Fabrication Drawing Notes That Prevent Substitution

An engineering drawing is a communication, and most of the cost it creates comes from what it does not say. A note that leaves a value to the shop is a value that will be chosen for the shop rather than for the design.

What the Drawing Should Fix

The drawing should fix the values that the design depends on and leave free the ones that do not matter. Fixing everything is expensive; fixing nothing produces a board that cannot be compared with the design.

The distinction is the tolerance. A dimension that matters needs a tolerance; one that does not should be left to the general note rather than given a tight one. Our manufacturability review notes describe how the tolerance is judged.

Common Omissions

The omissions that cause the most trouble are the finish, the mask and its expansion, the impedance requirement and its test structure, the material and its glass transition, the stackup and the panel requirement.

Each of those is a value that the fabricator will otherwise default. The default is chosen for the process, and it may be a lower grade than the design assumed.

Stackup and Its Presentation

The stackup should be drawn rather than described, with the material, the dielectric thickness and the copper weight of every layer, and with the total thickness and its tolerance.

A stackup that is drawn lets the fabricator price the board against the materials that will actually be used, and it removes the substitution that otherwise appears at quotation. Our layer count notes describe how the arrangement is chosen.

Impedance Requirements

Where impedance is controlled, the drawing should state the target, the tolerance and the layer, and it should include the test structure on the coupon.

A requirement expressed only as a net class in the design file is not visible to the fabricator, and it will not be measured. Our test coupon notes describe the structure that carries the measurement.

Notes That Are Read as Instructions

A note should state a requirement rather than a method. A note that says how to do something will be followed literally, and it becomes wrong as soon as the process changes.

Where a method is genuinely required, it should be stated as such and the reason given, so that a later change can be evaluated rather than blocked.

Revision and Change Recording

The drawing should carry a revision and a history, and the history should state what changed rather than only that something did.

Where the same part number is built to two revisions at once, the cut-in should be recorded on the drawing so that a board can be identified from the drawing alone. Our change control notes describe the cut-in.

Verification

The drawing should be reviewed against the assembly drawing and the bill of materials before release, because the three are read by different people at different times.

Where a question is raised during production and the answer is not on the drawing, the drawing should be updated rather than the answer being given by message. Our fabrication notes notes list the attributes that should be stated.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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.

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.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. 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.

FAQ

Is a general tolerance note enough? It covers the dimensions that do not have a specific requirement. It does not cover the attributes a fabricator needs in order to build the board the design simulated.

Should the drawing state the process? It should state the requirements. Where a process is required, it should be named together with the reason.

What does gopcb provide for drawing review? We provide a check of the attributes that affect the build, a drawn stackup with materials and tolerances, impedance targets with coupon structures, mask and finish specification, revision and change history with the cut-in, and an update route when a question is answered during production.