance rule applies: a heavy layer should be mirrored by a comparable layer or balanced by thieving, or the board will bow.
Cost and Availability
Heavy copper costs more in material and, more significantly, in process: the etching takes longer, the plating is more difficult, the minimum features are coarser and the yield is lower. Availability also matters, because heavy copper laminates and the plating capability to go with them are not offered by every shop, and the lead time may be longer. The decision should be made on the current and thermal requirement rather than on a preference for a particular weight, and the cheaper alternatives of widening traces or using parallel layers should be considered first.
Copper Weight and Plating
Heavy copper is normally produced by a combination of starting foil and plating, and the balance between the two matters for the way the trace forms. A layer that reaches two ounces purely by plating has a different grain structure and a different surface than one rolled to that weight, and the difference shows in the etching and in the adhesion of the resist. For very heavy copper, above about four ounces, the pattern is often defined by a plating-and-etch process in which the traces are built up rather than etched down, which places the tolerance on the plating rather than the etchant and allows features that etching could not hold. The designer does not need to choose the method, but the tolerance the fabricator quotes does depend on it, and the same nominal weight can therefore come with different achievable feature sizes from two different shops.
FAQ
What is one-ounce copper? A foil weighing one ounce per square foot, about 35 micrometres thick. Half-ounce is about 18 micrometres, two-ounce about 70.
How much current can a trace carry? It depends on the cross section and the acceptable temperature rise. Heavier copper or a wider trace carries more for the same rise.
Does heavy copper affect fine features? Yes. Thicker copper etches with more undercut, so the minimum line width and spacing increase.
Does copper weight affect impedance? Yes, noticeably on thin lines, so it belongs in the stackup document alongside the dielectric thickness.
Should the whole board use the same weight? No. Copper weight is per layer, and a good power design uses heavy copper only where the current or the heat requires it.
Conclusion
Copper weight is a per-layer decision driven by current, heat and the fine-feature requirement, and it interacts with etching, impedance and cost. Calculate the current capacity from the real temperature rise, split heavy and fine features onto different layers, and record the weight of every layer in the stackup. Material and process capability is listed under PCB capabilities, the etching consequences are described in PCB manufacturing, and the trace width and stackup decisions belong to PCB design and layout. Power boards are normally proven through prototype PCB assembly in 2026.
