PCB Fabrication Notes: What to Send With Your Gerbers

Most fabrication problems begin with a file that was complete in one respect and silent in another. The Gerber data describes the artwork, but it does not describe the material, the finish, the tolerance or the way the boards should be panelised. Those decisions live in the fabrication notes, and a design that ships without them is a design whose details will be guessed.

Why the Notes Are Part of the Design

Two suppliers can produce very different boards from the same Gerber set. The difference is not the artwork but the construction: the laminate, the copper weight, the surface finish and the tolerance. Where those are not specified, the supplier will choose the combination that is most economical to produce, which is a reasonable behaviour and not necessarily the one the designer intended.

A note that specifies the construction also gives the manufacturer something to check the design against. A stack-up that cannot be built with the specified copper weight, or a hole size that is outside the process capability for the specified thickness, is much easier to raise when the requirements are written down.

Material and Construction

The notes should state the base material, the material brand where the electrical performance depends on it, the board thickness and the copper weight of each layer. Copper weight deserves care because it changes the minimum trace width and spacing that can be produced, and because it determines the current-carrying capability the designer has assumed.

Where a controlled impedance is required, the notes should give the target impedance and identify the layers and the reference planes involved. The laminate supplier’s dielectric constant should be quoted, since the same nominal material from different sources can differ enough to move the impedance outside its tolerance.

Finish, Mask and Legend

The surface finish should be named rather than implied. A hot-air solder levelled finish, an electroless nickel immersion gold finish, an immersion silver or an organic protective coating all behave differently in assembly, in storage and in cost. Because the finish also interacts with the assembly process, it should be chosen with the soldering method in mind and recorded with the storage window that applies to it.

The solder mask colour and the legend colour should be specified, along with which layer of the data set represents the mask and which represents the legend. A datasheet sent without that mapping forces the manufacturer to infer the intent, which is one of the more common causes of a board arriving with the legend printed on the wrong side.

Layer Identification and Special Requirements

Where the data set contains layers that are not self-explanatory, the notes should explain what each one is for. This is particularly important for designs that include a mask layer used as a keep-out, a paste layer used for a stencil, or a mechanical layer that defines a cutout. If the deliverable is intended for turnkey assembly as well as fabrication, the assembly drawing and the bill of materials should be listed in the same note.

Any requirement that is not visible in the artwork belongs here: press-fit holes with a specified tolerance, a particular hole to be left unplated, a stiffener or a heat-spreading coin to be bonded into a cavity, or an edge to be bevelled.

Tolerances and Quantity

Board dimensions carry a tolerance, and the tolerance should be realistic. A requirement tighter than the process can hold reliably will either be quoted at a higher price or met inconsistently. The same applies to hole position and to the registration between layers: a stated tolerance tells the supplier which process route to use, and it also tells the designer what the design can rely on.

The order quantity should be stated together with whether the boards are required as individual pieces or in a panel. For a prototype, the quantity of panels matters as much as the number of boards, because a small panel may contain only a few units.

Panelisation and Breakaway Tabs

Panelisation is a manufacturing decision with electrical consequences, so the requirements should be stated rather than assumed. The notes should say whether the boards are to be delivered as a panel, whether a working border is required for the assembly equipment, and how the units should be separated. A design that will be assembled on an automated line needs a panel whose dimensions and fiducials suit the machine, and asking for a panel without specifying that is likely to produce a panel that is convenient to fabricate and awkward to assemble.

Where the boards are unusual in shape, panelisation also determines how efficiently the base material is used, which affects both the price and the delivery time.

Delivery and Special Handling

Finally, record the delivery date and any handling requirement. A board with an organic finish has a shorter storage window than one with a metallic finish, and a board that will be assembled months after fabrication may need a different finish or a specified packaging arrangement. Stating the assembly date lets the supplier propose a construction that will still be solderable when the boards are used.

Our design release checklist covers the content of the fabrication note, the HASL surface finish material explains one common finish in detail, and the cost reduction notes describe which of these decisions change the price.

A Short Note Is Better Than a Long One

The list of requirements in a fabrication note is finite, and a single page is normally enough to cover it. What matters is that each item is unambiguous: a material name rather than a category, a finish named explicitly rather than implied, a tolerance with a value, and a quantity with the unit stated. A note that raises questions costs a day in email correspondence; a note that answers them lets the order go straight into the process.

Where the same product is ordered more than once, keeping the note as a controlled document makes the reorder simpler and prevents a specification from drifting between builds. That habit also makes it visible when a requirement changes, which is useful when the change affects the price or the lead time.

FAQ

Can the fabrication notes be included in the design file itself instead of a separate document? Yes, and for many projects it is the better choice, because a note embedded in the file cannot be separated from the data. Whatever the medium, the content is what matters, and the same list of requirements should be present. A short text file sent alongside the archive is the most common approach because it is easy to read and easy to revise.

Which tolerance should be quoted for hole position? The tolerance the design actually needs, which is normally driven by the smallest component pitch and the most demanding connector. Quoting a tighter tolerance than required raises the cost without improving the product, while quoting a looser one may produce boards that cannot be assembled. Where the design is not sensitive, a standard tolerance is the appropriate answer.

What does gopcb do when a note is silent? We ask, or we state the assumption in the quotation so that the customer can correct it before the panels are released. Almost every fabrication dispute we see traces back to a requirement that was neither specified nor asked about, so a short note that covers the ten points above removes the ambiguity at the point where it is cheapest to resolve.