What copper weight is needed to carry a given current without overheating?

Current capacity is governed by the conductor cross-sectional area and the allowable temperature rise, modeled by IPC-2152. Cross-section equals trace width times copper thickness, so doubling copper weight from 1 oz to 2 oz doubles the area but only raises the allowable current by roughly 65 percent for the same temperature rise, since current scales with area to the 0.725 power. A 1 oz external trace 50 mils wide carries about 4.6 A for a 30 degree C rise; the same width in 2 oz carries about 7.6 A. For high-current RF power amplifier bias and ground returns, 2 oz or 3 oz copper is common, while signal and impedance-controlled microwave layers usually stay at 0.5 oz or 1 oz to preserve fine line-and-space etch resolution.

RF Design

Copper Weight

Q: How thick is 1 oz copper in microns and mils?

One ounce of copper spread evenly over one square foot of laminate produces a base-copper thickness of about 34.8 microns, which is 1.37 mils or 0.00137 inch. Common multiples scale linearly: 0.5 oz/ft is roughly 17.4 microns, 2 oz/ft is about 69.6 microns, and 3 oz/ft is about 104 microns. These figures refer to the foil weight before plating. On a finished plated-through-hole board the actual surface copper is the base foil plus the electroplated copper added during the via metalization step, so a 1 oz starting foil with 1 mil (25 micron) of plating finishes near 60 microns. RF designers must use the finished thickness, not the foil weight, when modeling trace cross-section and impedance.

Q: Does copper weight affect RF insertion loss at millimeter-wave frequencies?

Indirectly, yes. At RF and microwave frequencies the current concentrates in a thin surface layer set by the skin depth, which for copper is about 2.1 microns at 1 GHz, 0.66 micron at 10 GHz, and 0.21 micron at 100 GHz. Because the skin depth is far thinner than even a 0.5 oz foil, increasing copper weight alone does not lower conductor loss once the foil is several skin depths thick. What matters far more is the copper surface roughness profile, since the high-frequency current must follow the rough tooth profile of the foil. Heavier foils are often produced with rougher treated sides for adhesion, so for low-loss millimeter-wave boards designers frequently choose thinner, very-low-profile (VLP) or hyper-very-low-profile (HVLP) foils rather than heavy copper.

/KOP-er wayt/

Specifying the thickness of the copper foil bonded to a printed circuit board, this parameter is quoted as the mass of copper spread over one square foot, in ounces. A weight of 1 oz/ft equals about 34.8 μm (1.37 mils) of base copper, so the value scales linearly: 0.5 oz is roughly 17 μm and 2 oz is roughly 70 μm. Copper weight sets the current-carrying cross-section, the minimum line-and-space an etcher can hold, and, together with surface roughness, the conductor loss of a transmission line. Because RF current flows in a shallow surface layer governed by the skin effect, foils only a few skin depths thick behave identically, so microwave designers pick copper weight for manufacturability and current handling rather than to reduce loss.

1 oz/ft: ≈ 34.8 μm

Common weights: 0.5, 1, 2 oz

RF default: 0.5 to 1 oz

How Ounce-per-Square-Foot Maps to Real Copper Thickness

Copper weight is an old unit inherited from the foil industry: it states how many ounces of copper, by mass, are rolled or electrodeposited across one square foot of carrier. Because copper has a fixed density of 8.96 g/cm³, that mass-per-area converts directly to a uniform thickness. One ounce over a square foot works out to 34.8 micrometers, or 1.37 mils, of base copper. The shorthand has stuck even though most fabricators and CAD tools now want a thickness in microns, so an RF engineer must be fluent in both. A stackup that calls for 0.5 oz outer layers and 1 oz inner planes is really asking for about 17 μm of treated foil outside and 35 μm on the buried microstrip reference planes.

The distinction between foil weight and finished thickness trips up many stackup calculations. The ounce figure describes the starting foil only. During plated-through-hole metalization the shop electroplates additional copper, typically 20 to 30 μm, onto every external surface and into the barrels. So a board ordered with 0.5 oz outer foil can finish at 40 to 50 μm of total surface copper once plating is added. For controlled-impedance microwave traces this finished number, not the nominal ounce, must drive the field solver, because trace cross-section and the etched sidewall trapezoid both shift the characteristic impedance by several ohms.

