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A printed circuit board is used to power the electronic devices. However, they also serve as informational devices to electronic manufacturers. How? These printed circuit boards carry information pertaining to components and their uses. This is known as legend text or silkscreen text. Silkscreen text is usually printed on the solder mask of the printed circuit board and it is usually done towards the ending stage of printed circuit board manufacturing. This silkscreen printing is performed in a specific way to achieve the desired results. This post will help you gain an understanding on PCB legend in detail.An Introduction to PCB Legend
Although a legend doesn’t contribute to the performance of the PCB, it conveys important information to users who can refer to it during testing, troubleshooting, and assembly. The information printed on the PCB legend can be broadly classified into different types depending on their purpose. A user may be exposed to the following information:
This may include serial numbers, manufacturer information, and other information, which protects against copying of design.
These are warning signs that are added to protect users from electric shocks. A few examples of these may include grounding, high voltage, and so on.
Part numbers are added on the circuit board to enable easy replacement or design evolution whenever needed.
Often troubleshooting is performed during repair or as a part of testing. PCB engineers add the information on the PCB to intimate users about replacing a burnt or damaged part.
Users can follow circuit identification information to understand ground points, testing points, polarity indicators, and component interconnect, and so on.
This may feature information on rating verifications or component replacement testing, and so on.
Although PCB legend text printing guidelines may vary across manufacturers, the following are a few common points that remain same across all of them:
Legend texts must be legible and readable with naked eyes. The PCB is viewed vertically and is usually read from top to bottom. The legend text at the top must be readable, whereas the text at the bottom is often mirrored in corresponding layers or Gerber files. The text at the bottom is usually non-readable.
The ink used for marking should not affect the annual ring.
The PCB legend printing can be done through manual screen printing, liquid photo imaging, and direct legend printing.
Manual screen printing is one of the earliest methods of PCB legend text printing, where epoxy ink is used for printing. The ink is made to pass through the laminate and allowed to dry.
Liquid photo imaging process uses epoxy ink and silkscreen is exposed to UV light for drying.
Direct legend printing is the process, where inkjet printers are used to print on the PCB.
Legend line width: May range from 7 mil to 4 mil
Text height: Set at 30 mil
Maximum text width at minimum height: 17 mil
The areas or the texts on the legend must be clipped to match the soldermask. The clipping clearance of legend is set at 10 mil to 2 mil. This means, the legend is clipped 2 mil from the opening of the soldermask.
The outline of the board must be present in the layers of the legend.
A legend layer is not recommended on the copper layer of the PCB, without a soldermask.
Encroachment is permitted on only one side of the land on SMT pads with pitch 1.25 mm, and it should not exceed 2 mil or 50 microns.
Encroachment is permitted on only one side of the land on SMT pads with a pitch < 1.25 mm, and it should not exceed 1 mil or 25 microns.
Legend texts are usually printed in colors such as white, green, yellow, and black. The choice of colors usually depend on customer preferences. In addition to the above-discussed guidelines there are a few more factors need to be considered when designing a PCB silkscreen or PCB legend. The next post focusses on some tips for PCB legend printing.