The short answer
A heavy copper PCB is a board carrying 3 oz per square foot or more of finished copper on at least one layer. Three ounces is the accepted threshold because it is roughly where the standard process stops being comfortable: etching, plating, lamination and solder mask all start to behave differently above it.
Ordinary boards run 1 oz (35 µm) on outer layers and 0.5 to 1 oz on inner layers. The range above that is usually split into heavy copper at 3 to 6 oz, and ultra-heavy at 10 to 12 oz, where the board starts to behave less like a circuit and more like a machined busbar.
What copper weight actually measures
Copper weight is the mass of copper per square foot of area, expressed in ounces. It is a thickness in disguise: 1 oz spread over one square foot is about 35 µm thick.
The number that matters for acceptance is finished copper, after plating, not the base foil you bought. A 1 oz start plated to 3 oz is a 3 oz board, and the two routes are not priced the same.
| Copper weight | Finished thickness | Where it appears |
|---|---|---|
| 0.5 oz | 17 µm | Fine-pitch inner layers, signal-only |
| 1 oz | 35 µm | Default outer layer on standard boards |
| 2 oz | 70 µm | Power planes, motor drives, thick copper |
| 3 oz | 105 µm | Lowest heavy copper build |
| 6 oz | 210 µm | High-current distribution, industrial and EV |
| 10 oz | 350 µm | On-board busbars, welding and inverter hardware |
| 12 oz | 420 µm | Upper end of standard wet processing |
Where fabrication breaks
Etching: traces turn trapezoidal
Etchant removes copper from the top down, and sideways at the same time. On 1 oz foil the sideways bite is small enough that a 4 mil trace comes out close to 4 mil. On 6 oz it is not. The trace cross-section ends up trapezoidal, the top is narrower than the base, and minimum line width has to widen with the copper.
These are starting points, not capability statements. The real figure depends on the shop's etchant line, resist type and whether the copper is heavy foil or plated up. A design that needs 6 oz and 6 mil traces at the same time is asking for two processes that fight each other.
Plating: hours, not minutes
There are two routes. Heavy foil starts with 3 to 6 oz copper laminated on and is etched down, which gives consistent thickness but poor fine features. Plate-up starts thin and electroplates to final thickness, which protects fine features but takes hours in the plating line and produces uneven thickness across the panel, thickest at the edges and around holes.
That unevenness is what drives the second problem: hole walls. A 12 oz surface and a 25 µm hole wall cannot be plated in one pass without a fight, so heavy copper builds often need a two-step plate or a button-plate process. It is also where "dog-boning" appears, copper building up at the hole mouth and thinning along the barrel.
Lamination: resin has to go somewhere
Copper that is 200 µm tall creates a cavity. The prepreg has to fill it, and if resin content is too low you get voids at the copper edge, resin starvation and measurable thickness variation across the panel.
The fix is more prepreg plies with higher resin content and a stackup balanced front to back. Unbalanced copper is also the main cause of warp on heavy builds, which then fails at assembly because the board will not sit flat.
Drilling, routing and scoring
Drills pass through more metal, so bits wear faster and registration drifts. V-score depth tolerance tightens because the remaining web is often thinner than on a normal board. If the design has heavy copper in a connector edge, the profile is usually routed rather than punched.
Solder mask
A 6 oz trace is a 200 µm step. Liquid mask flows off the edge and pools at the base, leaving thin coverage on the sidewall and a thickness gradient that can crack during thermal cycling. Where the mask must cover heavy traces, the finish and the mask type have to be chosen together, which is why the finish decision sits in the surface finish comparison rather than in a separate conversation.
| Copper weight | Typical minimum line and space |
|---|---|
| 1 oz | 4–5 mil |
| 2 oz | 6–7 mil |
| 3 oz | 8 mil |
| 6 oz | 12 mil |
| 10 oz and above | 20 mil and up |
Copper weight is not ampacity
The most common mistake in a heavy copper RFQ is treating ounces as a current rating. It is not. Current capacity depends on trace width, copper thickness after plating, whether the trace is on an outer or inner layer, the permitted temperature rise and the ambient.
IPC-2152 is the current standard for this calculation, and it replaced the older charts that most people still quote from memory. Its main practical lesson is that inner layers run hotter than outer layers at the same geometry, because the laminate insulates them.
So the useful input is not "6 oz" but "40 A continuous on this net, 10 °C rise, 60 °C ambient, 8 mm of board width available". From that, copper weight and width can be solved together. Copper weight alone proves nothing.
When copper is the wrong answer
Heavy copper spreads heat, but it is not always the cheapest way to move it.
Mixed construction, with different copper on different layers, is normal on power boards. It needs a stackup drawing that states finished copper per layer, not a single headline number.
| Requirement | Usually better than 12 oz copper |
|---|---|
| Local hotspot under one component | Copper coin or inlay, thermal vias to a plane |
| Whole-board heat spreading for LEDs | Aluminium or IMS metal-core board |
| Very high current off-board | Busbar, cable lug, separate conductor |
| High current plus fine-pitch logic | Split the board: heavy copper power section, 1 oz signal section |
What to send with the RFQ
Finished copper per layer, inner and outer separately. Actual minimum line and space the layout needs. Current per net with the permitted temperature rise. Layer count and stackup. Finish. Acceptance class. Quantity and date. If a copper coin or inlay is involved, its dimensions and the keep-out around it.
Our heavy-copper fabrication page covers the review sequence, and the stackup reference builds show how copper is distributed on 4, 6 and 8 layer boards. Lamination behaviour at the top of the range is in the delamination analysis, and the laminate grades themselves are on the materials page and in the materials hub.
A copper weight is a starting condition, not a specification. Two boards both described as 6 oz can differ by 40 percent in finished thickness and by a factor of two in cost.
Send the stackup with finished copper stated per layer and we will review the construction against your current and thermal requirement before quoting. Where heavy copper is the expensive answer, we will say so and propose the alternative.
Questions, answered
Before your next build
Is 2 oz copper considered heavy copper?
No. Two ounces is thick copper but the industry threshold for heavy is 3 oz (105 µm). The distinction matters commercially because 2 oz still runs on a standard etch and plate line, while 3 oz and above usually triggers a process review.
Can a heavy copper board have fine-pitch components?
On the same layer, usually not. The etching constraint pushes minimum line and space out as copper grows, so a 6 oz layer cannot hold 4 mil traces. The workable answer is a mixed stackup: heavy copper on the power layers, 1 oz on the signal layers.
Does more copper mean the board carries more current?
Only if the width and the thermal path allow it. Ampacity comes from geometry, layer position and permitted temperature rise, calculated per IPC-2152. A narrow 6 oz trace can carry less than a wide 2 oz one.
How much more does heavy copper cost?
There is no single multiplier. Cost rises from copper foil, longer plating, more prepreg, wider spacing that reduces panel utilisation, and yield loss, and those five move independently. A 3 oz board is often 1.5 to 2 times the 1 oz price on the same layer count; 6 oz and above can be three times or more once yield is included. The only reliable number comes from a stackup-specific quote.
How much longer does a 12 oz build take?
Plan on additional plating time, extra lamination cycles and lower yield rather than a single added step. The exact schedule depends on layer count and hole density, so it is quoted per construction, not per ounce.