On most printed circuit boards (PCBs), the designators, symbols, markings and other legends on the exterior layers of the board are known as PCB Silkscreen or PCB Legend. The silkscreen is printed with non-conductive epoxy ink and cured to permanent identifiers, which form a “map”, so that assembly, rework and debug technicians can find the correct components as well as other information on the board that they are working with. PCB silkscreen colors are commonly green, and there are many glossy and matt colors are available.
PCB Silkscreen Guidelines
Before a printed circuit board (PCB) is installed in a product, it must be assembled and tested first. Some of this work may be done manually, and for the technicians, doing this work to find specific components, they need to be labeled. Even after the PCB board is completed, there may still be maintenance, setups, or even field repairs that requires simple information printed on the circuit board. This is accomplished with an ink that is silkscreen onto the board.
These silkscreen markings are laid out in Gerber file, and the completed artwork is usually referred to simply as the silkscreen. It is important that the correct data be included on the silkscreen, and that the correct sizes and shapes be used, as well. If the sizes are too small, or the wrong font is used, it may be unreadable.
The data should put into silkscreen layer(s) will usually include the following:
Reference Designators: Each component and assembled part on the board should have a unique reference designator.
Component Symbols: While most components may have a simple rectangle or line to show how they are aligned on the board, some, such as BGA packages, may require more complex symbols to designate their orientation.
Pin Markings: Many components will need their first pins marked, and in the case of large components like connectors with hundreds of pins, there may be additional markings as well.
Special Markings: In some cases, a silkscreen may contain specific user instructions, such as switch settings or assembly instructions.
Documentations: Silkscreens are also used for marking information, such as company names and contact information, the board name and part number, bar codes, date codes, and other information.
As you can see, the silkscreen is much more important to the assembled circuit board than it is often given credit for. Such as, it is critical that PCB designers spend time to ensure it is created correctly. Here are some of the details to keep in mind as you finalize the silkscreen in your PCB design.
The first step in creating a good PCB silkscreen is to use the most optimum font sizes and line widths. Font sizes that are too small or are drawn with too wide of a line will end up looking like an ink spot instead of readable text. Line widths that are too narrow may not silkscreen on the board correctly.
Another concern is silkscreen spacing to pads or other PCB objects. Not only will silkscreen be unreadable if it ends up on a pad, but it could also affect the solderability of the board. At MADPCB, we recommend the following values for the best PCB silkscreen results in daily PCB manufacturing and assembly experience:
Font size: For best results, use a 1.27mm (50mil) font size and absolutely no smaller than 0.89mm (35mil).
Line width: Silkscreen fonts should use no smaller than 0.15mm (6mil) line widths. Keep in mind that although wider lines can be used with larger font sizes for company names, part numbers, and other user information, the line widths must be reduced when using smaller fonts for reference designators, pin numbers, and polarity markings.
Clearance: Silkscreen should be kept at a minimum of 0.13mm (5mil) from PCB pads and other objects.
The next important step is arranging the different items on the silkscreen. Reference designators should be close to their part and rotated to be easily readable. Unless it just isn’t possible to fit it in, a good practice is to only use two rotations with designators—0 and 90 degrees.
It can be difficult if someone constantly must rotate the board in their hands to read the next silkscreen element. Also, make sure that important silkscreen information is not covered up by placed components. It is very difficult for technicians and inspectors to find components or pin markings if they are covered by the part.
Common Silkscreen Errors and How to Avoid
One of the most common errors found on silkscreens is either ignoring to mark polarized parts or not clearly marking them. For electrolytic capacitors, the silkscreen should clearly show which pin is positive. For diodes, it is usually preferred to mark the anode pin with an “A” or “+” and the cathode with a “C” or “-“. Another option for the diode would be to draw the complete diode symbol on the silkscreen so that there is no mistaking which pin is what.
Another silkscreen problem can occur when designers use a silkscreen line as a separator between solder pads. This is often seen on two-pin discrete components such as capacitors and resistors, and a silkscreen line is drawn between the two pads. These silkscreen lines may cause the formation of micro solder balls during the solder reflow process. The micro solder balls may then be deposited on the surface of the board and cause inadvertent shorts.
When design the silkscreen layer(s), you can follow the component manufacturer’s recommendations on how to mark their part on a silkscreen. Then submit your Gerber files to your PCB manufacturer who can conduct a free DFM check for your design before an order. At MADPCB, if some errors found, we will tell and recommend how to revise.