PCB copper weight describes the mass of copper spread over a unit area. In North American fabrication language, 1 oz copper means one ounce per square foot, which corresponds to a nominal foil thickness of about 35 µm. Designers use 0.5 oz, 1 oz, 2 oz and heavier copper to balance current capacity, heat spreading, feature resolution, impedance and cost.
The number is not always the final measured thickness on every trace. Outer layers may begin with a base foil and gain copper during through-hole plating, while inner layers are normally imaged and etched from the starting foil. A complete drawing must distinguish base copper from finished copper.
PCB Copper Weight Conversion Table#
| Nominal copper weight | Approximate nominal thickness | Typical design use |
|---|---|---|
| 0.5 oz/ft² | 17–18 µm (0.0007 in) | Fine features, some multilayer inner layers |
| 1 oz/ft² | 34–35 µm (0.0014 in) | Common general-purpose circuitry |
| 2 oz/ft² | 68–70 µm (0.0028 in) | Higher-current and improved heat spreading |
| 3 oz/ft² | About 105 µm (0.0041 in) | Power distribution and heavy-copper designs |
| 4 oz/ft² | About 140 µm (0.0055 in) | Specialized high-current construction |
These are useful conversions, not guaranteed finished values. Copper-foil tolerances, etching and plating change the delivered conductor. IPC-2152 defines copper weight as mass per unit area and treats conductor thickness as the finished conductor including metallic coatings; its official table of contents and scope also frames current capacity around finished conductor size and acceptable temperature rise.
Base Copper vs. Finished Copper#
Base copper is the foil or deposited copper present before pattern plating and final etching. Finished copper is the conductor thickness after fabrication. They can be close on inner layers but different on plated outer layers.
For a plated-through-hole board, the fabricator may start outer layers with lighter foil, image the circuit, and electroplate copper onto exposed traces and hole barrels. A drawing that says only “2 oz copper” may be interpreted as 2 oz starting foil, 2 oz finished conductor, or a supplier's standard build that produces an approximate result. Write the intended final requirement and let the fabricator propose a controlled starting construction.
Finished thickness is not perfectly uniform. Current distribution, panel position, feature density and copper balancing affect plating. Minimum finished copper and permitted tolerance are more useful than an unrealistic exact value at every point.
External vs. Internal Layer Copper#
Internal layers are patterned and etched before lamination. Their copper is not normally increased by the later through-hole electroplating step. Outer layers, by contrast, receive additional copper when hole barrels are plated. Therefore a stack-up might use 1 oz inner copper and a different finished outer-layer requirement.
Put copper information in a layer stack table. For each layer, identify copper type, starting or finished requirement, thickness units and tolerance. Also state the finished board thickness separately; our PCB thickness guide explains how copper, cores and prepregs combine.
How Copper Weight Affects Current Capacity#
Thicker or wider conductors have lower DC resistance and generally carry more current for the same temperature rise. But “amps per ounce” is not a valid universal rule. Current capacity depends on finished width and thickness, copper area, allowed temperature rise, ambient conditions, board construction, nearby planes, duty cycle and cooling.
IPC lists IPC-2152 among its PCB design standards for determining current-carrying capacity. Use an analysis method appropriate to the product, then validate critical power paths with prototypes or thermal measurements. Fuse behavior, connector ratings, via arrays and solder joints must also be checked; a thick trace cannot compensate for an undersized terminal.
Resistance and Voltage Drop#
For a uniform copper trace, resistance follows:
R = ρL ÷ (W × T)
where ρ is copper resistivity at the relevant temperature, L is trace length, W is finished width and T is finished thickness. Doubling thickness approximately halves the trace resistance when all other factors stay equal. Real layouts include neck-downs, pads, vias, thermal spokes and temperature-dependent resistivity, so model the complete current path.
High-current planes should be reviewed for bottlenecks around cutouts and connector pads. Parallel layers only share current predictably when their interconnections and geometry are balanced.
Copper Weight and Heat Spreading#
More copper can spread heat laterally and lower local electrical loss. It does not, by itself, create a complete cooling path. Heat still needs to cross the dielectric and reach a heat sink, chassis or airflow. Plane area, board material, component attachment and thermal interfaces can matter more than copper weight alone.
For hot components, combine copper area with a suitable thermal via array or another qualified construction. Evaluate solder voiding and component package limits as part of the assembly.
Why Heavy Copper Needs Larger Features#
Etching removes copper laterally as well as vertically. As copper gets thicker, maintaining very small traces and spaces becomes more difficult. The fabricator may require wider conductors, larger clearances, adjusted artwork or a different etch process. Dense BGAs and heavy-current areas can therefore pull a design in opposite directions.
