PCB Silkscreen: Design Rules and Ink Selection

The silkscreen is the last process step on most boards and the one that receives the least design attention, which is why it is also a frequent source of manufacturing complaints. Ink printed over a pad ruins solderability, text that is too small to read is useless to the operator, and a legend that disappears after a few thermal cycles was never properly cured. A PCB silkscreen is a functional layer, not decoration.

What the Legend Carries

The legend identifies components and their orientation, marks connectors and test points, shows polarity and pin one, and often carries the board part number, revision and a warning or certification mark. It is the first thing an assembly operator, a test technician and a service engineer look at.

That makes readability a requirement rather than a preference. The information has to be legible at the magnification the operator actually uses, and it has to be positioned where it can be read once the component is fitted, not underneath it.

Text Height and Line Width

Minimum text height is usually specified by the fabricator and is typically around zero point eight millimetres, with one millimetre preferred where the space allows. Text below that limit becomes a suggestion on the finished board, because the printing resolution and the ink flow close the gaps in the characters.

Line width follows the same logic. A stroke of a hundred and fifty micrometres is comfortable, a hundred is workable, and below that the characters fill in. Bold text is not simply thicker text; it is a different font weight that the screen printer has to be able to reproduce with consistent resolution.

Clearance to Pads and Mask Openings

Ink must not print on a solderable pad. A pad contaminated with silkscreen ink wets poorly and the joint forms badly, and the fault is intermittent because it depends on how much ink reached the pad. The design rule is a clearance from the pad edge, typically a hundred micrometres, measured to the mask opening rather than to the copper.

The reason is that the mask opening is larger than the pad, and the ink is printed on top of the mask. Ink on the mask near the opening can flow into it during curing, so the clearance grows as the opening grows. Where space is tight, the reference designator should be moved rather than the clearance reduced.

Ink Types and Curing

Silkscreen ink is a pigmented epoxy or a similar resin, printed through a screen and then cured. Thermal curing produces the most durable film, while ultraviolet curing is faster and suits high volume production, at some cost in adhesion and in resistance to high temperature.

Ink adhesion to the solder mask matters as much as adhesion to the board. Ink printed over a fully cured mask bonds to the mask rather than to the laminate, and a mask that is too smooth or contaminated gives the ink nothing to grip. The two layers should be specified as a system, since the interaction is described in the material on solder mask ink thixotropy.

Colour and Contrast

White ink on a green or blue mask gives the best contrast for inspection, which is why it is the most common combination. Black ink on a white mask is equally readable and is used on lighting boards where the reflector has to stay clean.

Contrast is the parameter that matters, not the colour itself. A dark ink on a dark mask may look elegant in a rendering and be unreadable on the bench, and any combination that lowers contrast will be worked around by the operators, usually by adding their own markings.

Placement and Assembly Usability

A reference designator should sit next to the component it names and should not be covered when the part is fitted. For a small component between taller neighbours, the sensible place is beside the part rather than above it, and the orientation mark should be adjacent to the polarity feature it identifies.

Pin one markers, connector numbering and test point labels save far more time than their cost, particularly on a board that will be debugged or repaired. Markings that help assembly include the direction of a connector opening, the location of the fiducials and the position of a switch that is hidden under a housing.

Traceability and Variable Data

Part number, revision, date code and serial number are commonly printed in the silkscreen. Variable data, such as a serial number or a batch code, is applied by inkjet or by laser rather than by screen printing, and the choice of method affects where on the board the marking can go.

Laser marking removes or carbonises mask rather than adding ink, and it needs an area where the removal will not expose a conductor. Inkjet marking adds a dot of ink and requires a compatible surface. Both need a defined area on the drawing, and both have resolution limits that should be confirmed before the text is specified.

Problems and Their Causes

Bridging between characters, filled in letters and missing strokes are the visible defects, and they come from ink viscosity, screen tension and squeegee parameters. Poor adhesion shows up later as a marking that lifts when tape is applied or when the board is cleaned.

Ink adhesion also degrades with the number of thermal excursions the assembly sees, so a board that is reflowed twice should be specified with an ink that is qualified for both passes. Ink on a pad is the defect with the worst consequences, because it affects the joint rather than the appearance. It is also the easiest to prevent, since it is a layout decision. A design that leaves adequate clearance and does not place text in the shadow of a pad has no such defect regardless of how the printing process behaves.

Design Review Checklist

Before release, the legend should be checked for minimum text height and stroke width, clearance to every mask opening, readability after assembly, correct orientation marks and the presence of the part number and revision. Where the board carries high voltage, the required warning markings should be verified as well.

It is also worth printing a sample and reading it with the light and magnification the line actually uses, because a rendering on a screen hides exactly the defects that the printer produces. The wider set of quality characteristics that a board should satisfy is described in PCB design quality characteristics.

FAQ

Can text be printed on a pad if there is no alternative? It should not be. Ink on a pad reduces solderability and the joint quality becomes unpredictable. Moving the text or reducing the number of designators is the correct answer.

What is the smallest readable text? Around zero point eight millimetres in height with a stroke of about a hundred and fifty micrometres, assuming adequate contrast. Below that, legibility depends on the fabricator’s process rather than on the designer’s intent.

Does the ink colour affect anything else? Only the contrast and the appearance. The ink is not a functional electrical layer and cannot be relied on for insulation, as noted in the manufacturing guidance on PCB design guidelines for manufacturability.