PCB Silkscreen

PCB is an essential part of electronic devices, while PCB silkscreen is an important step in the PCB manufacturing process. This article will introduce the definition, function, design principles, and production methods of PCB silkscreen.

The Definition of PCB Silkscreen

PCB silkscreen, also known as legend printing or marking printing, refers to the process of printing text, symbols, or graphics on the surface of a PCB. PCB silkscreen is typically printed on the component side (assembly side) or the non-component side (soldering side) to facilitate the assembly, debugging, and maintenance of electronic components.

• Silkscreen markings include:

• Component Reference designators

• Polarity markings

• Component outline dimensions

• Pin 1 indicator

• Revision number

• Company logo

• Manufacturer identification

• Warning labels

The Function of PCB Silkscreen

• Facilitate electronic component assembly

Marking designators, component values, and polarities on PCB silkscreen helps to quickly identify component locations and quantities for accurate component assembly. In addition, PCB silkscreen can indicate the direction of electronic components, ensuring circuit connection accuracy and reliability.

• Provide PCB information

In addition to component information, PCB silkscreen can also display important information such as PCB version numbers, manufacturing dates, manufacturer information, and PCB usage, which can provide strong support for maintaining, upgrading, and tracing the history of the PCB.

• Enhance aesthetic appeal

PCB silkscreen can make the PCB look more neat and aesthetically pleasing. A carefully designed PCB silkscreen can increase the PCB's texture and also improve product recognition.

The Design Principles of PCB Silkscreen

• Clear and legible font

The font on the PCB silkscreen should be clear and legible to ensure that personnel can accurately understand the information. It is recommended to use only two directions in the silkscreen for ease of reading. The direction of the reference designators should be readable from left to right and from bottom to top in the layout (as shown in the figure below), which makes the silkscreen look even and clear.

• Avoid overlap as much as possible

The component names, numbers, and other information on the PCB silkscreen should be avoided from overlapping as much as possible to avoid ambiguity or confusion. At the same time, the silkscreen should avoid overlapping with vias or pads to prevent partial silkscreen loss during the opening of solder mask windows, which could affect recognition. The spacing between silkscreen elements should be greater than 5mil.

• Clearly indicate the location of polarized or directionally oriented components

For polarized or directionally oriented components, their polarity and direction should be reflected on the silkscreen, and the polarity direction indicator and installation direction indicator on the silkscreen should be clearly marked to prevent installation errors.

• Avoid placing silkscreen over high-speed signal lines

This recommendation is for high-speed signal lines on the top or bottom layer, which can be considered microstrip lines. The signal propagation speed (phase velocity) on a microstrip line is dependent on the dielectric material. If silkscreen is placed over the line,the dielectric will become non-uniform, leading to a change in phase velocity and ultimately resulting in impedance discontinuity and degradation of signal quality.

The Production Method of PCB Silkscreen

• Manual screen printing

Screen printing is a traditional and relatively economical printing method that applies epoxy resin ink to the laminate and dry to obtain the imprint on the board. Screen printing is easy and cost-effective, but also has drawbacks, such as the need for larger spacing between the boards and longer drying time for the stencil. Screen printing can be considered when the budget is low and board spacing is not a problem.

• Liquid photo imaging (LPI)

Liquid photo imaging is very similar to the process of applying soldermask. In this case, liquid photo-imageable epoxy resin is applied to the laminate and then exposed to ultraviolet light. The circuit board is developed and cured, which requires more preparation than manual screen printing.

• Direct legend printing (DLP)

DLP is the most accurate but also the most expensive of these three screen printing production methods.

In this process, an inkjet projector is used together with acrylic ink, which is applied directly from CAD data to the original PCB. The ink is cured with ultraviolet light during printing. It should be noted that acrylic ink will not cure on boards with silver finishes.

DLP uses acrylic ink to save drying time. This method has the highest accuracy and best time efficiency and can accommodate very low text heights, but it is not recommended to go below 0.02 inches as it becomes unreadable.

The table below summarizes the advantages, disadvantages, and DFM considerations of the three screen printing methods.

In the following video, there is an introduction to screen printing and direct legend printing, you can take a look.

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