Silkscreen PCB Printing: All You Need to Know

Silkscreen PCB printing is a significant step of its manufacturing process. It creates the layer that identifies components for the assembly process, testing steps, and servicing. This article overviews the layer and its key elements, including how we print it. To start us off, let’s define and examine its function.

What is Silkscreen Printing in PCB?

Silkscreen printing in PCB is the process of creating the topmost layer. This layer labels the board and its components. It contains printed text, graphics, and other markings. These provide valuable information for anyone handling the board.

For example, it tells assemblers where to place parts, ensuring their correct placement. It also guides testing and rework experts on where to connect probes. Sometimes, the PCB maker uses the layer for branding or printing regulatory information.

The layer can be many colors, with standard options being black, white, and yellow. Text sizes also vary depending on the desired look or legibility requirements. Below is a more detailed look at the silkscreen marks and symbols.

PCB Silkscreen Symbols and Other Elements

The silkscreen layer comprises various elements. The print can be a text, part outline or value, logo, graphics, or an identifiable symbol like part polarity. Each marking has a specific purpose, which can be functional or aesthetic.

• Manufacturer Symbols – company logo, serial number, date code, copyrights
  
• Assembly Symbols – part number, value, polarity, reference designators, alignments, pin one indicator
  
• Testing Symbols – testing points, jumper markings, mode indicators
  
• Regulatory Symbols – warning symbols, approval markings
  

PCB designers identify and locate text, values, and symbols when creating the board on the CAD program. This data later guides the printing process, which can be manual or automated.

How is PCB Silkscreen Printed?

The layer derives its name from its initial printing method. It traditionally involved squeezing ink through a fine mesh of silk. The mesh transferred this pattern to the bare circuit board.

The silkscreen printing process is still popular today, but other technologies are also available. These include inkjet printing or laser light and photosensitive ink.

Ultimately, the text or graphics must adhere to the PCB surface and last long. The following section takes you through the silkscreen printing options in PCB manufacturing.

Silkscreen PCB Printing Methods

Your PCB maker uses one of these methods to produce the layer: liquid photo imaging (LPI), direct legend printing (DLP), or manual printing. Let’s examine the various silkscreen methods, their benefits, and drawbacks.

Liquid Photo Imaging

LPI involves coating the board with a photo-imageable resin and exposing it to UV light through a patterned film (photomask). This action hardens the exposed sections, setting the ink at the marked locations.

Washing the soft, unexposed parts develops the print, leaving the hardened regions intact. We usually use it for wider lines or spacing exceeding 4 mils.

LPI is the most accurate, which makes it suitable for high-legibility applications. However, it costs more than other methods since it uses complex technologies like laser and photomask.

Direct Legend Printing

The DLP method uses an inkjet printer to apply ink, directly creating the required symbols or text. It requires no masking, making it cost less than the laser-based method.

On the downside, the direct legend-printed layer has a lower resolution than laser-printed types. However, the accuracy is high enough for high-density boards. Another limitation is that you cannot use it on silver surfaces.

Manual Screen Printing

Manual printing requires a physical stencil to print the layer. The stencil is a fine mesh that transfers the print and ink to the board and desired sections.

Manual printing offers cost benefits. Its drawbacks include reduced accuracy, speed, and laborious steps. As such, it mainly suits low volumes and low-density PCBs.

PCB Silkscreen Guidelines

A few rules apply when designing the silkscreen for a printed circuit board, PCB. For instance, the engineer must carefully select the layer material, location, and thickness. Below are key considerations when creating one, including best practices for each.

Material Choice

Some materials provide better contrast and durability, making their selection essential. Some are also more expensive, while some are usable on many surfaces.

The typical PCB silkscreen material is a conductive or non-conductive epoxy ink or acrylic. Some inks are inkjet-printed, while some are photo-imageable with the help of lasers.

Location

Depending on space availability and other factors, you can print the layer on the board’s top, bottom, or both. Additionally, it’s always good practice to place it on the components’ side but away from pads or vias.

On the other hand, symbols like polarity must be close to the marked part. The markings must not overlap or obscure other graphics. A 5-mil clearance is advisable during PCB layout and component placement.

Thickness

PCB silkscreen thickness determines its ability to resist application stresses like abrasion, heat, or light. Various printing technologies produce different thicknesses. Dry and liquid photo imaging methods create thicker layers than inkjet and direct laser imaging.

Font Size

The font must be legible for the assembly process, testing, or other activities like repair and rework. However, it must not take too much space or obstruct part placement steps. The most popular fonts are 25-35 mils with 5 mil-line widths.

Colors and Contrast

Your chosen color must provide sufficient contrast to make the text or markings adequately visible. Many manufacturers use black or white on green circuit boards, making the colors standard across the PCB industry.

You can also customize PCB silkscreen colors for branding and visual appeal. That means choices that reflect your business or make the board attractive and professional.

What Can Damage Silkscreen PCB Layers?

Certain harsh conditions and materials can damage the silkscreen layer. The causes include exposure to ultraviolet light, some chemicals, and excess humidity or heat.

The above conditions can fade the print, making it hard to read. They can also cause cracking and abrasion, reducing their look and functionality.

Thicker layers perform better than thin layers, offering increased resistance to damage. Users can also extend the layer lifespan by avoiding conditions that harm the delicate print or using better inks.

Silkscreen PCB Repair and Rework

Various methods can repair damaged silkscreen PCB layers. One uses paint to re-mark the text, logos, or symbols and make them identifiable. Other options include using adhesive strips printed with the required text or another mark.

Sometimes, the damage is too extensive to fix using the above techniques. In such cases, the circuit board may have its parts removed and taken through a fresh printing process.

Conclusion

Silkscreen PCB printing requires a careful approach. The PCB maker and assembler must carefully select the print material and layer creation method. That also includes observing industry rules when printing it. The aim is to ensure the print is visible, within desired size limits, and resistant to damage.