A silkscreen may not have any role in the functioning of a PCB; however, it is valued for the information that it delivers. For instance, it will help anyone to:
- Understand the warning symbols
- Polarity of parts
- Identify the test points
- Locate the parts through reference designatorsf
- Identify the numbers that are unique to the printed circuit board.
- Identify manufacturer marks, version numbers, and so on
How to apply PCB silkscreen?
Manual screen-printing is done when the line widths are greater than 7 mil (0.007”) and the registration tolerance is 5 mil. For this method you require a stencil of the text and traces made of nylon. The ink is pushed through the stencil onto the laminate. Next the board is baked in a curing oven for the ink to cure.
Liquid Photo Imaging (LPI)
This method is used when the line widths are greater than 4 mil. Liquid Photo Imaging is quite like the process used for the application of solder mask. In this a liquid photo-imageable epoxy is coated on to the laminate and then exposed with UV light. After this the board is developed and cured. It is much more accurate than manual screening.
Direct Legend Printing (DLP)
DLP is the most accurate of all these processes but is more expensive. In this process an inkjet projector is used with acrylic ink that is applied onto the raw PCB directly from the CAD data. The ink is cured with UV light as it is printed. It should be noted though that the acrylic ink does not cure on boards with silver finishes.
What are the considerations for PCB silkscreen design?
As discussed earlier, using a silkscreen PCB layout is an important aspect of board design and not including it can cause serious problems for PCB assembly and testing. The silkscreen is also used for labeling, which may include manufacturer information, testing certifications, model or serial numbers. In order to be effective and useful, the silkscreen must have clearly readable text, provide sufficient image resolution and, most importantly, be accurate. Here are a few tips to help you ensure that your silkscreen PCB layout can be used for its important roles in PCB assembly, testing and identification:
- Only use fonts approved by your CM as some fonts, although easily created in software, are difficult to generate during board manufacture.
- Use white or black instead of other colors to provide the best contrast with your board’s solder mask color.
- Make sure to follow your CM’s clearance and dimension guidelines. This will ensure that your silkscreen items do not get covered by components and are easily visible.