PCB Silkscreen: Process, Materials and Design Rules

The PCB silkscreen is the printed layer that carries reference designators, polarity marks, connector pin numbering and the board revision. It is the least electrically significant layer on the board and the one most often read by the people who assemble, test and repair it.

Because it is printed rather than etched, its limits are different from copper. Understanding the process explains why small text fills in, why a silkscreen over a pad is a defect and why legend placement is a design decision rather than an afterthought.

What the Legend Is For

The legend exists for assembly and service. A reference designator next to a component lets an operator verify a placement, and a polarity mark prevents a reversed capacitor or diode. Pin one markers on connectors are among the most valuable marks on any board.

It also carries information that has no other home: the board revision, the assembly variant, the date code if required and the manufacturer identification. Those marks are the first thing consulted when a production question arises.

Screen Printing

Screen printing is the traditional method. A mesh screen is coated with emulsion and imaged so that ink passes only where the legend should appear. Ink is pushed through the open areas with a squeegee, then cured.

Resolution is limited by the mesh count and by the ink rheology. Fine text below about 0.15 mm line width becomes unreliable, and the ink tends to spread, filling in the counters of letters and closing gaps between characters.

Screen printing remains the standard for volume production because it is fast, durable and inexpensive per panel. Its limitation is that every design change requires a new screen, which matters for prototypes and for frequent revisions.

Inkjet and Direct Legend Printing

Inkjet legend printing deposits ink directly, driven by the same data used for the copper layers, so no screen is required. It is well suited to prototypes and to small batches where the cost of a screen is disproportionate.

Resolution is comparable to screen printing for practical designs, and modern machines reach finer detail than a coarse mesh screen. The main limitations are ink adhesion and cure, which must be controlled to survive assembly and handling.

Because the legend is generated from the layout database, inkjet printing eliminates the film and screen preparation step entirely. That shortens the fabrication cycle for a revision and removes one opportunity for a tooling error.

Ink Chemistry and Curing

Epoxy ink is the most common legend material. It is a two-part or thermally cured system that adheres to solder mask and resists cleaning and handling. Cure temperature and time determine adhesion and chemical resistance, and an undercured legend wipes off during assembly.

UV-curable inks offer faster processing but may require a thermal post-cure to reach full adhesion and solvent resistance. Whichever system is used, the legend is applied after solder mask curing and before surface finish in most flows, although the order varies with the fabricator.

Colour is largely a cosmetic choice, with white standard on a green mask and black common on white or light masks. Contrast between legend and mask determines whether automated optical inspection can read the marks.

Resolution and Legibility Limits

Practical minimums for legend are a line width around 0.15 mm with a character height of at least 1 mm. Below that, the risk of an unreadable mark rises quickly and the legend becomes decoration rather than documentation.

Character stroke width and spacing both matter. A single-pixel font rendered with a hairline stroke may look precise on screen and print as a broken line. Bold, simple strokes survive the printing process and remain legible after the assembly process adds flux and heat.

Keep text orientation consistent across the board, typically readable from one direction, so that an inspector does not have to rotate the assembly to read a designator.

Design Rules for Legend

Never print legend on a pad. The ink reduces solderability, and the joint that forms over it is unreliable. Legend should also stay clear of fiducials, test points and gold fingers, all of which have their own keep-out requirements.

Legend may be printed on vias only if they are tented or covered. Printing across a via that will be exposed creates an ink plug that can trap flux and interfere with cleaning.

Where space is tight, prioritise. Reference designators for the parts most likely to be misplaced, polarity marks and connector pin numbering should survive; a decorative border or a version note can be dropped.

Legend and Assembly Interaction

Legend and solder mask registration must be considered together. If the mask shifts and the legend does not, text can creep onto a pad even when the drawing shows clearance.

The assembly drawing and the legend should agree. A reference designator that differs between the two produces a question on the line, and questions on the line cost time on every order rather than once.

Reviewing Legend Before Release

Check the legend layer at the same scale and in the same orientation as the assembly drawing. Verify that every component has a visible designator after assembly, that polarity is unambiguous, and that no text sits on a pad, a fiducial or a test point.

Then check the revision. A board with an obsolete revision mark in the legend is worse than a board with no mark at all, because it will be identified incorrectly during a production investigation.

Further reading: board outline and mounting design, placement order and pad positioning, and PCB circuit board explained.

Cost and Turnaround Considerations

Screen printing has a fixed tooling cost that is spread across the panel, so it becomes inexpensive at volume and expensive for a single prototype. Inkjet legend has no tooling and is therefore cheaper for small quantities and revisions.

Legend colour and the number of colours affect cost as well. A second colour requires a second screen or a second print pass, and the benefit is usually cosmetic rather than functional.

Where a design changes frequently, direct printing removes the need to remake a screen on every revision, which shortens the fabrication cycle and eliminates an entire class of tooling error.

Prototype Versus Production Legend

Prototype boards often carry a simplified legend because the layout is still in flux. That is acceptable as long as the assembly drawing remains authoritative, since the designators on the board and the designators on the drawing must be the same set.

Production legend should be complete and reviewed against the bill of materials. A board that cannot be assembled by reference to its own markings forces the operator back to the drawing for every placement, which slows the line and increases the chance of a misload.

FAQ

What is the minimum legible text size? Around 1 mm character height with 0.15 mm stroke width for screen printing. Finer text is possible with inkjet but becomes fragile and difficult to read on an assembled board.

Can legend be printed over vias? Only if the vias are covered or tented. Printing across an exposed via leaves an ink plug that can trap flux and interfere with cleaning, and the mark itself will not be legible.

Does legend colour affect anything besides appearance? Yes. Contrast against the solder mask determines whether inspection systems and operators can read the marks reliably, which matters on boards with dense component placement.