In the past few years, the primary method of silkscreen referred to using a stencil to directly apply ink to the board surface. Nowadays, there are the two most common silkscreen PCB printing techniques, such as liquid photo imaging (LPI) and direct legend printing (DLP).
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 similar to 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.
The application of PCB silkscreen is done during PCB fabrication, but it’s used for the PCB assembly process steps, which is mainly in referring to placing or mounting components. There are some silkscreen markings used to accurately place components as follows:
Reference indicators – these identify the component types from the BOM.
Pin 1 indicator – this indicates which pins connect to what pads of the footprint.
Component outline – this, in conjunction with the landing pattern, indicates where the component should be placed.
Polarity indicators – these provide the orientation of polarized components.
For components with small numbers of pads or simple shapes, it may be acceptable for general placement to have the absence of the component outline. However, there still exists the possibility of asymmetrical placement, which may lead to problems during soldering, for example tombstoning. The absence of reference, pin 1 and polarity indicators may lead to incorrect component orientation or placement, which could have a negative effect on device functionality.