Silkscreen Design Rules for Assembly and Repair

Silkscreen is the only part of the board that explains itself to a person. It carries the reference designators, the polarity marks, the connector pin numbering and the warnings, and it is usually the last item to be considered in the layout.

What the Legend Has to Do

The legend identifies the parts, shows the orientation of the parts that can be fitted the wrong way, and marks the points a technician needs to find under time pressure. Anything else is decoration.

The test of a good legend is whether a technician can assemble, test and repair the board without a drawing. If the drawing is needed for orientation, the legend has failed. Our quality notes describe how the finished board is judged.

Legibility Limits

The minimum line width and text height are set by the printing process, and they depend on the mask surface the text sits on. Text on bare laminate prints differently from text on a mask covered plane.

Below the limit the characters fill in, and the reference becomes a smudge. The limit should be taken from the shop that will print the board rather than from a general figure.

Placement of Reference Designators

The designator should be outside the component footprint and close enough that there is no ambiguity about which part it belongs to. A designator that sits under the part is invisible once the board is assembled.

Where several small parts are grouped, the designators should follow the same order as the placement data so that a missing part can be found by counting. Our fabrication notes notes list the attributes that should be stated.

Polarity and Pin One Marks

Polarised parts need a mark that is unambiguous at a glance and that cannot be confused with the designator beside it. A filled dot, a bar and a chamfered outline are all used, and the choice should be consistent across the board.

Connectors need pin numbers or at least pin one, because a reversed connector is the most common assembly error on a new design. Our component selection notes describe how orientation is defined.

Silkscreen Over Pads

Silkscreen must never print on a pad. Ink on a pad reduces the solderable area and it can prevent the joint from forming, and the problem is invisible in the legend drawing.

The layout tool should be checked for this before the data is released rather than after the first build. Where the space is insufficient, the text should be moved or shortened rather than trimmed to fit. Our inspection notes describe what the optical system can detect.

Other Markings

The board usually carries a part number, a revision and sometimes a date code, a serial number field and a warning. These should be placed so that they survive the assembly process and remain readable after coating.

A revision mark that is covered by a connector is a mark that will be read from the wrong board, which is how a wrong revision reaches a test bench. Our design release notes cover the marking requirement.

Verification

The legend is verified against the assembly drawing and against the physical board on the first article. The check looks for missing designators, ambiguous marks and ink on pads.

The technician who will use the board is the best judge of the legend, and the first article is the last opportunity to change it cheaply. Our fabrication notes describe how the result is recorded.

Process Control and Verification

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. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

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.

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

Should the silkscreen be removed under a component? The reference designator should be visible, so it should be moved rather than deleted. Where it must stay, it should not print on a pad.

Is a white legend always best? It gives the best contrast on most mask colours, but the finish matters too. A legend that sits on a bright finish is harder to read than one on a dark mask.

What does gopcb provide for silkscreen design? We provide line width and text height limits for the process, designator placement outside the footprint, polarity and pin one marking conventions, ink on pad checks, part number and revision marking placement, and first article verification of the legend.