Silkscreen Legend: Preparation, Placement and Process Control

The silkscreen legend is the only documentation that stays with the board after it leaves the design office. It tells the assembly operator where each part goes, it tells the inspector what the board should look like, and it tells the repair technician which component is which.

It is also the layer that is treated as cosmetic in most projects and printed last, which is why legends are frequently unreadable, misplaced or missing on the parts that matter most. Treating it as a functional layer costs nothing and saves time in every subsequent operation.

What the Legend Is For

The primary purpose is identification. Every component needs a reference designator printed close enough to the part that the association is unambiguous, and clear enough to be read at the magnification the operator actually uses.

The secondary purposes follow: a polarity marking for the parts that can be fitted the wrong way round, a pin one indicator for a connector, and a boundary that shows where a shielded or a potted area begins. Each of these prevents a specific error.

Reference Designator Placement

The designator belongs beside the component, not underneath it, and on the side that will be visible when the board is being assembled. A designator that disappears under the body is useless precisely when it is needed.

Where the density makes that impossible, the designator can be placed at the end of a short line that points to the part, or grouped with its neighbours in a block that is printed in a clear area. What should not happen is for the designator to be moved somewhere convenient and left unconnected to the part it identifies.

Polarity Marking and Pin One

A polarity marking has to be unambiguous at a glance. A bar, a dot or a plus sign placed next to the correct end of a component works; a small mark placed between two components does not.

For a package with a pin one convention, the marking should be placed at the corner where the numbering begins and should be distinct from the marking used for polarity, so that the two cannot be confused on a board that carries both types of part.

Clearance from Pads and Features

The legend is printed on the solder mask, and the clearance rule is what keeps it off the pads. Ink on a pad prevents the solder from wetting, which produces an open joint that cannot be seen after assembly.

The clearance between the legend and the mask openings should be checked by the design rule set, using the minimum the fabricator can print, because the fabrication process will remove or clip any legend that overlaps an opening in the mask. A legend that was drawn across a pad will simply not be there on the finished board, and the designator that was meant to identify a part will be missing.

Reading Direction and Consistency

A reading direction convention keeps the legend legible whatever the orientation of the board. The usual rule is that designators are readable from one side of the board, or from the bottom edge of the panel, and that rotated text is avoided.

Consistency matters more than the specific convention. A board where some designators read left to right and others are rotated by ninety degrees forces the operator to turn the board, and turning the board is how components get placed on the wrong side.

Font Size and Line Width

The font size and the line width determine whether the legend can be printed at all. A text height below the process minimum will produce an unreadable smudge, and a line width that is too thin will break up during printing.

The limits come from the fabricator and should be in the design rules. Where the density forces text below the minimum, the honest answer is to move the designator to a clear area rather than to print a mark that cannot be read.

Legend on Both Sides and Panel Markings

A double sided assembly needs a legend on both sides, and each side should identify its own components. A designator that refers to a part on the other side of the board is a source of confusion, and the convention should be written down.

Panel level markings are equally useful: the product name, the revision, the date code and the panel identifier make the board traceable through the line. Serialisation, where it is required, is usually printed or laser marked at this stage, and the position has to be clear of the assembly area.

Legend and Test and Coating Requirements

A legend that covers a test point or a fiducial defeats the purpose of both. The mask openings for those features have to be kept clear, and the rule set should treat them as prohibited areas for the legend layer.

Where the board will be coated or potted, the legend has to remain legible through the material, or the identification has to be applied in another way. A dark coating over a white legend can leave the board unidentifiable, which is a problem for a product that has to be serviced.

Common Legend Defects and How to Avoid Them

The defects are predictable: text on a pad, text clipped by a mask opening, designators that have been moved and no longer point at anything, and characters that are too small to print. All of them are visible in a plot of the legend layer against the mask layer.

Reviewing that plot before release is a five minute exercise that removes a class of problems, and it is worth doing again whenever a component is moved or replaced on the layout. The same review is part of the general check of the data described for pad and mask design, and it belongs with the requirements recorded for the fabrication package.

Legend in the Assembly Documentation

The legend is the visible part of the documentation, and it should agree with the drawings that accompany the build. Where the assembly drawing shows a component in one orientation and the legend suggests another, the operator has to choose, and the choice is not always the right one.

Checking the legend against the assembly drawing and the bill of materials before release is part of the general review described for manufacturable design. The three documents describe the same assembly, and they should not contradict each other on the side that is read by a person under time pressure.

Process Control and Verification

On a design of this kind, reference designator is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Can the legend be removed to save cost? It is a separate printing step on some processes, but the saving is small and the cost of a missing identification in assembly or repair is much larger.

Does the legend affect the electrical performance? No, provided it stays off the pads and the mask openings. Ink on a pad is a manufacturing defect, not an electrical one.

Where should the board revision be printed? In a clear area away from the assembly, visible from the side that faces the operator during inspection, and large enough to read without magnification.