Fabrication Notes and Drawing Requirements

Fabrication notes are the sentences on a drawing that carry the requirements the graphics cannot. They are short, they are read by the shop before the panel is made, and a missing one moves a decision from the designer to the process.

What the Graphics Already Say

The layer images carry the shape, the drill file carries the positions and the stack-up carries the build. Those three describe most of the board and they do not describe the intent behind it.

The intent is what the notes carry. Our fabrication notes notes describe the list that is normally used.

The Notes That Always Earn Their Place

The surface finish and its thickness, the solder mask type and colour, the finished thickness with its tolerance, the impedance requirement with its tolerance, the annular ring minimum and the array and singulation method.

Each of those answers a question the shop would otherwise answer itself, and each has a consequence if it is answered wrongly. Our plating thickness notes describe one of them in more detail.

A note is a requirement rather than an explanation: it names a figure, a standard or a method, and it does not describe why. A note that explains is a note that is read once and then skipped.

Where a value sits close to a process limit, the note should say so, because the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The notes that cause trouble are the ones that are ambiguous or that conflict with a general note elsewhere on the drawing. A specific note and a general note that disagree leave the shop to choose.

The second class is the note that refers to another document without naming the revision. A reference to a specification that has been updated is a reference to a requirement that may have changed.

The notes are reviewed with the graphics rather than separately, because a note that has been superseded by a change to the layout is a common source of error. The review is a read of the drawing as a whole.

A short review at release is cheaper than the first article that catches it. Our board quality notes describe how the finished board is checked against the notes.

A change to a note is a change to the product, and it is released with the same control as a change to a layer. The revision is carried on the drawing and on the fabrication panel identification.

Where the note changes and the stack-up does not, the two disagree and the shop follows the drawing. That is the reason the two are reviewed together.

Acceptance and Its Evidence

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. 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.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

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 an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure. The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

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. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Checks Before Release

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

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

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. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

Is a general note enough? It covers the default cases and it does not cover the requirements that are specific to the product.

Should notes repeat the drawing dimensions? They should not. A note that repeats a dimension creates a second source for the same figure.

What does gopcba provide for fabrication notes? We provide a standard list of requirements, notes written as figures and methods rather than explanations, a review of the notes with the graphics before release, a documented list of the assumptions we had to make, and change control that releases a note and a layer together.

Points to Confirm at First Article

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. 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.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.