6oz and 8oz Heavy Copper PCB: What Copper Weight Does to Trace Width and Spacing

The two numbers on the product page do not form a combination

The heavy copper page publishes a specification table. Two rows from it matter here:

Read as a list, these are two independent statements about the process: copper weights from 3oz to 20oz are available, and the finest line width the heavy copper process holds is 6mil. Both are true.

Read as a combination — "so I can have 6mil traces on 20oz copper" — the sentence stops being supported by the table. A minimum in a capability row is the edge of the envelope, not a constant that applies at every point inside it. The 6mil figure sits at one end of that copper weight range, and the achievable minimum moves upward as the copper gets heavier.

The page itself says why, in one line: *heavier copper needs wider spacing and etch compensation*. That sentence is the whole subject of this article, compressed.

The same reading error shows up on the capabilities page, which publishes 0.5oz – 20oz copper weight and 2mil/2mil minimum line width and space for standard PCB work. Those are two envelope edges across the whole product mix. A 2mil line and 20oz copper are both inside the published capability. They are not inside it together.

So the honest version of the question is not *what is your minimum line width* but *what is the minimum line width at the copper weight I have chosen, on the layer I need it on*. Only the second version can be answered.

Why heavier copper pushes the minimum width up

Etching removes copper by chemical attack, and the attack does not arrive strictly from above. It works on the sidewall of the conductor as it works down through the thickness, so the conductor comes out narrower at the top than the pattern intended, with a sloped rather than vertical wall. That undercut is not a defect; it is how subtractive etching behaves.

The consequence for heavy copper is arithmetic. More copper thickness means more material to remove, which means more time in the etchant, which means more sidewall exposure while it happens. A 3oz conductor and a 12oz conductor are not the same problem scaled up — the heavier one spends longer being attacked from the side.

Two things follow, and both are practical rather than theoretical:

A conductor has a minimum width below which it stops being reliable at a given copper weight. Squeeze the width too far on heavy copper and what you designed as a conductor arrives as something thinner than intended, or with a cross-section that no longer carries what you sized it for. This is the reason the achievable minimum width rises with the copper weight, and it is why "6oz plus the finest line on the page" needs an engineering answer before it gets a price.

The gap between conductors has its own floor, and it is not the same number as the width. Because the walls are sloped, the gap at the top of two adjacent conductors and the gap at their base are different measurements. Fill the space too tightly and the bases approach each other while the tops still look comfortable in the layout view.

We do not publish an oz-by-oz table of minimum widths, and this article is not going to invent one. What it can tell you is the shape of the relationship and the fact that your specific combination has to be confirmed rather than assumed. That confirmation is a five-minute question at RFQ stage and an expensive one after tooling.

Etch compensation is the supplier's action, not yours

Here is the part that most often surprises buyers seeing a heavy copper DFM report for the first time.

Because etching narrows conductors predictably, manufacturers compensate for it: the artwork that goes to production is adjusted so that after etching, the finished conductor lands on the dimension the design asked for. That adjustment is etch compensation, and on heavy copper it is not optional — the product page lists it as part of what heavier copper requires.

Three consequences matter commercially:

Your Gerber is not modified. Compensation happens on the production artwork the supplier derives from your data. Your design files stay as you sent them. Nothing in your repository changes.

"My design says 6mil" and "the finished conductor is 6mil" are two different statements. One is your input, the other is the manufactured outcome after compensation. Which one your specification is written against determines what the supplier is actually committing to. This is the same structural gap that exists between base foil weight and finished copper thickness, where a document naming only one of them has named half a requirement — PCB168 covers that field pair in finished copper thickness versus base copper in a PO.

Compensation is a target, so it needs an acceptance rule. If the finished conductor is the thing being committed to, then the specification has to say where it gets measured, against what datum, and within what tolerance. Without those three, "6mil finished" is a sentiment. With them it is an inspectable requirement, and the capabilities page publishes what the measurement infrastructure supports: automated optical inspection on every layer, 100% electrical testing by flying probe or fixture, and cross-section microsection for quality validation.

Note the asymmetry this creates in quote comparison. A supplier who assumes your dimension is a design-intent figure and a supplier who assumes it is a finished-conductor requirement are pricing two different jobs. Neither has done anything wrong. Your document left it open. PCB168 covers how a specification sheet becomes binding rather than indicative in PCB fabrication specifications that bind the quote.

Spacing is a separate field, not a consequence of width

Buyers routinely send a single number: minimum trace width. It feels complete, because in a layout tool width and clearance often move together under one design rule set.

For a heavy copper quote it is half an input.

