Procurement Documentation: Specifications, Drawings and Supplier Dialogue
Most disagreements between a customer and a fabricator come from a document that is incomplete rather than from a process that is incapable. The specification is the interface, and it is worth more attention than it usually receives.
What the Fabrication Drawing Must Contain
The drawing must define the outline, the hole sizes and their tolerances, the layer stack, the copper weights, the surface finish, the mask and legend requirements, and the electrical requirements such as controlled impedance.
It must also state the standard the board is built to and the acceptance criteria that apply where the standard offers a choice.
A drawing that states only the outline and the finish leaves the rest to the fabricator’s default, and the default is chosen for manufacturability rather than for the design. Our fabrication notes checklist describes the full set.
Stating Tolerances Properly
A tolerance is only useful where it corresponds to something that can be measured. A requirement that a feature be held to an unmeasurable value costs money and produces a dispute.
Where a feature is critical, the tolerance should be stated and the measurement method agreed. Where it is not critical, a wider tolerance reduces the price without affecting the function.
The distinction between a requirement and a preference should be made explicit. A note that asks for a nominal value without a tolerance is a preference, and it will be treated as one. Our cost reduction notes describe the effect of the requirement on the price.
Data Package and Its Contents
The data package includes the artwork in a standard format, the drill file, the stack-up drawing and the fabrication drawing. Each must be at the same revision.
The artwork should be checked against the netlist before it is issued, since a fabricator builds what is in the file and does not verify the design.
The revision correspondence is a common failure: an artwork file from one revision with a drawing from another produces a board that matches neither. Our quality notes describe the incoming checks that catch it.
Asking the Right Questions
Where a requirement is at the edge of the process, the fabricator’s opinion is worth more than the quotation. A short question about the narrowest feature or the tightest registration often prevents a yield problem.
The question should be specific: what is the minimum annular ring that can be held on this stack, rather than whether the design is manufacturable.
The answers should be recorded, because the next revision of the same design will face the same questions.
Deviation and Concession
Where the fabricator cannot meet a requirement, the deviation should be raised before the boards are built rather than after. A deviation accepted in advance is a decision; the same deviation discovered later is a dispute.
The decision should be made against the function of the feature, not against the drawing. A registration that is outside the drawing tolerance may still be acceptable if the remaining ring is sufficient for the current.
Where the deviation is accepted, the record should state the deviation and the reason, so that a later review can see the basis. Our component selection notes describe the same principle applied to a part.
Qualification and Repeat Orders
The first order establishes the process and the second order should reproduce it. The documents that make that possible are the approved stack-up, the process notes and any deviation accepted.
Where the same design is ordered from a second fabricator, the documents are the specification and the second fabricator’s process is different. The design must be reviewed against that difference rather than assumed to transfer.
A repeat order that quietly changes a material or a finish is a change of product, and the record should show it.
Process Control and Verification
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
Verification and Records
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.
Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.
FAQ
Is a fabrication drawing needed for a simple board? Even a simple board needs an outline, a stack-up and a finish. The drawing is the only place those are stated unambiguously.
Can the requirements be given in an email? They can, and an email is not a controlled document and does not carry the revision.
What does gopcb provide for procurement documentation? We provide a fabrication drawing template with the requirements that matter, stack-up drawings, a review of the data package for revision consistency, specific answers on process limits, and a deviation process that records the decision before the boards are built.