One solution is to use lighter copper where fine pitch is required and distribute power on other layers. Another is a hybrid or selective construction approved by the supplier. Do not assume that the minimum trace/space published for 1 oz also applies to 3 oz or 4 oz.
Heavy Copper PCB: What Does It Mean?#
“Heavy copper” is an industry description, not one universal threshold. Many suppliers use it for 3 oz or greater, while service limits and definitions differ. Always replace the label with numeric layer-by-layer requirements.
Heavy-copper boards can support power conversion, battery systems, automotive controls, industrial drives and high-current distribution. They may also need special plating, etching, lamination and resin-fill controls. The thicker topology changes prepreg flow around traces and can affect finished flatness and dielectric spacing.
Copper Weight and Controlled Impedance#
Impedance depends on finished trace thickness as well as width, dielectric thickness, dielectric constant and reference-plane geometry. With thicker copper, the conductor cross-section and etch shape become more important. A calculator that assumes zero-thickness traces or nominal starting foil may miss the actual geometry.
Provide target impedance, tolerance, layer stack and coupon requirements, then allow the fabricator to model the production construction. Avoid locking trace width and dielectric values independently if the supplier must adjust them to meet impedance.
Copper in Plated Holes and Vias#
Surface copper weight does not fully specify via-barrel copper. Hole-wall plating has its own process and acceptance requirements. Reliability depends on barrel thickness, aspect ratio, material expansion, hole preparation and thermal cycling. State the applicable performance standard and plating requirement rather than assuming a “2 oz board” automatically has a particular via-wall thickness.
Smaller drills and higher aspect ratios make plating distribution harder. Coordinate copper and hole decisions using the PCB drill-size guide.
Choosing 1 oz vs. 2 oz Copper#
| Design priority | 1 oz often fits | 2 oz often fits |
|---|---|---|
| General logic and signals | Common, economical baseline | Usually unnecessary unless power/thermal needs justify it |
| Fine trace/space | Easier to etch | May require wider rules |
| Power distribution | Works with sufficient width and thermal margin | Reduces resistance where area is limited |
| Cost and availability | Broadly standard | Higher cost; capability varies by stack-up |
| Board thickness impact | Lower copper contribution | Adds thickness across multiple layers |
Select from calculations, not habit. A wide 1 oz plane may outperform a narrow 2 oz trace, and layout changes can cost less than upgrading every layer.
Fabrication Drawing Checklist#
- List every signal and plane layer in order.
- State whether copper values are starting or finished.
- Use unambiguous units: oz/ft², micrometres or both.
- Define minimum finished copper and tolerances where function requires them.
- Specify hole-wall copper separately.
- Include finished board thickness and dielectric requirements.
- Identify controlled-impedance layers and test coupons.
- Agree minimum trace, spacing, annular ring and copper-to-edge rules for the selected weight.
- Request a production stack-up for approval before layout freeze.
Common Copper-Weight Mistakes#
Assuming 1 oz means exactly 35 µm everywhere
It is a nominal mass-to-thickness conversion. Foil, etch and plating tolerances create real variation.
Using the same copper on every layer without analysis
Signal layers may benefit from finer copper while power layers need more cross-sectional area.
Ignoring finished outer-layer plating
Starting foil plus plated copper determines final geometry, impedance and spacing.
Increasing copper instead of fixing bottlenecks
Vias, neck-downs, thermal spokes and connectors may still control current and temperature.
Ordering “heavy copper” without numbers
The term varies by supplier. Define numeric requirements for each layer.
A Practical Copper-Weight Workflow#
Calculate current, voltage drop and temperature rise using the finished geometry. Separate the power, signal, impedance and thermal needs by layer. Ask candidate fabricators for standard stack-ups and heavy-copper design rules, then update trace widths, spacing, vias and dielectric thickness together. Finally, prototype and measure the worst-case operating condition.
The broader PCB plating guide explains deposited copper and surface finishes. For a manufacturability review, send PCBPit the stack-up, current table, Gerbers and fabrication drawing.
Frequently Asked Questions#
How thick is 1 oz copper on a PCB?
Its nominal mass converts to roughly 34–35 µm, or about 0.0014 inch. Finished conductors vary with foil tolerance, etching and plating.
How thick is 2 oz copper?
The nominal conversion is roughly 68–70 µm, or about 0.0028 inch. Confirm whether the fabricator means starting foil or finished copper.
Does 2 oz copper carry twice the current of 1 oz?
Not as a general rule. Doubling thickness lowers resistance for the same geometry, but allowable current depends on trace width, temperature rise, environment, board construction and cooling.
Is thicker copper always better?
No. It can improve current and heat spreading, but it reduces fine-feature capability, affects impedance and stack thickness, and adds cost.
What counts as heavy copper?
Definitions vary. Many suppliers start around 3 oz, but the purchase drawing should state numeric copper requirements instead of relying on the term.