is a layer of ink traces used to identify components, test points, parts of the
PCB, warning symbols, logos and marks etc. This silkscreen is usually applied
on the component side; however using silkscreen on the solder side is also not
uncommon. But this may increase the cost. Essentially a detailed PCB silkscreen
can help both the manufacturer and the engineer to locate and identify all the
is a non-conductive epoxy ink. The ink used for these markings is highly formulated.
The standard colors we normally see are black, white and yellow. PCB software
also uses standard fonts in silkscreen layers but you can choose other fonts
from the system too. For traditional silk-screening you require a polyester
screen stretched on aluminum frames, a laser photo plotter, spray developer and
Method of Silk=screening:
are three basic ways to apply silkscreen.
1: Manual Screen-printing:
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. The application and set up is easy but the result is least precise.
2: Liquid Photo Imaging (LPI):
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 photoimageable 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.
3: Direct Legend Printing (DLP):
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
Importance of Silk-Screen:
A properly designed silk-screen can prove to be highly
useful as it can reduce the chance of error and can reduce time taken to spot
the error. The silkscreen can easily label the passive components of the PCB no
matter how packed the board may be.
are a few things you should keep in mind regarding silk-screening. For example,
the silkscreen epoxy should not be printed over pads or land PCBs that will be
soldered as it will melt into the solder joint. This is why it should be
applied over the solder mask. While designing you should keep the component
outlines away from the pins (almost at a distance of 0.25 mm). Also you should
check to see if the width of the silkscreen graphics is suitable to the design.
Try to keep the line widths no less than 6 mil.
more about PCB and silk-screening can help you to reduce your PCB costs. For
example, applying silk screen on only one side and choosing the standard colors
will give you better overall pricing and give you the same benefits.