When it comes to PCB design and eventual prototyping, two aspects that are very important, but may not get the discussion they deserve are Solder Mask & PCB silkscreen. Let us look at the difference between solder masks and silkscreen and each of them in detail.
Silkscreen VS Solder Mask
What is solder mask in PCB?
Also referred to as a solder resist, a solder mask is a thin polymer. When applied to the surface of the PCB it helps seal the copper from environmental conditions. Essentially a solder mask performs two essential tasks. First, it helps prevent oxidation & second it ensures that solder bridges are not formed. If oxidation does happen it impacts the electrical property of copper. In turn, this affects the functioning of the PCB.
A solder bridge refers to an electrical connection between two conductors. When a solder mask is used it prevents this from occurring. A solder mask becomes particularly important for mass-produced boards where the soldering is done automatically by way of reflow or wave solder techniques.
A solder mask is available in a wide variety of colors. Additionally, it also comes in different media ranging from epoxy liquid to liquid photo imageable solder mask inks as well as dry film photo imageable solder masks. LPSM can be either sprayed on the PCB or silkscreened. When it comes to DFSM, it is typically vacuum laminated on the PCB followed by developing it.
Function of Solder Mask in PCB Design
To sum up, the functions of a solder mask layer include:
- Ensuring that the conductor circuit doesn’t break physically.
- Ensuring that there is no short circuit from the bridge connection during the process of soldering.
- Ensuring that oxidation doesn’t happen on the copper layer.
- Ensuring that there is no short circuit between conductive lines and solder joints.
Silkscreen
While a silkscreen is often seen as a not so important layer at first glance since it does not impact the electrical functionality of a PCB, the fact remains that it provides valuable information that can make or break the functioning of the PCB.
A silkscreen in PCB is typically used to transmit the following information:
- It states the location of the parts with the use of reference designators.
- It helps identify test points.
- It offers identification numbers unique to each board.
- It offers warning symbols.
- Ensures polarity of parts.
When a PCB is being assembled, the information provided by the silkscreen is invaluable as among other things, it ensures that the components are correctly placed. In cases where a silkscreen is not available an assembly drawing becomes imperative.
The silkscreen also plays an important role in PCB testing as it provides the engineer with the location where measurements are to be taken, during the process of testing. This can go a long way in finding out the causes of potential failure. Some of the markings of the silkscreen used for testing include:
- Test points – These indicate the location on the board where the desired values are known.
- Warning symbols – These warning markings come in handy as they indicate areas such as a high voltage point.
- Part numbers – In case a datasheet is not present it provides device information.
How silkscreen applied on PCB?
A silkscreen is applied on the PCB in three ways:
- Manual Screen Printing (MSP)
- Liquid Photo Imaging (LPI)
- Direct Legend Printing (DLP)
How to ensure accuracy of silkscreen?
The importance of a silkscreen in ensuring PCB accuracy is undisputed. What is important is that the silkscreen should be accurate. Some tips to be followed that ensure accuracy of a silkscreen include:
- Using fonts that work for the PCB contract manufacturer.
- Using colors that are readable.
- Following the dimension guidelines so that the silkscreen is not covered by components.
To sum up, both solder masks & PCB silkscreens play important roles in ensuring that the functioning of the PCB isn’t affected, albeit in different ways.
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