Silkscreen is an important process in printed circuit board manufacturing. The silkscreen layer may not be the most functional part of a PCB, it just ought to be present for users’ convenience.
In this post you will learn more about what silkscreen printing actually is, its methods, and the reason it is that essential.
What is Silkscreen in a PCB?
Silkscreen printing dates from early 1911, when the technique of applying a dye onto a substrate was invented. Such a process is traditionally well-known as serigraphy printing or just serigraphy.
Nowadays, we know silkscreen as the topmost or outer layer of PCBs. It may be applied to both the sandwich-like component and the soldering. They are actually the substrates. However, extra printing on the soldering layer may increase the cost of a PCB.
The dye can easily be deposited on a substrate using the methods we will discuss in the last section of the post.
Silkscreen Ink
Silkscreen can be regarded as a thin layer of ink traces. Its basic color – white, as such ink ensures great contrast with most PCB colors (green, blue, black, red, etc.). Although, you may choose any silkscreen color you like – black, yellow, blue, or any other.
It is also good to know that ink is a non-conductive, epoxy one. It does not have any effect on PCBs or any other electronics. It is also durable and rarely deteriorates over time.
Intended Use of Silkscreen
When it comes to its intended use, silkscreen is a “legend” – a basic reference indicator for placing components of PCBs. This layer is utilized to provide text-based, vital data to engineers and users. Here is the list of information to indicate:
- Reference designators.
- The polarity of parts.
- Company logos.
- PCB identification number.
- PCB version number.
- Warning symbols.
- Test points.
- Other, if necessary.
The presence of silkscreen is considered great assistance during the assembling or testing of electronics.
Why Should We Use Silkscreen Printing?
It is nearly impossible to find an industrial PCB fabricated without silkscreen printing being involved. But electronics enthusiasts are ones who truly do not have a rich opportunity to neglect serigraphy.
Let’s review several essential reasons why we should employ silkscreen printing:
- Assistance to technicians
As claimed previously, specialists cannot identify PCB components and test points without such a “legend” as silkscreen.
Basically, the absence of silkscreen makes manual assembling impossible. Some companies tend not to apply silkscreen to avoid stealing their innovative PCB designs – not to counterfeit their electronics. In this case, automated production lines solely do the job.
In case there is no silkscreen, quality testing is also greatly complicated. Repair shops and independent laboratories may refuse to test PCBs without the screen.
- Quality Assurance/Safety Policies compliance
As a matter of fact, silkscreen “legend” contains other important information such as warning symbols. The presence of them onto PCBs is a form of safety measure. It also helps to comply with Quality Assurance requirements.
As such, most factories are obligated to include at least the PCB identification number and the PCB version number. Otherwise, it would be impossible to detect a batch of malfunctioning parts. Companies may be penalized if silkscreens are absent on their PCBs.
Next, in case users are not warned with special symbols, they may approach electronics incorrectly. In this case, the quilty would be on a company that produced PCBs without silkscreens.
- Error reduction
There is always a risk of an error during both the assembly and the rework phases in production. Silkscreens help to navigate the PCBs.
For example, personnel may incorrectly position a batch of PCBs on the production line. They may just accidentally swipe the sides. The presence of a “legend” lowers the likelihood of this happening.
In general, labeling PCBs with a “legend” is beneficial for any factory overall.
How Can We Apply Silkscreen Printing?
There are only three methods to apply silkscreen. They differ in the quality of text printed and the involvement of labor.
1. Manual Screen-printing.
Manual printing is the very first and most primitive method of screen application. It can be utilized when the allowed registration tolerance for the “legend” is 0.005″. In other words – when the width of lines is greater than 0.007″ (0.18 mm.).
The method is grounded on the use of a stencil that has the text engraved in it. Stencil also comprises traces that are made using nylon. Ink is pushed through the stencil and spread over the text’s engraved counters and traces.
In the next step – the ink is cured. This procedure is performed in a baking oven. As a result, the deposited ink is inextricably linked to the substrate.
As you could have understood, the quality of such printing may be questioned. In any case, it is the simplest printing method. It is still widely spread among small-sized factories and completely suits DIY fabrication.
2. Liquid Photo Imaging (LPI)
The more advanced method – LPI, is somewhat similar to solder mask application. It can be used to produce the text and traces with lines of 0.004″ (0.1 mm) or preferably greater.
Basically, you can use film tools to block areas that need to be coated. They, a contact printed, applies an epoxy, photo-imageable, liquid mixed with dye onto a PCB. Then, it is treated with UV light instead of curing with heat.
As a result, you obtain a coating that comprises the text and traces – or the “legend” of a PCB. This method is semi-automated and still requires some involvement of labor. The quality of text using LPI is much better than using manual printing.
3. Direct Legend Printing (DLP)
DLP is considered the most technologically advanced method of silkscreen printing so far. It supports the print of the thinnest lines – 0.004″ (0.1 mm).
The method is somewhat similar to CNC machining or 3D printing. It involves the use of an inkjet projector and acrylic ink. Lines of text and traces are created in accordance with a computer-aided design (CAD) file.
The post-treatment procedures include obligatory curing of the silkscreen layer with UV light.
DLP produces the greatest outcomes. It is widely incorporated by large factories. It is also the most expensive method of silkscreen printing. Another drawback – acrylic inks cannot be applied to PCBs with silver finishes.