When it comes to PCB Manufacturing, the importance of Silkscreens cannot be overstated. Not only does it contain valuable information about the location of components, it also contains other indispensible information such as that on identifiers, labels and more. An effective silkscreen is therefore like an efficient map with the right legends and calibrations. The silkscreen provides information of various components as well as of the circuitry. The role of PCB silk screening therefore spans a host of areas, including but not limited to:
- Identification of circuitry including testing points, interconnections and more
- Parts identification
- Safety precautions such as high voltage
- Rating verifications and more.
There are 3 Silkscreen printing technology that are currently available:
This is typically used when the line width is greater than 7mil and registration tolerance is 5 mil. From the accuracy point of view, however this isn’t the most desirable choice as the output is determined by aspects such as the viscosity of the ink, humidity, pressure of the stroke and more.
Liquid Photo Imaging (LPI)
This is used when the line width is more than 4mil. In terms of accuracy, this turns out to be more accurate than manual printing, even though the approach is time intensive.
Direct Legend Printing (DLP)
In terms of accuracy this method is rated the highest. It is also faster than LPI, even though it comes at a higher cost.
While the importance of Silkscreen is undisputed, what also comes in its wake are a number of potential issues, which if not taken care of can lead to costly errors.
Below are some of the commonly faced silkscreen issues and potential ways to avoid them:
Components not represented correctly– Any small error in the silkscreen with regard to its intended components, their shape, location, polarity indicators etc can lead to a whole lot of issues for the technicians, when they are doing any modification or a debug.
Non- Readability– Another commonly faced issue is to do with the legibility of the silkscreen text. Using a very small font size or too narrow or too wide a line width are all potential causes of unreadability.
Wrongly placed Reference designators– Another common mistake which leads to error sand a lot of angst among technicians, is to do with erroneously placed reference designators. In testing the board, technicians then have to grapple with the issue of the schematic not matching the board. The other issue with reference designators often is to do with dense designs, where reference designators are hidden by assembled components.
Spilt silkscreen ink– Ever so often the silkscreen ink can cover the surface mount pins and can cause damage to the board.
To avoid these common errors, it is imperative that the design guidelines are followed carefully. Whether it is to do with font sizes or line widths, do stick to the specifications. Communication with the fabricator and understanding the requirements in detail will also go a long way in ensuring that the manufacturing errors are minimized if not totally eradicated.
Doing some common hygiene checks including checking to see if the reference indicators are in place and that all fonts are readable, will ensure that the technicians do not face any issues too. Other checks such as ensuring that silkscreens clearances are fine will also reduce chances of error.
Other important aspects that an experienced silkscreen manufacturer will follow, include:
- Marking using standard colours and larger shapes
- Allow for common printing errors in fixing the distance on the board.
- Adding extra marks to show the orientation of components on the circuit board
- Placing polarity marks to reduce component orientation mistakes during board assembly
- Measurement marks of pins and lines
Choosing the right silkscreen manufacturer, therefore, who has a strong learning curve is imperative to the success of the PCB Assembly. Going with a manufacturer who has extensive experience can save you both valuable time as well as costly errors.