PCB Finished Copper Thickness vs Base Copper: What Your 4oz Callout Actually Means

One number on the drawing, three numbers in the process

A buyer writes "4oz copper" on the fabrication drawing. The quote comes back. The boards arrive. Someone measures a cross-section, gets a value below 140µm on an inner layer, and a dispute opens.

Nobody lied. The drawing simply asked a question the process cannot answer with a single number, because copper on a finished board is built in stages:

• Base copper is the foil laminated onto the dielectric before any imaging or plating. This is the number a fabricator buys, stocks and schedules against. When a quotation line says "4oz", this is very often what it means.
• Plated copper is electrolytic copper added during through-hole and pattern plating. It lands on outer-layer conductors and hole walls. It is a process contribution, not a material you order.
• Finished copper is what remains on the board after imaging, etching and plating are complete — the only one of the three your circuit actually carries current through.

The relationship is not the same on every layer, and that is the whole problem:

So "4oz" on a mixed build is not one number. On a board with outer and inner heavy copper layers it is at least two, plus a third for hole-wall copper. PCB168's published heavy copper range on the heavy copper PCB product page is 3oz–20oz on 1–16 layers, and the site-wide copper range on the capabilities page is 0.5oz–20oz — but neither page can tell you which basis *your* drawing intended. Only your drawing can.

The fix is not a better number. It is a stated basis, a stated location and a named acceptance document. Everything below is about writing those three things down.

Weight, thickness and the two units that get mixed

Before the basis question, clear the units, because half of the ambiguity is here.

Copper is specified two ways, and buyers routinely mix them inside one document:

• Weight per unit area — the "oz" convention. Historically it describes foil mass spread over one square foot.
• Linear thickness — µm or mil, a direct dimensional value.

PCB168's heavy copper product page states the pairing it uses in its own copy: 3oz corresponds to 105µm, and it describes 6oz as roughly 210µm. Those are the page's own expressions, and they are useful precisely because they are written down somewhere you can point at.

The trap is that a weight figure describes foil as supplied, while a thickness figure is usually what an inspector measures on your board. A drawing that says "4oz minimum" and a report that says "128µm measured" are not automatically comparable — they are two conventions applied at two different moments in the process, and converting between them requires stating which basis you are converting *from*.

Three rules keep this clean in your own documents:

1. Pick one primary unit per requirement line and put the other in brackets if you want it readable by both sides. Do not let the bracketed value become a second, independently enforceable requirement.
2. Never write a bare "oz" without a layer scope. "4oz" alone is a material note. "4oz base copper, inner layers 3 and 4" is a requirement.
3. If your electrical design was calculated against a thickness, state that thickness as the controlling requirement and let the weight figure be informative. Your current-carrying calculation was run on cross-sectional area, which is a dimensional quantity, not a purchase description.

Where your design calls for a value outside a published range, or a combination the published tables do not cover — heavy copper together with the tighter impedance class, for instance — treat it as requiring engineering review confirmation rather than assuming the two page values combine.

Where the measurement happens decides what the number means

A basis without a location is still ambiguous, because copper thickness is not uniform across a board. Imaging, etching and plating all vary with feature geometry and position. A wide plane, a 6mil trace and a hole wall on the same panel will not return the same value.

So each copper requirement needs a measurement location defined by type of feature, not by a coordinate you picked off the layout:

• Conductor surface — a non-destructive reading taken on top of copper. Fast, but on an outer layer it includes plating and any finish above it unless the method excludes them.
• Conductor cross-section — a microsection through a defined conductor, read under magnification. This is the value that corresponds to your current-carrying calculation, because it shows actual conductor area.
• Plated hole wall — a separate measurement in the barrel, answering a different question from surface copper.
• A named local feature — where a specific pad, plane region or high-current path must hold a value that a generic panel average would hide.

PCB168's capabilities page lists cross-section analysis for quality validation, AOI on every layer, 100% electrical testing, TDR coupon verification for impedance, X-ray for BGA and via fill, and ionic contamination testing. Those are the inspection methods published as available. What the page does not do — and cannot do — is decide which of them your copper requirement is accepted against. That is a choice you make in your own document.

Two practical consequences that catch people out:

Surface readings and cross-sections are not interchangeable. If your drawing does not say which one governs, you have effectively agreed to whichever one produces a passing result. Write the method you want into the requirement line.

