Prototype Order: PCB Prototype Checklist Before the Order Is Placed

A prototype order is small, quick and usually urgent, which is exactly why the details are skipped and the boards come back wrong. Working through a short checklist before the data is released costs minutes and prevents a second cycle.

Confirm the Data Package

The package includes the copper layers, the solder mask, the silkscreen, the drill file, the outline and the stackup drawing. Every one of those has to be exported from the same revision of the design.

Mixed revisions are the most common cause of a board that does not match its own documentation. Freeze the design, export everything once, and check the file list against the previous order.

Check the Units and the Format

Metric and imperial data look similar in a file browser and behave very differently on the line. Confirm the units, the number of decimal places and the coordinate format that the shop expects.

A drill file that is interpreted in the wrong units produces either a board with no holes or one with holes that no longer fit the pads. It is a mistake that wastes an entire order.

Stackup and Material

The stackup defines the layer order, the dielectric thickness and the copper weight. Specify it explicitly rather than leaving the shop to choose, because the default is selected for manufacturability rather than for impedance.

Where controlled impedance is needed, state the target, the tolerance and the reference plane for each group. Include the calculation or the field solver result so that the shop can verify the stackup before the panel is made.

Panel Design and Break Away

A small board cannot be assembled on its own, so it is placed in a panel with rails and break away tabs. The panel size, the rail width and the tab positions follow from the assembly machine.

Fiducials belong on the panel as well as on the individual board. Their position relative to the outline is more important than their number, and they have to be free of solder mask.

Tolerances That Matter

Not every dimension needs a tight tolerance. State the ones that matter, such as the finished hole size for a press fit part, the board thickness for a connector and the impedance for a high speed trace.

An over specified drawing raises the price without improving the product. A drawing that specifies nothing lets the process decide, and the process will choose the middle of its own window rather than the middle of the design window.

Surface Finish and Solder Mask

The surface finish follows from the assembly process and the storage time. A finish that is suitable for hand soldering may not be suitable for a fine pitch assembly, and one that is ideal for fine pitch may not survive long storage.

Solder mask colour has no electrical effect, but its thickness and its clearance around pads do. Where a mask dam between fine pitch pads is required, the width has to be stated and checked.

Silkscreen and Legibility

Silkscreen is printed at a limited resolution, so the minimum line width and the minimum character height are process limits. Text that is smaller than the limit will be illegible or missing.

Keep the silkscreen off pads and off the fiducials, and use it for the information assembly actually needs: the reference designator, the orientation mark and the revision.

Drill and Via Considerations

The drill table lists every hole size, the quantity and whether the hole is plated. A drill size that is not in the standard list adds a tooling step and a cost.

Via in pad, filled vias and plugged vias each require a different process, so they should be identified on the drawing rather than inferred from the copper layers.

Component and Mechanical Fit

Check the connectors, the mounting holes and the height limits against the enclosure before the boards are ordered. A prototype that does not fit its case wastes the schedule it was meant to save.

Where a part is a new package, verify the land pattern against the data sheet one more time. A footprint error found on the bench costs a revision, and a footprint error found during assembly costs the whole build.

Assembly Data

The assembly drawing, the bill of materials and the paste layer are part of the same release. The paste layer defines the stencil, so its apertures should be reviewed with the same care as the pads.

Mark the polarity of every polarised part on the silkscreen and in the assembly drawing. Most prototype failures that are not electrical are one component fitted the wrong way round.

Communication With the Shop

State the lead time requirement, the quantity and the inspection the board needs. Where the schedule is tight, ask which steps are on the critical path rather than assuming that everything can be compressed.

Raise questions before the order rather than after the coupon has been measured. A shop that knows the impedance target and the finish requirement will flag a conflict instead of producing the default.

Cost and Lead Time

Prototype cost is dominated by setup: tooling, engineering review and test. Increasing the quantity reduces the price per board because the setup is spread over more units.

Lead time depends on the layer count, the finish and the queue. A standard stackup with a standard finish moves fastest, and any deviation adds a step that may run once a day.

First Article Review

When the boards arrive, inspect them before assembling anything. Check the outline, the hole sizes, the plating, the finish and the mask registration against the drawing.

Measure the impedance coupon if the design has one, and keep the measurement with the design record. The data is what makes the next order predictable, and the approach follows the design and fabrication flow.

Records for the Next Revision

Record what was ordered: the stackup, the finish, the tolerance that mattered and the shop that produced it. A prototype is a measurement of the design as much as it is a working sample.

That record is what turns a second order into a repeat rather than a new project. Keeping the released data and the inspection result together is the same practice described in our notes on design quality.

Additional Considerations for This Build

Practical attention to fabrication data pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating fabrication data explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, stackup is the item that decides how the rest of the board is arranged. 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

How much data does a prototype order need? The complete fabrication set: copper, mask, silkscreen, drill, outline and stackup, exported from one frozen revision with the units stated.

Should the panel be designed by the shop? Often, yes, because the panel must fit their equipment. Where the designer provides one, the rail and tab dimensions should match the assembly requirement.

Is a tight tolerance always better? No. It raises the price and can reduce the yield. Specify tolerances only where the function requires them, and use the process default elsewhere.