Heavy Copper Multilayer PCB: How to Declare a Layer-by-Layer Copper Weight Table in Your RFQ | PCB168

Two numbers do not describe one purchasable object

Write "16 layers, 6oz heavy copper" in an RFQ and you have named a layer count and a maximum copper weight. You have not named a board.

That single line can describe a stackup where every one of the 16 layers carries 6oz copper. It can equally describe a stackup where two internal power layers carry 6oz and the remaining fourteen carry 1oz signal copper. Both are honest readings of the same sentence. They are not the same product: they differ in total copper volume, in how many lamination steps the panel goes through, in achievable line width on the thick layers, and in the finished board thickness they land on.

When three suppliers each pick a different reading, you get three prices that cannot be compared. The usual reaction is to suspect the cheapest one. The more likely explanation is that the cheapest supplier read your line as "two heavy layers" and the most expensive read it as "all layers heavy". Nobody misquoted. The RFQ did not contain enough information to have a single answer.

The purchasable object is the table, not the pair of numbers. A heavy copper multilayer RFQ becomes comparable the moment it carries a row per layer.

What this article does not cover

This site already covers adjacent decisions, and this article deliberately does not repeat them:

• Where the layer-count ceiling sits and which combinations need feasibility review at the count level — see thick copper multilayer layer count limits.
• Which oz grade to choose from your current and temperature-rise budget — see heavy copper current and thermal budget.
• Whether the oz figure means base or finished copper, how it is measured and which document decides — see finished copper thickness vs base copper declaration.
• Purchase specification and acceptance wording after the build is agreed — see heavy copper purchase specification and acceptance.

This article answers one question only: what a layer-by-layer copper weight table has to look like so that every supplier quotes the same object.

The minimum information unit: one row per layer

A usable declaration has four fields per layer. Anything less and the supplier has to assume.

A four-field table for a mixed build looks like this — note that this is an illustrative declaration format, not a PCB168 capability statement:

```

L1 1oz signal pairs with L16

L2 1oz ground pairs with L15

L3 6oz power pairs with L14

...

L14 6oz power pairs with L3

L15 1oz ground pairs with L2

L16 1oz signal pairs with L1

```

Two observations about this format. First, it takes about as long to write as the ambiguous one-liner. Second, it forces you to discover your own undecided items before a supplier discovers them for you. If you cannot fill the "layer role" column, that is an internal design question, and finding it during RFQ preparation costs far less than finding it after a first article.

State the units you are using in the same block. Copper weight in oz and in µm both appear in circulation; declaring "oz, finished" or "oz, base" removes a second layer of ambiguity that has nothing to do with the layer table itself.

Mixed-weight versus all-layer heavy copper

These are the two families your table will land in, and they push cost in different directions.

All-layer heavy copper means every layer sits at the same elevated weight. The consequence is volume: copper cost scales with the whole panel, the dielectric has to fill more topography on every layer, and the finished thickness climbs with each layer you add. The design side is comparatively simple, because the same line-width and spacing rules apply everywhere.

Mixed-weight keeps heavy copper only where current flows and leaves signal layers thin. It buys back board thickness and lets fine features live on the thin layers. What it costs you is uniformity. Two different copper weights in one stack mean two different sets of design rules inside a single board, pressing behaviour differs between thick and thin regions, and the stackup has to be balanced deliberately rather than by default.

The practical consequence for the RFQ: a mixed-weight build is where an incomplete table does the most damage. In an all-layer build the maximum weight is the whole story. In a mixed-weight build the maximum weight is almost no information at all, because the interesting question is how many layers carry it and where they sit.

Reading the three published ranges as a set

The heavy copper PCB page publishes three ranges: copper weight 3oz–20oz, 1–16 layers, board thickness 0.8mm–6.0mm. Capabilities publishes the wider standard envelope: 1–40 layers, copper weight 0.5oz–20oz, board thickness 0.2mm–6.0mm, minimum line width/space 2mil/2mil, maximum board size 600mm × 1100mm.

The mistake worth naming: reading three ranges as three independent dials.

