PCB Silkscreen: All You Need to Know

Silk screen is the topmost layer of a PCB, adding a silkscreen layer is the final step in PCB manufacturing. This layer adds ink markings to identify PCB components, test points, symbols, logos, and other important details. Silkscreen plays a crucial role in helping manufacturers and designers easily identify components by annotating them on PCB.

The silkscreen ink is a specially non-conductive formulated epoxy. It comes in various colors, the most common in the industry are black and white. PCB design software typically uses standard fonts for silkscreen layers, though designers have the flexibility to choose other fonts as needed. In this article, we’ll explore the importance of silkscreen, design guidelines, useful tips and tricks, and how to incorporate custom fonts into the silkscreen layer. See the full comprehensive guide, how PCB are manufactured in JLCPCB fab.

What is a PCB Silk Screen?

Think of roads and streets that lack direction/distance texts and landmarks. Do they make any sense? Similarly a PCB silkscreen plays the same role as the landmarks and text. It offers text-based information about various circuit points, parts, and extra circuitry as per the requirement. Silkscreen is mainly used in marking test points and annotating component prefix names. Silkscreen can be placed during fabrication using a UV liquid photo-imaging process, which is similar to that used for solder mask application. If you need very small linewidths, your fabricator may use an alternative process called “direct legend printing”.

What Should Go on PCB Silkscreen?

Here is some of the information that should be included in your PCB silkscreen:

• Print company logos, the name of the designer/ manufacturer
• Warning signs, such as High Voltage
• Version numbers, such as Ver1.0, V2.0, V3.0, V4.0, and others
• Testing points
• Component numbers and orientation
• Connector pinouts, especially for pin headers

All of this information can get very cramped on a dense board; we’ll look at some ways you can organize your silkscreen markings later in this article. The example above shows an example PCB assembly board containing some of the information in the above list.

Industry Standard Product Based Silkscreen markings:

Regulatory markings indicate that a PCB design complies with specific regulations or has passed inspection. Common markings include RoHS, FCC, CE, and E-waste disposal symbols, typically found on consumer and commercial products that meet the required standards. Another important marking is the UL symbol, which signifies that the design was manufactured and assembled by a UL-certified facility, adhering to the UL 796 and UL 94 safety standards. These markings assure that the product meets industry and safety requirements.

How to Clean Up Silkscreen:

When you’re working in a dense design, there is the possibility that your reference designators and other silkscreen markings overlap. With the current demand for miniaturized and compact boards, it is almost impossible to include every bit of information. This means you’ll likely have to clean up and rearrange some designators so that they do not overlap, are easy to read, and clearly indicate component positions. We can place some designators farther away from the components. Avoid the silkscreen over pads, via’s and exposed tracks.

In this regard, prioritize your markings as described below:

1. Start with regulatory requirements ROHS, Lead-Free, safety ratings and other IPC standards.
2. Consider manufacturer identification like PCB serial number, labels, and logo.
3. Prioritize assembly aids such as polarity and placement indicators.
4. Consider component designators and names.
5. Consider testing points and jumper indicators.

According to the above guide, if you prioritize your marking, you will undoubtedly make maximum use of PCB identification and labeling, even if you are working with a small board.

Different Methods to Create PCB Silkscreens:

There are several methods for silkscreen printing on a PCB, including:

Screen Printing: This is the most common method for creating the silkscreen on a PCB. It involves using a fine mesh screen to transfer the print onto the PCB.

Inkjet Printing: This method uses an inkjet printer to print the silkscreen directly onto the PCB. The printer is equipped with a special printhead that can print on the surface of the PCB.

Laser Printing: This method uses a laser to etch the silkscreen onto the PCB. The laser is guided by a computer-generated design, which burns away the PCB's surface to create the silkscreen.

Screen printing method is most commonly used and adopted by a lot of manufacturers, the full silkscreen printing process is shown below.

PCB Screen Printing Process:

The PCB screen printing process involves applying non-conductive ink, typically white, black, or yellow, to label components, test points, and other important markings. Here are all the steps from design to final product:

1. Design Preparation: The silkscreen layer is created in PCB design software and turned into a Gerber file. This Gerber file includes all the information regarding PCB.

2. Screen Preparation: Print the design you prepared in step one on transparent paper. After that, place them on dry silk fabric the way you would want them to look on the actual PCB.

3. Printing: Then place the fabric on board and mount the design firmly on the surface with clear tape. Ink is applied through the screen onto the PCB using a squeegee.

4. Curing: Expose the board design to a UV machine covering the other side with opaque material. The ink is cured via UV light or heat to solidify the markings.

5. Quality Check: The board is inspected to ensure clear, accurate labels.

This process ensures quick identification of components and aids assembly and troubleshooting. To know more about the PCB design see our ultimate guide on PCB manufacturing process.

Types of PCB Silkscreen:

In PCB manufacturing, there are primarily two types of silkscreen processes utilized:

• Manual Screen Printing
• Liquid Photo Imaging (LPI)
• Direct Legend Printing (DLP)

Each of these methods has its unique characteristics, advantages, and disadvantages, and their application largely depends on the specific requirements of the PCB design.

1. Manual Screen Printing: Involves using a stencil to manually apply ink. This method is cost-effective for simple designs but less precise for complex PCBs. The process involves:

• Coating the screen with a layer of photosensitive emulsion
• Exposing it to light through a film positive of the design
• Washing away the emulsion to create a stencil of the design on the screen.
• Applying the ink to the screen, through the stencil onto the surface to be printed.

2. Liquid Photo Imaging (LPI): A photosensitive liquid epoxy is applied onto the PCB, which is then exposed to ultraviolet light through a film that carries the desired legend pattern. Which creates the fine and detailed markings on the PCB. It’s ideal for high-density boards. However, the process is more time-consuming and costly compared to other methods. The need for UV exposure and development adds to the manufacturing time, and the use of photosensitive epoxy and films increases the material costs.

3. Direct Legend Printing (DLP): Uses inkjet printing technology to apply the ink directly onto the PCB. DLP is fast, accurate, and eliminates the need for stencils, making it a preferred option for modern PCB production. Since it does not require any films or UV exposure, the DLP process can significantly reduce the manufacturing time. Although it may not match the high resolution offered by the LPI process. Therefore, DLP may not be suitable for PCBs that require high durability of the silkscreen.

Each method has its advantages in terms of cost, accuracy, and complexity.

JLCPCB Manufacturing Notes to Silkscreen Printing:

Spacing and Linewidth:

To make the silkscreen clear on the board, the width of the texts ( filled lines) and the space between letters need to be no less than 0.15 mm with a height no less than 1 mm. The preferred ratio of width to height is 1:6.

Overlapping and Prioritizing:

When the silkscreen overlaps with the openings, we will follow the principle of prioritizing the openings. In another word, we will ignore the silkscreen and make the openings on the boards only. Under the circumstances, if you want to keep the silkscreen on the openings, please kindly make a note in the remark column so that we can let our engineers and factories pay attention to it and meet your needs.

Silkscreen on Aluminum PCBs:

Currently, we support white and black silkscreen colors for Aluminum boards. White silkscreen will be applied to green and black solder mask ink while black silkscreen for white solder mask ink. Please kindly note, silkscreen is not recommended on bare Aluminum surfaces due to its low level of adhesion and hence will easily fall off from the surface. To know more about PCB ordering and files used in PCBs, see our file format blog explaining these facts.