PCB Silkscreen – is it really that easy?

PCB Silkscreen – is it really that easy?

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Screen printing contains human and machine-readable markings on the surface of the PCB that indicate the location and polarity of PCB components, test points, warning and standardized symbols, logos, and other information about the board. Screen printing uses a non-conductive ink, the standard color is white, but other colors are also available. Screen printing is applied to bare boards at the final manufacturing stage, primarily as a guide for board assembly, testing, and subsequent maintenance and repair.

What's wrong with silkscreen?

During data preparation for the PCB manufacturing process, the CAM engineer should consider the following objectives when treating the screen print layers:

l The screen printing should keep a sufficient distance from the solder resist gap.

l Screen printing should be readable.

The first aspect is critical for manufacturers. Since screen printing ink is non-conductive, it can damage the quality of the electrical contacts when it strays into the solder mask gap by mistake, potentially leading to scrapped assembled boards if no defects are found during electrical testing. Therefore, to ensure the quality of the board, standard procedure in most manufacturing pre-production processes is to clamp the wire mesh to keep it a minimum distance from the solder mask gap, as shown in Figure 1. The required distance between the screen elements and the solder mask gap depends on the screen printing process and its accuracy, which may vary from manufacturer to manufacturer. Naturally, the silkscreen of the clips becomes less readable, in extreme cases - not at all.

In the example above, the silkscreen (blue) is close to the left side of the soldermask gap (green). On the right, the silkscreen is cropped away from the solder mask. Red indicates copper features.

However, even if the gap between the screen print and the solder mask is kept a safe distance, its readability can be affected by other factors, such as being close to the edge of the board, close to tools or mounting holes, or under components. All of these factors should be kept in mind when placing screen printed components on the board.

Last but not least: Different screen printing processes and machinery impose various constraints on minimum line width and character size. For example, liquid photographic imaging (LPI) processes are typically limited to four mil linewidths. There are techniques for printing finer lines, but they are more expensive and may not be available when your board is manufactured.

So what to do?

To ensure readability of the screen print, make sure that the screen print is placed a safe distance away from all other boards that may affect its readability (especially the solder mask clearance). At the same time, you should ensure that the line width of the screen printed components is suitable for the manufacturer's manufacturing capabilities, as well as the minimum height and width of the individual characters.