PCB Silkscreen Printing Technology
Though silkscreen is not the top priority in high-density PCB board, detailed PCB silkscreen can help manufacturers and engineers easily locate and identify components.
The name of the PCB screen is derived from the actual process of using a stencil to apply ink to the PCB. This method is still used in some cases, but new methods have been designed to increase readability and allow for smaller text height and line width.
Liquid photo imaging or LPI is similar to the process used for soldering masks. Basically, the photoimageable film is exposed to ultraviolet light. After exposure, the board must be developed and cured. This approach allows for better resolution of text and artwork but the whole process will require more time.
Direct legend printing or DLP is considered to be the best and fastest way to PCB silkscreen layer. Special inkjet printers are used in conjunction with acrylic ink that is cured while the actual printing is done, with UV light.
The manual method requires a stencil of text and artwork that are usually made of nylon. The epoxy ink (paste) is pushed onto the laminate through a stencil. After this process is complete, the board must be baked to cure the ink. Although this method is quite fast, the stencil is easy to produce, it gives the most inaccurate results. As a comparison, the minimum text height for manual screen printing is 35 mils, some manufacturers have a 50 mil limit, while the DLP method only allows 20 mils text height.
In today's high-density design, if you can reduce the size of the text, and maintain readability, then the direct legend printing seems to be a good choice. However, you should note that not all manufacturers are equipped with DLP technology. In addition, the acrylic ink does not cure on boards with silver finishes.