One measurement cannot represent every point. If you need a minimum held at a specific high-current region, name that region. If a panel-representative value is acceptable, say so. Silence here reads as "any single location will do", which is rarely what a power design intends.

For how these values sit alongside the rest of your layer information, the multi-layer PCB stackup guide covers how to organise per-layer data in a form a fabricator can quote from.

Writing it into the fabrication drawing

The fabrication drawing is where the requirement lives. The PO points at it. Get the drawing right and the rest follows.

A copper callout that survives acceptance has four parts. Each line should read:

> [Basis] — [value with unit] — [layer scope] — [measurement method and location] — [acceptance document]

Worked examples in that shape, using representative values a buyer might hold:

• *Finished copper, minimum 140µm, outer layers 1 and 6, verified by conductor cross-section, accepted per IPC-6012 at the class stated in this drawing.*
• *Base copper 4oz as laminated, inner layers 3 and 4; finished inner copper accepted by cross-section against the minimum stated in the table below.*
• *Plated hole-wall copper accepted separately per the class stated in this drawing; not covered by the surface copper requirement above.*

Notice what these do. Each one names a basis, so it cannot be read the other way. Each one scopes to layers, so a mixed build does not collapse into one figure. Each one names how it is verified, so the report and the requirement compare like with like. And each one points at an acceptance document rather than restating criteria in prose.

On standards, name them and stop. IPC-4562 is the reference for the metal foil you start from. IPC-6012 covers performance and qualification requirements for the finished board. IPC-A-600 is the acceptability reference. IPC-TM-650 is where test methods live. Citing a standard and its class is enough — do not transcribe values out of it into your drawing, because a transcription error becomes a contradiction between your own document and the standard it claims to follow, and then nobody knows which governs.

Two things to avoid writing:

• "Copper thickness: 4oz" with no other qualifier. This is the sentence that starts disputes.
• A tolerance you invented. If you need a minimum, state a minimum. Inventing a symmetric tolerance around a nominal creates an upper limit you probably do not care about and may not be achievable in combination with your other requirements.

If a required combination falls outside what the product and capabilities pages publish, mark it for engineering review confirmation in the RFQ rather than asserting it as a requirement the supplier has already accepted.

Carrying the same basis into the PO and the quote confirmation

Three documents will contain a copper figure: your fabrication drawing, the supplier's quotation confirmation, and the inspection report. If any two of them use different bases, you have a dispute waiting on delivery.

The PO's job is not to restate the requirement. It is to bind the drawing revision and name the governing order:

• Reference the drawing by number and revision. "Per FAB-1042 Rev C" is enforceable. A copper figure retyped into the PO body is a second source of truth that will eventually disagree with the first.
• State the governing order explicitly. One sentence: *in case of conflict, the fabrication drawing governs over the PO body, and the PO body governs over quotation correspondence.* Without this line, a conflict is resolved by argument rather than by document.
• Require the quotation confirmation to restate the basis. Ask the supplier to write, in the quotation they return, which basis their price was built on and which method they will report against. A confirmation that says only "4oz as specified" carries no information and leaves the ambiguity intact.
• Name the report you expect. If cross-section evidence matters to you, say the requirement is verified by cross-section. Do not specify report turnaround or delivery timing you have not agreed.

On PCB168's quote page and through sales@pcb168.com, the site states responses within 24 hours; the footer lists ISO 9001, ISO 14001, UL Certified and RoHS. Use the engineering contact on the contact page for the basis question specifically — it is a document question, and answering it in writing before the PO issues costs nothing.

One boundary worth keeping: this is your own paperwork, not a supplier audit. If you also want to evaluate how a fabricator handles heavy copper as a process, that is a separate exercise and it has its own request list in heavy copper supplier process evidence.

Reconciling the three documents at incoming inspection

When boards arrive, the reconciliation is mechanical if you did the work upstream and improvised if you did not.

Read the three documents in this order:

1. The report — what basis and method does it state? If the report gives a surface reading and your drawing required cross-section, the report has not tested your requirement. It is not a failure; it is the wrong measurement.
2. The drawing — what did you actually require, by layer? Compare only against lines that share the report's basis and method.
3. The quotation confirmation — did the supplier restate your basis, or substitute their own? If they restated a different basis and you accepted that quotation, the accepted basis is arguably theirs.

