The silkscreen layer is a text layer at the top layer of a PCB and it's generally used for annotation. The core principle behind a successful silkscreen layer is: Make it look beautiful, leave no room for ambiguity.
The beautiful part is especially true for products that will be sold as standalone PCBs like Arduino shields etc. The ambiguity part is especially important for devices that have hand-soldered components. In the absence of a pick and place machine, you should have on-PCB guidance for the worker to correctly identify components and polarity.
Here's our checklist for a great silkscreen design based on our years of experience manufacturing PCBs for hardware startups worldwide:
Only 2 reading directions shall be used for silkscreen instead of 4 directions for easy reading. Generally speaking, the layout of the silkscreen should be from left to right and from bottom to top. The layout of the silkscreen shall be tidy and clear.
Try to use the chip in the same direction as the reading direction: the first character of the horizontal component is on the left, and the first character of the vertical component is on the bottom.
Common font width and sizes are 4/20mil, 5/25mil, 6/30mil. The font size should be selected by the space availability of the PCB, and the distance between silkscreen elements should be greater than 5mil.
When fonts are being chosen, the result should be confirmed according to the font width and size to facilitate the reading.
The silkscreen text should be placed close to the corresponding elements to identify them clearly. If the component density is high, silkscreen could be placed inside the elements, or boxes and arrows can be used to indicate which component the text belongs to.
Keep in mind that placing silkscreen text under the components will deem them invisible once the components are soldered. If your PCB requires high serviceability, you should try using this method
In addition to components, there should be some information about the mounting hole position and dimensions. In case your PCB is screw mounted, information regarding the type and length of the screw should also be noted.
Indicate polarity such as signal direction outside the component area. You may also add descriptions about the component's role. If the polarity mark is unclear, additional signs could be added, as this is one of the most common reasons for part failure.
Electrolytic capacitors, diodes, and other polarized components should be oriented in the same direction within each functional unit as much as possible to reduce human error in manufacturing and quality control.
There should be no silkscreen markings on top of a pad or exposed part. Doing so will reduce the solderability of the pad and will reduce readability alike.
No silkscreen should be present on top of high-speed signal cables to prevent it from negatively affecting the impedance of the cables.
If there are pads enclosed by packaged components such as BGA or QFN chips, the silkscreen should have at least 6mil clearance from the pads.
Connector (such as USB) directions should be clearly indicated on the silkscreen and pin numbers should also be present
Board name, manufacturing date, version number, and other identifying information is non-negotiable. Place them at the corner, with enough clearance from the edge of the board and other components.
If the space allows it, you can add your company logo, tracking barcodes, QR codes, anti-static markings, etc..
Quality verification signs should be added according to related requirements and warning signs for high temperature or pressure should also be added according to your needs.