Silkscreen and Solder Mask: What Each Layer Does
Two Different Layers
The solder mask and the silkscreen are both printed onto the surface of a board and they are often drawn on the same screen by a designer, but they do completely different jobs. The solder mask is a permanent polymer coating that covers the copper the board does not want soldered. The silkscreen, also called the legend, is ink printed on top to label the parts, show orientation and give reference designators. One is a functional coating that affects electrical and assembly behaviour, the other is documentation applied to the board, and confusing them leads to design rules that are applied to the wrong layer.
What the Solder Mask Does
The mask separates adjacent pads so that solder does not bridge between them, it protects the copper beneath from oxidation and handling damage, it prevents the solder from wicking along traces during wave and selective soldering, and it provides a defined area for the paste deposit to sit on. On a fine-pitch board the mask also acts as a dam that keeps the paste from spreading, which is why the mask dam width is a design rule with a process number behind it. The mask is applied over the whole board and removed from the pads, so the artwork is a negative: an opening in the mask file means copper exposed.
What the Silkscreen Does
The legend identifies the component positions, their orientation and the board reference. At assembly it tells the operator where each part goes, at inspection it allows a fault to be located, and in the field it lets a service engineer identify a test point or a connector. It contributes nothing to electrical function, and a board with no legend is perfectly functional, which is why legend rules are sometimes relaxed and then regretted: an assembly with an ambiguous orientation mark is a board that gets built wrong.
Clearance Between Legend and Pad
The mask opening is larger than the copper pad, and the legend has to be kept clear of both. If the ink overlaps a pad, it lands on solderable metal and interferes with the joint; if it sits too close to the mask opening, the printing tolerance can push it onto the pad. The rule that avoids this is to keep the legend clear of the mask opening by a margin that covers the printing registration error, typically in the region of 0.1 to 0.15 mm, and to keep the ink away from vias and test points that must remain clean. Where space is tight, the legend is moved rather than shrunk, because a line that is too thin disappears during printing and a line that is too dense fills in.
Mask Over Vias and Test Points
Vias are usually covered by mask, which prevents solder from being drawn into them and keeps the surface flat. A via that must be probed, however, needs to be open, and a via under a component pad needs the filling treatment described elsewhere. Test points need a mask opening slightly larger than the pad so that the probe can make contact without the mask interfering. Where a via is tented, the mask covers both ends and the barrel is sealed by the coating; where it is plugged, the mask is pushed into the hole. The two are different processes and the drawing should say which is required, because a tent is not a plug and vice versa.
Legend Readability at Assembly
The value of a legend is decided by how legible it is at the assembly station. Text that is too small, that is printed across a mask opening, or that is placed under a component body cannot be read. Orientation marks that are ambiguous, such as a dot that could be pin one or a polarity mark drawn on the wrong side, cause assembly errors. The practical rules are to keep text away from component bodies, to keep characters above the minimum the printer can reproduce, to place pin one markers where they will not be covered, and to verify the legend on a photograph of the assembled board rather than on the layout screen.
Design Rules
Specify mask expansion as a process number agreed with the fabricator rather than a default. Keep the legend clear of mask openings by the printing tolerance. Do not put legend on a pad, a test point or a via that must be probed. Do not use the mask to create a decorative pattern that produces narrow dams. Confirm whether vias are tented or plugged, and mark the ones that must remain open. Finally, check the legend after the mask is drawn, because a legend that was placed before the mask openings were finalised is a legend that will be printed onto metal.
FAQ
What is the difference between solder mask and silkscreen? Solder mask is a permanent coating that defines where solder may go; the silkscreen is printed ink that labels the board.
Does the silkscreen affect the circuit? No. It is documentation only, but it affects assembly accuracy and inspection, so its rules still matter.
How far should the legend stay from a pad? Enough to cover the printing tolerance, typically around 0.1 to 0.15 mm from the mask opening.
Should vias be covered? Usually yes, with mask, unless the via must be probed or soldered, in which case it needs an opening or a plug.
Can legend go on a test point? No. Ink on the pad interferes with the probe contact and with solderability.
Conclusion
The mask is a functional coating with process-driven rules, and the legend is documentation with legibility-driven rules; treating them separately is what keeps both correct. Set the mask expansion with the fabricator, keep the legend clear of every opening, state whether vias are tented or plugged, and check readability on the assembled board. Mask and legend capability is listed under PCB capabilities, the imaging steps are described in PCB manufacturing, and the clearance rules belong in PCB design and layout. Legend errors are usually caught during PCB assembly in 2026.
