Silkscreen Legibility Rules for Densely Packed Boards

Silkscreen is the part of a board that people read when something has gone wrong. A technician repairing a dense assembly needs to find R47 among a hundred passives, and the legend is what makes that possible. On tightly packed boards every millimetre of legend competes with copper, mask and pads, so legibility has to be designed rather than assumed. This article covers the width, height and placement rules that keep a legend readable after the ink has been printed.

Why Legend Quality Matters More on Dense Boards

On a simple board a slightly thin character still reads. On a dense board the same character sits near a pad edge, and the combination of a narrow stroke and low contrast turns it into a smudge. The cost appears later, when assembly, test and repair all take longer because the reference designator cannot be resolved quickly.

Legibility also affects safety and compliance. Polarity marks, pin one indicators and connector labels are read under time pressure, and an ambiguous mark is a genuine source of assembly error rather than an inconvenience.

Minimum Line Width and Text Height

The stroke width of a legend character is the single most important variable. Below about 0.15 mm the ink tends to spread or break up depending on the printing method, and anything under roughly 0.12 mm should be treated as a special request rather than a default. Text height should be at least 0.8 mm for comfortable reading, with 1.0 mm preferred where space allows.

Character spacing follows from the stroke. A practical rule is that the gap between characters should be at least the stroke width, and the gap between lines at least one and a half times the text height. Squeezing those gaps produces a block of ink that looks like a bar code rather than a label. A practical test is to print the legend at final size and read it from arm’s length before release.

Always verify the minimum against the fabricator’s process capability, because the achievable stroke depends on the printer, the ink and the surface it is printed on.

Character Shapes That Stay Readable

Some typefaces survive small sizes better than others. Simple sans-serif characters with open counters and uniform strokes read best, while decorative fonts and serif detail fill in and merge. Numbers such as six, eight and nine are the usual first casualties and deserve particular attention.

The artwork should also avoid very small strokes that taper, and it should keep the same stroke width throughout a character. A font that looks crisp on a monitor at fifty percent zoom may be unreadable at production size, so judge it at final size on a printed sample.

Contrast Between Ink and Solder Mask

Contrast is produced by the difference in tone between the legend ink and the mask beneath it. White ink on a green, blue or black mask gives strong contrast, while white on a light grey or yellow mask is weak. Matt finishes generally read better than glossy ones because they scatter light instead of reflecting the inspection lamp.

Contrast also changes with the surface. Legend printed over copper sits on a slightly different level than legend printed over bare laminate, and where the process prints over both, the ink film may vary in thickness and therefore in apparent tone.

Placing Reference Designators Without Clutter

Reference designators should sit close to the component they identify, consistently oriented, and outside the keep-out around the pads. A designer who places every designator in the same corner of the board creates a legend that is complete and useless, because finding the part requires a map of its own.

Reading direction matters as well. Where the manufacturing flow defines a preferred orientation, align the text so that most designators are read from the same direction. Mixed orientation forces the inspector to rotate the board mentally on every line.

Keep-Outs Above Pads and Test Points

Legend must not print on pads, and it should stay clear of the solder paste printing area and the stencil aperture. Printing across a pad contaminates the joint and can cause a solder ball or a void, and the ink is usually removed by the soldering process anyway. The same applies near the board edge, where routing can expose text that was meant to stay on the panel.

Test points, fiducials and via holes all need their own clearance. A designator printed partly over a test point is a common defect that reduces probe contact and, in the worst case, blocks the probe entirely. Define these keep-outs in the design rules so the layout tool enforces them.

Inkjet Printing Versus Screen Printing

Screen printing produces a thick, opaque legend and handles fine strokes well, but it needs a screen for each design and it struggles with the height differences introduced by heavy copper. Inkjet printing writes the legend directly from the data, so it needs no tooling and handles small batches economically.

Inkjet ink is thinner and may appear less opaque, and its adhesion depends on the cure that follows. Both methods can produce a good legend; the choice depends on volume, on the copper topography and on whether the design is likely to change between builds.

Registration Tolerance for Legend

Legend is printed with a registration tolerance of its own, and that tolerance has to be smaller than the clearance between the text and any feature it must avoid. If the pad clearance is 0.1 mm and the legend registration tolerance is also 0.1 mm, the design has no margin and will occasionally print onto the pad.

Where tolerance is tight, identify the critical characters and give them extra room rather than reducing the stroke width. A slightly larger text block costs nothing, while an unreadable mark costs time at every stage that follows.

Registration should be verified on the first article and checked periodically, since print registration drifts with screen tension and with machine setup.

Reviewing the Legend Before Release

Legend review belongs in the same quality sign-off as the copper and the mask. Print the silkscreen layer at one to one on paper, place it over the assembly drawing, and check that every designator is present, readable, correctly placed and clear of pads. It takes minutes and catches problems that would otherwise be found on the line.

gopcb includes legend legibility in its fabrication review because a legend that cannot be read becomes a fabrication complaint later. Where the design is too dense for a readable legend, the answer is usually to move or remove non-essential text rather than to shrink the strokes further. Any text that survives that test is doing real work; the rest is decoration.

FAQ

What is the minimum readable silkscreen line width? Around 0.15 mm is a practical production minimum for most methods, and 0.12 mm should be treated as a special request. Ask the fabricator what their process can hold reliably.

Can silkscreen be printed over pads? No. Ink on a pad interferes with paste printing and soldering and is usually removed during soldering anyway. Keep legend clear of pads, test points and fiducials by the design rule clearance.

Does silkscreen affect electrical performance? It should not, and any legend that crosses exposed copper creates a risk it will. Treat the legend as a printed feature that must obey clearance rules like any other layer.