They are not independent. Each published range states what is available on that axis; it does not promise that the endpoint of one axis is available simultaneously with the endpoint of another. Copper occupies vertical space. Layers occupy vertical space. Board thickness is the sum. So "20oz" and "16 layers" and "0.8mm" are each inside their own published range, while the combination of all three is a thickness arithmetic problem, not a capability question.

This is not a hedge — it is the reason a feasibility step exists at all. Any RFQ that approaches more than one endpoint at once should expect, and should explicitly ask for, a written feasibility confirmation rather than a price.

Minimum line width on a heavy copper layer

The heavy copper page publishes 6mil minimum line width; capabilities publishes 2mil/2mil for the standard envelope. Both are true, and copying the wrong one into your table is a common source of a quote that later becomes conditional.

The reason the figures differ is physical. Etching removes copper from the top down and from the sides inward at the same time. The thicker the copper, the longer the etch, and the more the sidewall is attacked while the bottom of the channel is still being cleared. A geometry that resolves cleanly in 1oz copper does not automatically resolve in 6oz copper — the same nominal line comes out with a different profile.

For the RFQ table, the operational rule is: do not inherit line width and spacing requirements across layers of different copper weight. If L3 is 6oz power and L1 is 1oz signal, they get separate line-width entries. Writing one global "min line width" for a mixed-weight board either over-constrains the thin layers or asks the impossible of the thick ones.

Etch compensation is the shop-side answer to this, and it is the supplier's engineering decision, not an RFQ input. What belongs to you is the requirement: state the target width, state which layer it applies to, and state the tolerance you can accept. What belongs to engineering is how they get there.

Board thickness budget

Board thickness is where a heavy copper multilayer RFQ most often turns out to be internally inconsistent — and usually the buyer finds out from a mechanical engineer, not from the supplier.

The logic is additive: each copper layer contributes its own thickness, each dielectric between layers contributes its own, and heavier copper needs more dielectric resin to fill the topography around it. Add heavy layers and the achievable minimum thickness rises. It cannot be negotiated downward, because nothing is being padded.

So treat thickness as a budget you spend, not a specification you assert. Before sending the RFQ, answer two questions internally: is the thickness figure a hard mechanical constraint (a connector height, a slot, an enclosure gap) or a preference carried over from a previous design? And if it is hard, which is more negotiable — the layer count or the copper weight?

If the arithmetic pushes past the published 6.0mm ceiling, the question stops being "can you make it thinner" and becomes a design conversation: fewer heavy layers carrying more current each, a different current-distribution strategy, or splitting functions across boards. Send the RFQ with that question stated rather than with a thickness number that cannot coexist with the rest of your table.

The impedance entry in a mixed-weight stack

The heavy copper page publishes impedance control ±10%; capabilities publishes ±5% with TDR coupon verification. For a mixed-weight board, the tolerance figure alone is not a complete declaration.

Impedance is a property of a specific trace on a specific layer with a specific dielectric above and below it. In a stack where copper weight changes between layers, the geometry that produces a target impedance is not the same on every layer. So the entry needs three things, not one:

1. Which nets or layers are impedance-controlled — not "the board".
2. The target value and tolerance for each of those groups.
3. How it will be verified and what gets delivered — capabilities lists TDR coupon verification; say whether you require the coupon data as a delivered record.

The third item is what converts impedance from an intention into something checkable at receipt. A tolerance with no stated verification method is a number both sides can believe they met.

Three-way classification: quote, confirm, or redesign

Sort your own table before sending it. Each row lands in one of three categories, and knowing which is which tells you what to ask for.

Category A — expect a direct quote. Layer count, copper weights and thickness all sit comfortably inside the published ranges, with margin on every axis. Line width requirements are stated per layer and are undemanding relative to the copper weight on that layer. Impedance control, if required, is scoped to named layers. Ask for a price.

Category B — expect written feasibility confirmation before a price. Anything that approaches an endpoint: the upper end of copper weight together with a high layer count; a thickness target near the published minimum for the number of layers you want; line widths on heavy layers close to the published minimum; a strongly asymmetric mixed-weight arrangement. Ask engineering to confirm the build in writing and to write back the stackup they priced. A price without the stackup attached is not a commitment to your table.