Three outcomes and what each one is:

• Conforming — same basis, same method, value meets the stated minimum on the scoped layers. Close it.
• Non-conforming — same basis and method, value misses. Now the decision is rejection, rework or concession, and that decision belongs to whoever owns the electrical and thermal margin on your side, not to purchasing and not to the fabricator. A concession is a design decision recorded against a specific lot.
• Not comparable — different basis or method. Do not negotiate a price adjustment on this. Get the measurement your requirement named, then return to the first two outcomes.

The "not comparable" case is the common one, and it is almost always a document gap rather than a manufacturing problem. Treating it as a quality failure poisons a supplier relationship over a sentence someone forgot to write.

Any construction or process change a supplier proposes in response should go to your design and quality review as an explicit exception, not be absorbed as a substitution. The full RFQ-to-acceptance sequence for heavy copper purchasing, of which this copper-basis question is one input, is laid out in heavy copper PCB purchase specification and acceptance.

A one-page checklist before the PO goes out

Run these seven lines against your own package. Each one is a yes/no:

1. Does every copper requirement state base or finished?
2. Does every requirement name a layer scope rather than applying globally by default?
3. Is there one primary unit per requirement line, with no second enforceable value hiding in brackets?
4. Does each requirement name a measurement method — surface, cross-section, or hole wall?
5. Where a specific region must hold a value, is that region named?
6. Does the PO reference the drawing by revision instead of retyping the copper figure?
7. Is there a governing-order sentence covering drawing vs PO vs correspondence?

Seven yeses and the copper figure in your package means exactly one thing to everyone who reads it.

What this checklist deliberately does not decide is how much copper you need. That is a separate calculation from current, duty cycle, ambient and allowed temperature rise, and it is covered in deriving copper weight from current and rise. The layer-count and board-thickness constraints that interact with heavy copper are in thick copper multilayer layer count limits. Published copper ranges and inspection methods are on the heavy copper PCB page and the capabilities page.

Decide the number with those. Then use this page to make sure the number you decided is the number that gets built and accepted.

FAQ

When a quote says "4oz copper", is that base copper or finished copper?

You cannot tell from the phrase, and that is the point. Different fabricators default differently, and on a mixed build the answer differs by layer. Ask for it in writing in the quotation confirmation, stated separately for inner and outer layers. Until a basis is stated, the figure is a material note rather than an acceptance criterion.

Why is finished copper on an inner layer lower than the base foil?

Inner layers are not plated after lamination, so nothing is added to them. Imaging and etching only remove copper. Outer layers go the other way: electrolytic copper is added during through-hole and pattern plating before etching defines the conductors, so outer finished copper is generally above the base foil. This is why one nominal cannot represent both.

Which measurement should I require — a surface reading or a cross-section?

If your requirement exists because of current-carrying or thermal area, require a conductor cross-section, because that is what shows actual conductor area. A surface reading is faster and fine where you only need a coarse confirmation, but on an outer layer it may include plating and finish above the conductor unless the method excludes them. State whichever one governs; do not leave both available.

Should I put the copper thickness figure in the PO body?

Reference the fabrication drawing by number and revision instead. A figure retyped into the PO becomes a second source of truth, and the two will eventually disagree after a drawing revision. Add one sentence stating the governing order — drawing over PO body, PO body over quotation correspondence.

What if my copper requirement falls outside the published ranges?

The heavy copper PCB page publishes 3oz–20oz on 1–16 layers; the capabilities page publishes 0.5oz–20oz across 1–40 layers. Where your combination sits outside either, or combines heavy copper with requirements the pages list separately, mark it for engineering review confirmation in the RFQ rather than stating it as already accepted. Send it to the engineering contact before the PO issues.

The inspection report and my drawing disagree. Is this a quality problem?

Check the basis and method first. If the report measured something your drawing did not specify — a surface reading against a cross-section requirement, or an outer-layer value against an inner-layer line — the result is not comparable, and that is a document gap rather than a manufacturing defect. Get the measurement your requirement named, then judge conformity. Only when basis and method match does the value become a conformity question, and at that point rejection, rework or concession is a decision for whoever owns the electrical and thermal margin on your side.