What PCB Copper Thickness Means
Printed circuit boards sit at the center of modern electronics. One small specification—copper thickness—affects how the board conducts current, spreads heat, and holds up mechanically over its life. The value is easy to leave at the fabricator's default. That default is not always the thickness the circuit, the thermal path, or the mechanical environment actually needs.
This article looks at the role of copper thickness in PCB fabrication: what the ounce rating means, how it sets current-carrying ability, how it spreads heat, and how it contributes to mechanical stability.
In fabrication, copper thickness is the thickness of the copper foil. It is commonly specified in ounces (oz), meaning the weight of copper per square foot. Typical values are:
- 17.5 μm (0.5 oz)
- 35 μm (1 oz)
- 70 μm (2 oz)
An ounce rating is a weight, not a direct micrometer callout, but the two are paired in practice. Half-ounce foil is about 17.5 μm. One-ounce foil is about 35 μm. Two-ounce foil is about 70 μm. Different thicknesses serve different electrical requirements and applications. Figures 1 through 3 cover three main effects on the board: current capacity, heat spreading, and mechanical stability.
Finished copper on an outer layer is not identical to the foil that went into the press. Plating adds copper in the barrel and on the surface. Inner-layer foil stays closer to the specified weight. A callout of 1 oz therefore describes the copper class, and the fabricator's process decides how much additional plate the outer layers receive. The design still has to start from the right class. A 0.5 oz inner layer cannot be plated into a 2 oz power plane.
Copper Thickness Sets Current-Carrying Ability
Copper is the main conductor on a PCB. Changing its thickness changes how well the board can carry current. For a given trace width, a thicker foil has more cross-section. Resistance falls, voltage drop along the trace falls, and the temperature rise for the same current falls with it.
If the copper is too thin, current capacity drops, and signal attenuation and unstable voltage become more likely. A power trace that was drawn wide enough for 1 oz copper will run hotter and drop more voltage on 0.5 oz foil. On a signal net, the same thin foil raises series loss. Attenuation grows with length, and the voltage that arrives at the load is less stable than the width on the plot seemed to promise.
If the copper is too thick, conduction improves, but cost and process difficulty rise. Heavy copper can also increase resin flow during lamination and create fabrication defects. Thick foil needs more etch time. Trace width and spacing become harder to hold, and fine features next to a 2 oz plane are more likely to undercut or to short. During lamination, the resin has to fill a deeper copper pattern. Excess flow, resin starvation, and voids are the defects that follow when the stack was not built for that weight.
Design note: 1 oz copper is the usual choice for ordinary boards. Special needs such as high current can justify evaluating 2 oz, provided the process can support it. Half-ounce foil remains useful where current is low and fine line width matters more than capacity. The choice is not a quality ranking. It is a match between cross-section, etch capability, and the current the net actually carries.
Copper Thickness Affects Heat Spreading
As device power rises, heat spreading becomes more important. Copper conducts current and also conducts heat. Thicker copper spreads heat faster and more evenly, which helps lower component surface temperature and can extend service life.
Heat leaving a package enters the pad, then the pour or plane attached to that pad. A thicker plane moves that heat sideways before it builds up under the part. The surface temperature of the component falls because the same power is spread over more copper and, through the dielectric, into a larger area of the board. Lower sustained temperature is one reason service life can improve on a board whose only change was copper weight.
Besides choosing an appropriate thickness, unused areas can be poured with copper to add heat-spreading paths and raise the board's overall thermal efficiency. A thick foil that exists only under the component does less than the same foil continued into a pour tied to that pad. Thickness and area work together. Thickness sets how easily heat moves through the copper. The pour sets how far that path extends.
Thermal improvement is not a reason to specify heavy copper on every layer. Inner signal layers that carry little current gain little from 2 oz foil and pay for it in etch and lamination difficulty. The layers that spread heat are the ones tied to power pads, ground pads, and thermal vias under hot parts.
Copper Thickness Affects Mechanical Stability
A PCB is a conductive network and also part of the mechanical structure.
- An appropriate copper thickness can raise bending resistance and reduce cracking and solder-joint lift over long use.
- It can also increase pad strength, improve solder reliability, and reduce cold or incomplete joints.
Copper is stiffer than the resin around it. A board with a suitable copper weight resists bending better than a board built on the thinnest foil the fabricator offers. That stiffness reduces the flex that cracks traces and lifts solder joints over long use, especially where the board is fastened at a few points and loaded in the middle.
Pad strength follows the same material. A thicker pad anchored into a heavier foil is harder to peel when a connector is inserted or when a joint is reworked. Solder reliability improves because the pad does not lift or crater as readily, and cold or incomplete joints caused by a pad that sinks heat unevenly are less likely when the copper class was chosen with both current and soldering in mind. Very heavy copper can still make a joint harder to solder, because the pad becomes a heat sink. Mechanical strength and solderability have to be read together, not as a reason to add ounces without limit.
Copper thickness is especially important on products that need higher mechanical strength, such as automotive and industrial-control boards. Those products see vibration, connector cycles, and temperature swings that a bench board does not. A copper weight chosen only for electrical capacity can still be too light for that mechanical life, or too heavy for the fine-pitch parts on the same panel.
Choosing a Thickness That Fits the Job
Copper thickness is a small specification with a large effect. A well-built PCB usually has a copper weight that fits the job:
- Stable conduction requires copper that is not too thin.
- Fast heat spreading requires copper that is not under-specified.
- Mechanical strength requires copper that is not ignored.
The usual starting point is 1 oz, about 35 μm, for ordinary boards. Half-ounce foil, about 17.5 μm, serves fine-line, lower-current layers. Two-ounce foil, about 70 μm, is the evaluation point for high current, provided the fabricator can etch and laminate it without resin-flow defects. Heat spreading can use that same heavier copper, extended by pours into unused areas. Mechanical strength on automotive and industrial-control boards is a reason to keep thickness in the review even when the current calculation would accept a thinner foil.
Choose thickness from function, current, thermal need, and fabrication cost together, and pick the value that fits the design. A single weight on every layer is rarely that value.
