Silkscreen refers to the white letters and numbers on the printed circuit boards(PCBs). White is the standard color because it is easier to be identified against the green solder mask, but other colors such as blue, yellow, red, black are also can be selected. Usually, silkscreen contains the valuable information of components position, identifiers, test points, company logos, PCB and PCBA part numbers, and other manufacturer marks. An effective silkscreen is like a clear map with detailed guides for workers to assemble the PCBs.
The usage of silkscreen on the PCBs as below:
l Identification of the location of components, interconnections and testing points, etc.
l Identification of part number, the board type, plate version, registration number
l Marking the safety precautions such as high voltage
l Certification mark such as EMC, RoHS
With this information, silkscreen can help PCB assembly personnel to recognize during the assembly testing and repair process. The silkscreen also provides the relevant details for authorities to identify.
Three typical technologies applied to silkscreen in PCB manufacturing.
l Manual Screen-printing
This method is used when the line width is greater than 7 mil, and the registration is 5 mil. The application and set up of Manual Screen-printing is easy and fast, but the result is least precise. The output of this method depends on many factors such as room temperature, the pressure of stroke, the ink viscosity, humidity and so on. Therefore, it is not a wise choice in the PCB manufacturing process.
l Liquid Photo Imaging (LPI)
This method is applied when the line width is more than 4 mil, and the process is much similar to the solder mask. Though it is more time-intensive and needs more ink, it provides more accuracy and legibility than manual screening.
l Direct Legend Printing (DLP)
DLP is the most accuracy, fastest, and expensive method for the process of PCB silkscreen printing. An inkjet printer with acrylic ink is used in this method, and the ink will be cured by UV light after the printing is finished. But the acrylic ink cannot be cured on boards with silver finish.
Among these three ways, LPI and DLP are the most widely used in silkscreen printing PCB because they have increased the legibility and allow smaller text height and line width in silkscreen design.
Some tips for silkscreen in PCB designs.
l Screen printing inks are the most important materials in screen printing. Proficiently in performance of all the inks is an indispensable condition for a successful silk screen printing process due to the quite numerous types of screen printing inks.
l PCB designers should double-check the silkscreen layers and avoid using excessively tiny text and putting ident over component pads.
l Silkscreen characters should be arranged from left to right, from bottom to top. Avoided using four directions when designing.
l PCB silkscreen printing should be clear, neat, no staggered.
In a word, silk screen printing in PCB manufacturing plays a very important role in PCB design, PCB prototype, PCB assembly, and testing. Customers should communicate the details and requirements with the PCB manufacturer to minimize the printing errors before the fabrication of the PCB prototype and PCB assembly.