Category C — expect a design conversation. The arithmetic does not close: thickness below what the layer count and copper weights can produce, or a combination past the published ranges. There is no price to give. Send the constraint set and the priority order instead, and ask what trade gets you closest.

The useful discipline here is that you can do this classification yourself, before the RFQ goes out. It costs one pass over your own table and it changes what you ask for, which changes what comes back.

Pre-RFQ self-check

Before sending a heavy copper multilayer RFQ, confirm each of these is present and unambiguous:

• Layer-by-layer copper weight table with layer number, weight, role and symmetry intent for every layer.
• Whether the oz figures are base or finished copper, stated once, explicitly.
• Line width and spacing requirements entered per layer group, not once for the board.
• Board thickness stated as a target with its tolerance, plus whether it is a hard mechanical constraint.
• Impedance-controlled layers or nets named, with target, tolerance and required verification record.
• Surface finish selected from the published options (HASL / ENIG / OSP) rather than left open.
• Material family and Tg grade stated (the heavy copper page publishes FR-4 TG170 / TG180).
• Board outline dimensions, checked against the published 600mm × 1100mm maximum panel size.
• Your own A/B/C classification of the build, so the supplier knows whether you are asking for a price or a feasibility opinion.
• Which document governs if the Gerber, the stackup drawing and the RFQ text disagree.

The last item is worth the thirty seconds. Most heavy copper disputes are not about copper — they are about which of three documents was authoritative.

Current-carrying capacity and temperature rise are deliberately absent from this checklist as numbers you declare. They are inputs you state as conditions — peak current, duty cycle, allowed rise, ambient, layer position — and the copper weight follows from engineering review of those conditions. Declaring a copper weight and a current rating simultaneously, without review, means one of the two is an assumption.

Where to go next

Bring your layer table to heavy copper PCB for the published copper weight, layer count and thickness ranges, multi-layer PCB for the general multilayer build context, and capabilities for the standard envelope and the inspection methods available as delivered records. When the table is complete, send it through quote; when a row lands in Category B or C, say so in the request so engineering responds with a feasibility confirmation instead of a conditional price.

FAQ

Is "16-layer 6oz heavy copper" enough to get a quote?

It is enough to get a price, but not enough to get a comparable one. The line does not say whether all sixteen layers are 6oz or only some. Suppliers reading it differently will quote different boards. Add a row per layer and the ambiguity disappears.

Should I declare copper weight per layer even if all layers are the same?

Yes, and it costs nothing. An explicit uniform table removes the possibility that "6oz" was read as a maximum rather than a value for every layer. It also gives the returned quote something specific to be attached to.

What is a mixed-weight stackup?

A build where different layers carry different copper weights — for example heavy internal power layers with thin signal layers elsewhere. It keeps board thickness down and allows finer features on the thin layers, at the cost of maintaining two sets of design rules inside one board.

Why does the heavy copper page say 6mil minimum line width when capabilities says 2mil?

They describe different contexts. 2mil/2mil is the standard envelope; 6mil is published on the heavy copper page. Thicker copper takes longer to etch and the sidewall is attacked while the channel is still clearing, so fine geometry does not transfer from thin copper to thick copper. Enter line width per layer group accordingly.

Can I get the maximum copper weight, the maximum layer count and the minimum board thickness together?

Each is inside its published range on its own axis. Together they are a thickness arithmetic problem, because copper and dielectric both consume vertical space. Ask for written feasibility confirmation rather than assuming the endpoints combine.

How should I write the impedance requirement for a mixed-weight board?

Name the layers or nets that are controlled, give each group its target and tolerance, and state whether you require the verification record as a deliverable. Capabilities lists TDR coupon verification; a tolerance with no stated verification method is not checkable at receipt.

What do I send if my thickness target does not fit my layer table?

Send the constraint set and your priority order — which of thickness, layer count and copper weight is least negotiable — and ask what trade gets closest. That is a design conversation, not a quote request, and framing it that way gets a useful answer faster.

Do I need to specify etch compensation?

No. Etch compensation is the manufacturer's engineering decision. Your side of the line is the requirement: target width, which layer it applies to, and the tolerance you can accept.