KEMI Docs · PCB practice

Silkscreen Rules — Text Size, Pad Clearance and Polarity Marks

Last updated: 2026-09-16

Silkscreen has no electrical effect, so it is handled last and least carefully — yet half of assembly mistakes start here: diode direction, connector pin 1, which designator belongs to which part. This article lists the smallest features a fab can actually print, and the marking conventions that keep the assembler from guessing.

Fab limits — what prints

Item	Safe value	Minimum (most fabs)
Line width	0.20 mm	0.15 mm
Text height	1.2 mm	1.0 mm (0.15 stroke)
Height / stroke ratio	6:1	5:1
Silk-to-pad clearance	0.20 mm	0.15 mm
Silk-to-board-edge	0.30 mm	0.20 mm

Minimums print, but smear or break. On the common green-mask/white-silk combination a 0.15 mm stroke is visibly faint. Design at the safe values, and when there is no room, delete the text rather than shrink it.

Silk on pads — why it gets clipped

Silk over exposed copper interferes with soldering, so fabs clip it unconditionally. The problem is what remains: a part outline with a bite taken out, which confuses orientation more than no outline at all. Keep silk at least 0.15 mm off pads and draw outlines outside the part, not between pads. For 0402 and smaller, drop the outline and keep only the designator.

Polarity and pin-1 conventions

Diodes/LEDs: a thick bar on the cathode side or a 'K'. Arrow symbols become unreadable at small sizes.
Electrolytic/tantalum capacitors: a single '+' is enough. Half-filled circle outlines vanish after reflow.
ICs: a dot (●) or a chamfered outline corner at pin 1. For QFN, put it in the same corner as the dot on the package.
Connectors: a '1' plus a triangle next to pin 1, oriented to be read from where the assembler stands (outside edge of the board).
Switches/jumpers: state in words — 'ON/OFF', 'BOOT0=1'. Symbols alone cannot be read on the bench.

Designator (R12, C34) placement rules

Next to its part, and in only two reading directions across the whole board (0° and 90°). 180°/270° text makes the assembler rotate the board.
Parts in a row get designators on the same side (e.g. all above). A designator wedged between two parts belongs to neither.
In dense areas, remove designators and rely on the assembly drawing. Unreadable designators are worse than none.
Bottom silk is mirrored automatically — it must read correctly when viewed from the bottom. If it looks reversed from the top, it is right.

Product information silk

Group board name, revision, date (YYMM) and maker logo in one corner. Without a revision you cannot tell a fixed board from an old one, and post-assembly fault tracing becomes impossible. Compliance marks (CE, KC) belong only on products that actually hold the certification.

KEMI's silk check

① Export Gerbers, show only the silk layer and look for pad overlap ② every polarised part marked ③ two-direction designator rule ④ revision present. Silk errors do not affect function, so DRC will not catch them — a human looks.

FAQ

Is black silk more readable?

Black on white mask reads well, but some fabs charge extra for mask/silk combinations. Green mask with white silk is the cheapest and most common.

Can I order without silkscreen?

Yes. The saving is tiny and the assembly-error risk is large, so it is not recommended for prototypes. Omit only on production boards that ship with an assembly drawing.

Designators only on the assembly drawing?

SMT houses work from the CPL and drawing, so that is fine. But repair and debugging slow down when the board carries no designators to match against the schematic.