Silkscreen: Design Rules and Process Limits

The legend on a board is not decoration. It is the only reliable way for an operator, a test technician, or a repair engineer to know which component goes where, how it is oriented, and which pin is the first one. A board without a legible legend can be assembled incorrectly by someone who has the schematic open, and a board with a misleading legend can be assembled incorrectly by someone who does not.

This article explains what the legend has to communicate, what sizes are legible, where the ink must not go, and how the printed legend is verified.

What The Legend Has To Do

The primary job is to identify each component position with a reference designator that matches the bill of materials. The second is to show orientation, which for a polarized component means a polarity mark, for an integrated circuit a first pin marker, and for a connector a pin numbering that follows the mating part. Without those marks, an assembly can be electrically correct at the schematic level and still be built with every diode reversed.

The third job is identification of the board itself. A part number, a revision, and a date code allow a board to be matched to its documentation after it has been separated from its paperwork, and a logo or a manufacturing mark identifies the source. Where the board will be potted, coated, or enclosed, the legend may be the only marking that survives, so its position has to be chosen with the final assembly in view.

Sizes And Legibility

Legibility is a function of the character height and of the stroke width, and both are limited by the printing process. A character height of about a millimetre with a stroke of about a sixth of that is a practical minimum for a standard silkscreen process, and smaller text is possible with a finer ink and a tighter process but is easily lost when the ink spreads or when the print is slightly heavy. A character that is too small is worse than no character, because it appears to be identifiable and is not.

The font matters as well. A simple sans serif style, or a stroke based vector font that the fabrication software can generate, prints more reliably than a filled or a decorative typeface at small sizes, because a filled character has thin sections that close up. Characters should be spaced so that the ink does not bridge between them, and a reference designator that has been rotated to fit should still read in the same direction as its neighbours.

Keepouts And Interference

Ink must never land on a solderable surface. A legend printed across a pad prevents the paste from wetting, which produces an open joint, and the defect is easy to create because the printing process and the mask opening are registered separately. The standard rule is that the legend clears a pad by a defined distance, and that distance has to be stated on the fabrication drawing rather than left to the supplier.

The same logic applies to every other functional surface. The legend should be kept off the gold fingers of an edge connector, off test points, off fiducials, off the areas where a stencil gasket seals, and off any surface that will be bonded or coated. It should also be kept off the board edge, where the router or the scoring wheel will remove it, and away from the areas that will be covered by a connector body, a shield can, or a heatsink, because a designator that is hidden after assembly has no value.

Designing For Readability

The first pin of a package should be marked in a way that remains visible after the component is placed, which usually means a dot or a bar outside the footprint rather than a mark under the body. The polarity of a diode or a capacitor should be indicated on the side where the operator can see it while placing the part, and a connector should have its pin one marked on the board rather than only in the drawing.

Text should follow the component it identifies. A designator placed at a distance, or in a position that suggests it belongs to a neighbouring part, is a source of error, and a designator that is rotated by ninety degrees relative to the component is difficult to associate with it. Where the component density is high, the answer is to group the designators in a consistent pattern rather than to shrink them further.

Process And Registration

The printing process has a registration tolerance, and the ink has a tendency to spread, so the drawn feature is not exactly the printed feature. The stroke width that the design uses should be at or above the supplier minimum, the clearance from a pad should allow for the registration error, and the ink type should suit the surface it is printed on and the processes it will see. An epoxy ink that is thermally cured resists reflow and cleaning better than a simple solvent based ink, and the choice matters where the legend must survive the assembly process intact.

Printing on both sides of the board is a separate operation, and the legend on the secondary side has to be considered during the layout rather than added afterwards. Where a board is printed with a varnish or with a mask over the ink, the appearance of the legend changes, and the contrast that made it readable may be lost.

Verifying The Legend

The check is a comparison between the assembly document and the printed board, performed on the first article and repeated when the design changes. It covers the presence of every designator, the polarity and pin one marks, the absence of ink on solderable and functional surfaces, and the legibility of the smallest characters at the magnification an operator will actually use.

A second check is performed after assembly, when the components are in place, because that is the state in which the legend has to be read. A designator that was legible on the bare board can end up under a component body or behind a connector, and the only way to find that is to look at the assembled unit. The geometry rules that keep the legend out of the pads are described under pad design standards, the process limits under manufacturable design guidelines, and the layout consequences under layout decisions that affect production.

FAQ

How small can silkscreen text be? About a millimetre of character height with a stroke of roughly a sixth of that is a practical limit for a standard process. Smaller text is possible but is easily lost to ink spread and registration error.

Can the legend be printed over a pad? No. Ink on a solderable surface prevents the paste from wetting and produces an open joint. The legend must clear every pad by the distance stated on the drawing.

Why does the pin one marking belong outside the footprint? Because a mark under the component body is hidden once the part is placed. The marking has to remain visible in the assembled state to be useful.