Silkscreen is the layer of printed text and symbols on a PCB. It includes reference designators, polarity markings, pin 1 indicators, test points, logos, and other assembly information. It serves as a critical visual map for component placement, assembly, inspection, troubleshooting, testing, and field servicing.
Some silkscreen legends are highlighted in red in the image below.
It is generally applied over the semi-cured solder mask using epoxy or acrylic inks through screen printing or liquid photoimageable (LPI) processes. Modern high-density designs typically employ direct legend printing (DLP) with industrial inkjet printers to achieve high registration accuracy and fine font resolution.
Silkscreen should not overlap exposed copper, solder mask openings, or component pads, as this can interfere with soldering and reduce marking legibility.
DFM guidelines to ensure legible silkscreen
- Use a minimum text height of 25 mil, character width of 25 mil, and text line width of 3 to 5 mil.
- Maintain at least 5 mil clearance between the silkscreen, copper features, and mask openings to prevent ink contamination during assembly.
- Place component reference designators close to their associated footprints without obstructing pads or assembly features.
- Clearly indicate polarized components, pin 1 locations, connectors, and test points to reduce assembly errors.
- Align all text designations in a uniform direction, typically readable from left-to-right and bottom-to-top. This improves scannability for automated optical inspection (AOI) systems and assembly technicians.
- Verify that the silkscreen ink complies with low-outgassing standards, such as NASA SP-R-0022A, when designing boards for high-reliability applications. This helps prevent volatile condensable material accumulation on nearby optical components and other sensitive electronic equipment in aerospace and defense environments.
- Configure the silkscreen design rules to automatically clip any text that overlaps solder mask openings. This prevents contamination of solderable areas.
- Dedicate an area for secondary markings such as corporate logos, board revision numbers, manufacturing date codes, or regulatory certification symbols (e.g., CE, RoHS, UL).