One of the more artistic processes in a PCB manufacturing facility is the application of Ident layers, also known as silk-screening.
Silkscreen is normally used on the component side to identify components, test points, PCB and PCBA part numbers, warning symbols, company logos and manufacturer marks. It isn’t uncommon to have silkscreen required for the solder side as well but if you are price sensitive, you need to know the work required to produce two silk-screens is simply twice as much as one.
Silk-screening requires specially formulated inks. Standard colours are Black, white and .yellow. Non-standard colours can be requested but will impact your cost and your lead-time.
At one time, PCB CAD software allowed for a few standard fonts to be used in silkscreen layers. Now, many packages allow for all available system fonts to be used. Be careful when choosing your font. It may seem interesting to pick a “Showcard Gothic” font but remember someone may have to try and read it later.
PCB manufacturers are required by IPC to provide both a manufacturer’s 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. Omni Circuit Boards prefers to place our logo and datecode on an ident layer.
Screen-printing is done when the boards are still panelized prior to the application of the final finish or routing/v-scoring.
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.
Currently three methods are available for applying the “Ident” to the PCB.
■Manual screen-printing can be utilized when line widths are greater than 7 mil (0.007”) 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.
The art of manual silk-screening takes time to develop. At Omni, our master craftspeople have been practicing their art for over 20 years. 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. Today, just like when I first joined Omni, I am amazed at the artworks we create in the screening department.
PCB layout designers sometimes forget to double-check their ident layers before sending designs to PCB fabrication. Designers should avoid putting ident overtop circuit pads, should avoid using excessively tiny text and should allow for registration tolerances.
So how can knowing more about Silk-screening help you reduce your PCB costs?
Requesting silk-screening on one side only will cut your artwork costs in half. Sticking with standard colours and larger geometries will also prove beneficial. Understanding these constraints can help you to better PCB pricing.
