PCB Silkscreen layer introduction

It is often necessary to print text and place other small printed idents onto a PCB. This can help in identifying the board, and also in marking component locations to aid in fault finding, etc. A silk screen generated by the PCB design software is sued to add the markings to the board, after the other manufacturing processes for the bare board have been completed.

Standard colors are Green, Black, white and .yellow. Non-standard colors can be requested but will impact your cost and your lead-time. Currently three methods are available for applying the legend to the PCB.

Manual screen-printing can be utilized when line widths are greater than 7 mil and the registration tolerance is 5 mil.
LPI (Liquid Photo Imaging) provides more accuracy and legibility than manual screening and is employed when line widths are greater than 4mil.
DLP (Direct Legend Printing) is the most accurate and legible of the 3 processes but also has the highest cost for consumables.

PCB manufacturers are required by IPC to provide both a manufacturers mark and an identifier which will allow for lot tracing should problems occur. Although there are no strict standards for where these markings appear, manufacturers will generally try to place them in non-critical areas. But just because you know the board will also double as a backlit equipment faceplate with your company logo doesn’t mean your PCB manufacturer will. If you require these markings to be placed in a specific area or omitted, you need to specify it in the PCB readme.txt file, in a fabrication drawing or in your communications with the manufacturer.

Conventional silk-screening requires polyester screens stretched across aluminum frames. Solvent resistant emulsions are used to coat the screens prior to imaging. Both the screen and panel must be registered precisely to ensure proper alignment. This method of screening requires the following equipment: Laser photo plotter to produce the initial film, UV printer, spray developer and curing ovens. Screens can be cleaned and reclaimed to be used again although not for jobs which require high resolutions.

The art of manual silk-screening takes time to develop. Some of the parameters to be considered include the viscosity of the ink, room temperature, humidity, age of the ink and hardener and line resolution. From these variables the correct amount and distribution of ink is derived and even the pressure of the stroke will impact the final outcome.

PCB layout designers sometimes forget to double-check their legend layers before sending designs to PCB fabrication. Designers should avoid putting legend overtop circuit pads, should avoid using excessively tiny text and should allow for registration tolerances. Sticking with standard colors and larger geometries will also prove beneficial. Understanding these constraints can help you to better PCB pricing.