PCB Manufacturing Specification: What to Include

Why a Gerber Set Is Not a Specification

A Gerber package describes the shape of each layer. It does not say how thick the board is, what the laminate is, how much copper is on each layer, what finish the pads carry, how tight the tolerances are or how the board is to be tested. Every one of those decisions changes the price and the performance, and if they are not written down, the fabricator makes them, using the cheapest option that meets the drawing.

The specification is the document that closes that gap. It is usually a fabrication drawing with a set of notes, and a well written one prevents the two most common causes of a rejected or disappointing board: a build that is technically compliant and functionally wrong.

The Stack

The specification begins with the layer stack, listed from the top to the bottom, with the function of each layer, the copper weight and the dielectric thickness between adjacent layers. Where impedance matters, the layer in which the impedance is referenced and the target value are stated in the stack rather than in a general note.

Two properties of the stack should always be stated. The finished board thickness with its tolerance, because it sets the mechanical fit and the impedance. And whether the stack is symmetrical, because an asymmetrical stack warps during lamination and again during reflow, and the fabricator needs to know whether that has been considered or whether it is free to balance it.

Materials

Name the laminate and, where it matters, the grade: the glass transition temperature, the dielectric constant, the dissipation factor, the flame rating and the tracking index. Where a specific manufacturer is not required, state the properties instead and let the fabricator propose a material, but require that the proposal is confirmed in writing before production.

Where a speciality material is used on some layers and ordinary laminate on others, say so and specify which is which. A hybrid stack is where a substitution is most likely to be made without asking, because the laminate is expensive and the temptation is real.

State the solder mask type and colour, the legend colour and, where the mask over a particular feature matters, the mask expansion. For a flexible or rigid flex board, state the coverlay material, the stiffener material and thickness, and the adhesive type.

Geometry and Tolerances

• Minimum trace width and spacing for each copper weight, expressed as what the design actually uses rather than as the plant capability.
• Minimum annular ring and the smallest hole, again as designed.
• Board outline tolerance and the outline method, whether routed, punched, scored or laser cut.
• Drill tolerances and the finished hole size for component holes where the lead fit matters.
• Registration tolerance between layers, which becomes the governing number on a fine line design.
• Bow and twist limits, which matter for any board that is placed in a connector or a housing.

Surface Finish

Name the finish and, where it matters, the thickness. Hot air levelling is the cheapest and is adequate for a through-hole board; electroless nickel immersion gold is used for fine pitch, for wire bonding and for flatness; immersion silver and immersion tin are alternatives with their own characteristics. Where a selective finish is required, state which areas receive which finish.

A finish note should also say whether the board will be reflowed more than once, because that determines whether an organic coating is appropriate or whether a metallic finish is needed for the second pass.

Electrical Requirements

Where the board carries controlled impedance traces, the specification states the target impedance, the tolerance and the layer stack reference, and requires a coupon to be built on the production panel and measured. The measurement result is part of the delivered data, not a separate request.

Where the board has a high voltage section, state the dielectric withstand voltage that must be met and the test method. Where the board is a radio board, state the insertion loss target if it matters. Where the board has a specific isolation requirement between the circuit and a metal base, state the voltage.

Our notes on PCB design and layout cover the design decisions that lead to these requirements and our notes on PCB capabilities describe the process window they have to fit inside.

Panelisation and Tooling

State whether the fabricator may panelise the design freely or whether the panel layout is defined, which matters where a specific depanelisation method or a specific panel size is required. State the tooling hole and fiducial requirements, the minimum border width, and the method of separation: routing, scoring or a punched edge.

Where the board goes through an assembly line, the panel has to carry fiducials for the placement machine and tooling holes for the conveyor. Those features belong in the specification rather than in a conversation after the first panel arrives.

Quality Class, Inspection and Test

State the acceptability standard, usually an IPC class, and the acceptance criteria that go with it for the conductor width, the annular ring, the plating thickness and the hole wall. State the minimum copper thickness in the holes and the measurement method, because the barrel plating is what a thermal cycle tests.

State the test regime: electrical test of every board or of a sample, the test voltage, the continuity criteria, and whether the boards are tested on the panel or as individual pieces. State whether a microsection, an impedance coupon report or a solderability test is required with the shipment.

The inspection requirements are what turn a specification into a contract. A requirement to supply a microsection with the order is enforceable; a requirement that the board should be good is not. Our notes on quality management describe the controls that sit behind these requirements, and our notes on PCB manufacturing describe the process.

Documentation and Deliverables

• Gerber files for every copper, mask, legend and paste layer, in a defined format and units.
• Drill file in the standard format, with the plated and non-plated holes separated.
• Fabrication drawing with the stack, the notes, the tolerances and the finish.
• Assembly drawing where the board is supplied with components.
• Panel drawing where the panelisation is specified.
• Test and inspection data as required by the specification: the coupon report, the microsection or the electrical test record.

It is worth stating the revision of the design and the revision of the specification in every document, and worth requiring that any change proposed by the fabricator is submitted for approval rather than implemented. A substitution of a laminate, a mask colour or a finish can look trivial and change the impedance, the solderability or the appearance of the product.

Common Omissions

• The copper weight of the inner layers, which is often left at the default and is then not what the design assumed for the current.
• The finish on the specific areas where a selective finish is needed.
• The impedance tolerance, given as a target with no permitted deviation.
• The maximum bow and twist, which is discovered when the board will not sit in its connector.
• The mask expansion over fine pitch pads, which decides whether the assembly produces bridges.
• The test voltage and the isolation requirement on a board with a mains or high voltage section.

FAQ

What is the difference between a Gerber file and a fabrication drawing? The Gerber files define the geometry of each layer. The drawing defines everything else: the materials, the stack, the tolerances, the finish and the test.

How detailed should the specification be? Detailed enough that a fabricator could build the board without asking a question and without making a decision that changes its performance or its price.

Should the specification name a specific laminate brand? Only where it is a requirement. Otherwise state the properties, and require the proposed material to be confirmed in writing.

What should accompany the first order? An impedance coupon report where impedance is controlled, a microsection where the copper thickness matters, and the electrical test record.

Conclusion

A fabrication specification is the document that turns a set of drawings into a contract. State the stack and the materials, the geometry and the tolerances, the finish and the electrical requirements, the panelisation, the quality class and the test regime, and the documents that must accompany the shipment. Require any proposed change to be approved in writing, and ask for the measurement data with the first order rather than trusting that the notes were followed. Our notes on PCB assembly cover the equivalent specification for the assembly stage.