PCB Copper Thickness Guide

Copper thickness on a PCB determines current-carrying capacity, thermal performance, impedance geometry and etch resolution. Specifying the right copper weight for each layer is a fundamental design decision that affects fabrication cost, manufacturing yield and board performance. This guide explains standard copper thickness options, how to choose the right weight for your application, and the manufacturing trade-offs involved.

Copper weight and thickness

PCB copper thickness is traditionally specified in ounces per square foot (oz/ft²). One ounce copper means that one ounce of copper is spread over one square foot of area, resulting in a foil thickness of approximately 1.4 mils (35 microns). Common copper weights and their approximate thicknesses are: 0.5 oz = 0.7 mil (17.5 um), 1 oz = 1.4 mil (35 um), 2 oz = 2.8 mil (70 um), 3 oz = 4.2 mil (105 um), 4 oz = 5.6 mil (140 um), and heavy copper from 5 oz up to 10 oz = 7.0 to 14.0 mil (175 to 350 um).

The most common copper weight for standard PCB designs is 1 oz (35 micron) on outer layers and 0.5 oz (17.5 micron) on inner layers. This combination provides adequate current capacity for most digital signal and moderate power designs while maintaining fine feature resolution for dense routing.

Current carrying capacity

The primary reason to increase copper thickness is to carry more current without excessive temperature rise. IPC-2152 provides the standard method for calculating trace current capacity based on copper cross-sectional area, trace length and allowable temperature rise above ambient. A 10-mil wide trace in 1 oz copper can safely carry approximately 1 ampere with a 10°C temperature rise in still air on an outer layer. The same trace in 2 oz copper carries approximately 1.5 amperes under the same conditions.

For power distribution, the choice is between wider traces in thinner copper or narrower traces in thicker copper. When routing density is constrained — many power nets competing for space on a limited number of layers — heavier copper allows adequate current capacity in narrower traces. When board area is available, wider traces in standard copper may be more cost-effective because heavy copper fabrication carries a price premium.

Impedance effects

Copper thickness affects characteristic impedance because it changes the geometry of the transmission line cross-section. Thicker copper requires wider traces to achieve the same impedance target, all else being equal. When a design specifies both controlled impedance and heavy copper on the same layer, the trace widths must be recalculated for the actual copper thickness — impedance values from a stackup calculator assuming 1 oz copper will be incorrect if the fabricated copper is 2 oz.

Etching and feature resolution

Thicker copper is harder to etch cleanly. Chemical etching removes copper both downward (desired) and laterally (undercut), producing a trapezoidal trace cross-section rather than a rectangular one. The amount of undercut increases with copper thickness. For 1 oz copper, a minimum trace/space of 3-4 mil is achievable. For 2 oz copper, the practical minimum increases to 5-6 mil. For 3 oz and above, minimum features are 8-10 mil or wider depending on the specific copper weight and etching process.

This etch limitation means that heavy copper layers cannot be used for fine-pitch signal routing. Designs that require both heavy copper power planes and fine-pitch signal routing must use different copper weights on different layers — for example, 2 oz on power layers and 0.5 oz on signal layers.

Thermal considerations

Copper is an excellent thermal conductor (approximately 400 W/mK). Heavier copper on inner planes spreads heat laterally more effectively than thin copper, reducing hot spots near high-power components. Thermal vias connecting heavy-copper inner planes to component pads provide a vertical heat conduction path from the component to the board’s thermal mass. In power electronics designs, the combination of heavy copper planes and thermal via arrays can eliminate the need for separate heatsinks in some applications.

Cost impact

Heavier copper increases fabrication cost through higher material cost (copper foil is priced by weight), longer etching time, increased chemical consumption and tighter process control requirements. The cost premium varies by copper weight: 2 oz typically adds 10-20% to the bare board cost, while 4 oz and above can add 30-50% or more. For designs where only one or two layers need heavy copper, specify the heavy weight on those layers only and use standard 1 oz or 0.5 oz on the remaining layers to control cost.

Choosing the right copper thickness

For standard digital designs with moderate current requirements: 1 oz outer, 0.5 oz inner. For power distribution with currents above 2-3 amps per trace: 2 oz on power layers. For power electronics, motor drives and high-current applications: 3-4 oz on power layers, with thick copper PCB constructions up to 10 oz available for extreme current density. Always verify current capacity using IPC-2152 calculations rather than rules of thumb.

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