Heavier copper improves current handling and thermal spreading but costs etch resolution. As foil thickness rises, the etchant undercuts the sidewalls more before clearing the field, so the achievable line-and-space widens: 0.5 oz copper holds 3 mil lines comfortably, 1 oz needs about 4 to 5 mil minimums, and 2 oz typically wants 8 mil or wider. For dense millimeter-wave layouts with tight coupled lines and gaps, that resolution penalty usually outweighs the thermal benefit, which is why most RF signal layers stay at 0.5 oz or 1 oz.

Copper Weight, Skin Depth, and Conductor Loss

At microwave frequencies the signal current crowds into a surface layer whose thickness is the skin depth, δ. For copper, δ falls from about 2.1 μm at 1 GHz to 0.66 μm at 10 GHz and only 0.21 μm at 100 GHz. Since even the thinnest practical 0.5 oz foil is roughly 17 μm, the foil is already dozens of skin depths thick at Ka-band, and adding more copper weight does nothing to lower loss. What does matter is the treated-side roughness profile, because the current must snake along that microscopic tooth structure, effectively lengthening its path. This is why low-loss boards specify very-low-profile foils rather than heavy copper.

Foil Weight and Thickness Conversions

Weight to base thickness:
t (μm) ≈ 34.8 × W_oz t (mils) ≈ 1.37 × W_oz

Copper skin depth:
δ = √( ρ / (π × f × μ₀) ) ≈ 2.1 μm / √(f_GHz)

Current capacity (IPC-2152 form):
I = k × (ΔT)^0.44 × A^0.725, A = W_trace × t_cu

Where W_oz = copper weight in oz/ft², ρ = 1.72×10⁻⁸ Ω·m for copper, μ₀ = 4π×10⁻⁷ H/m, A = cross-section in mil², k ≈ 0.048 (external) or 0.024 (internal). Example: 1 oz, 50 mil external trace, ΔT = 30°C → I ≈ 4.6 A.

Copper Weight Reference Table

Copper Weight	Base Thickness	Min Line/Space	Ext. Current (30°C, 50 mil)	Typical RF Use
0.25 oz	≈ 8.7 μm	2 mil	≈ 1.7 A	HDI mmWave fine lines
0.5 oz	≈ 17.4 μm	3 mil	≈ 2.8 A	Microstrip signal layers
1 oz	≈ 34.8 μm	4 to 5 mil	≈ 4.6 A	General RF, mixed signal/power
2 oz	≈ 69.6 μm	8 mil	≈ 7.6 A	PA bias rails, thermal spreading
3 oz	≈ 104 μm	10 to 12 mil	≈ 10.2 A	High-current power and ground

Common Questions

Frequently Asked Questions

How thick is 1 oz copper in microns and mils?

One ounce of copper over a square foot is about 34.8 μm, equal to 1.37 mils or 0.00137 inch. Multiples scale linearly: 0.5 oz ≈ 17.4 μm, 2 oz ≈ 69.6 μm, 3 oz ≈ 104 μm. These are foil-only figures. After plated-through-hole metalization, external surfaces gain 20 to 30 μm more, so a 1 oz foil finishes near 55 to 65 μm. Use the finished thickness, not the ounce value, for impedance modeling.

Does copper weight affect RF insertion loss at millimeter-wave frequencies?

Only indirectly. Copper skin depth is 2.1 μm at 1 GHz, 0.66 μm at 10 GHz, and 0.21 μm at 100 GHz, far thinner than any practical foil. Once the foil is several skin depths thick, more copper weight does not reduce loss. The dominant factor is the treated-side surface roughness, so low-loss mmWave boards specify very-low-profile (VLP/HVLP) foils rather than heavy copper.

What copper weight carries a given current without overheating?

Current capacity follows IPC-2152: cross-section is trace width times copper thickness, and current scales with area to about the 0.725 power. Doubling the area from 1 oz to 2 oz therefore gives roughly 65 percent more current at the same temperature rise, not double. A 50 mil, 1 oz external trace carries about 4.6 A for a 30°C rise; the same width in 2 oz carries about 7.6 A. PA bias rails often use 2 to 3 oz, while signal layers stay at 0.5 to 1 oz.

Related Terms

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