Width tells the supplier about the conductor. Spacing tells them about the gap, and on heavy copper the gap is the field with the sloped-wall problem in it. Two conductors with a comfortable gap at the top can have a considerably tighter gap where they meet the laminate. Which measurement your requirement refers to changes what has to be held.

Spacing also carries consequences width does not:

• Voltage. Clearance between conductors is a function of the potential difference between them, not of the copper weight. A heavy copper power board frequently has a voltage requirement driving spacing independently of anything etching does.
• Filling between conductors. The space between thick conductors has to be filled with resin during lamination. Tall conductors with narrow gaps are harder to fill completely than short ones with wide gaps, which is a multilayer construction question rather than an etching question. PCB168 covers the layer-count side of that in thick copper multilayer layer count limits.

State the two fields separately. If the tight spacing is only in one region of the board, say where, because quoting the whole board at its tightest gap prices work nobody asked for.

Layer position changes what an oz figure means

One more field that has to travel with the copper weight: which layer.

Outer-layer copper gets plating added during fabrication, so the finished thickness is the base foil plus what plating contributes. Inner-layer copper does not get that addition — the finished thickness is the foil you specified. The same "6oz" written without a layer position is therefore two different manufacturing situations depending on where it lands, and the etching problem differs with it.

For mixed builds — a high-current section on some layers and signal routing on others — the copper weight should be stated by layer position rather than as a board-level figure. The heavy copper page publishes 3oz to 20oz across 1–16 layers, which tells you the available band, not how your particular stackup is distributed. PCB168 covers declaring a mixed-weight build in heavy copper multilayer mixed-weight stackup in an RFQ, and the general multilayer construction frame sits on the multi-layer PCB page.

The six lines to add to a heavy copper RFQ

These replace a single "min trace 6mil" line. Written out, they take a paragraph and remove most of the back-and-forth.

1. Copper weight by layer position. Not one board-level number. Name the layers and the weight on each, and say whether each figure is base foil or required finished thickness.

2. Where the finest geometry actually occurs. Which layers, and ideally which region. If the tight area is a fan-out under one component, that is a very different quote from tight geometry everywhere.

3. Minimum spacing, stated separately from width. With the voltage requirement if one is driving it.

4. Measurement location for both fields. Top of conductor, base of conductor, or an average. This is the single line that most often prevents a dispute, because it is what an inspection result gets compared against.

5. Tolerance and acceptance basis. A dimension with no tolerance is design intent. A dimension with a tolerance and a named acceptance document is a purchase requirement.

6. Whether supplier etch compensation is expected and accepted. On heavy copper it will be applied. Saying so explicitly, and stating that the committed dimension is the finished conductor rather than the artwork, closes the gap that otherwise surfaces after tooling.

Send these with your Gerbers to the quote page and ask for the combination to be confirmed rather than the minimum to be quoted. If any of the six cannot be answered yet, say which — an open field that is marked open gets a question back, while an open field that looks closed gets an assumption.

If you are also comparing several heavy copper quotes against each other, the cost structure behind them is covered in heavy copper quote comparison and cost drivers.

FAQ

Can a 6oz heavy copper board be built with 6mil traces?

That combination needs engineering confirmation rather than a yes or no from a capability table. The heavy copper page publishes 3oz–20oz copper weight and 6mil minimum line width as two separate envelope edges, and the achievable minimum width rises as copper gets heavier. Send the copper weight, the layer position and the region where the fine geometry occurs, and ask for that specific combination to be confirmed.

Does etch compensation change my Gerber files?

No. Compensation is applied to the production artwork the supplier derives from your data. Your files are unchanged. What it does change is the relationship between the dimension in your design and the dimension on the finished board, which is why the specification needs to say which of the two is the committed requirement.

What happens if I only give a trace width and not a spacing?

The supplier has to choose a clearance assumption. Because heavy copper conductors have sloped sidewalls, the gap at the base of two conductors differs from the gap at the top, and because clearance also depends on the voltage between conductors, no copper weight implies a spacing. Two suppliers making different reasonable assumptions will return quotes that are not comparable.

Is 6oz on an outer layer the same as 6oz on an inner layer?

Not as a manufacturing problem. Outer-layer copper receives plating during fabrication, so finished thickness is base foil plus plating; inner-layer finished thickness is the foil as specified. State the copper weight with its layer position, and say whether each figure is base or finished.

Do I need to supply current figures at RFQ stage for this?

Not for the width and spacing confirmation itself, which is a process-capability question. You need them for the copper weight decision, which sits upstream — covered in current and thermal budget versus copper weight. If your copper weight was inherited rather than derived, supplying the current conditions anyway lets the review flag a mismatch before tooling.