Printed circuit boards are much smaller than they used to be. Despite their tiny form, it is still crucial to provide the necessary information to manufacturers and engineers. PCB silkscreen printing allows manufacturers to appropriately label these small components. Consider these six must-know things about silkscreen printing of PCBs.

PCB Silkscreen Printing Defined

In the world of printed circuit boards, silkscreen printing is the process of layering ink trace on the PCB. Ink is most commonly applied on the component side of the PCB, but it’s occasionally printed on the soldering side. Ink is printed to form letters, numbers, and symbols. This information helps users identify a variety of things, including warning symbols, test points, and component identifiers.

Ink Used in Silkscreen Printing

Manufacturers use specially formulated ink for silkscreening PCBs. This unique, epoxy-based ink is non-conductive and is very different from standard printing ink.

White is by far the most common color of ink used in PCB silkscreening. Black and yellow ink are also popular choices. However, companies do not have to adhere to the standard white, black, or yellow. It’s up to their discretion which colors they would like to use.

Text Design

In addition to the ability to choose which color ink they would like printed on their PCB, companies can choose which typeface they would like the information printed with. There are a variety of standard fonts used by PCB software specifically for silkscreen printing, but custom fonts are also permissible.

While the prospect of choosing a custom font may be an exciting one, it’s important to keep readability in mind. Engineers, manufacturers, quality inspectors, and others will need to read the typeface quickly and easily.

Considerations when designing text include:

• Character width
• Character height
• Character thickness
• Spacing
• Positioning
• Type of ink used (epoxy or acrylic)
• Ink color
• Special ink properties

Information Provided Through Silkscreening

As mentioned earlier, there is a plethora of information provided through PCB silkscreening. In addition to warning labels and date codes, other important information highlighted by silkscreening are:

• Test points
• Identification numbers
• Polarity indicators
• Components
• PCB and PCBA part numbers
• Company logos
• Date codes
• Warning and other symbols
• Manufacturer marks
• Reference indicators
• Pin 1 indicators
• Component outlines

Silkscreen Marking Placement

Placement is crucial when it comes to PCB silkscreening. The ICP, which is the electronic interconnection industry’s trade association, sets the industry standards for PCB manufacturing. Thus, the ICP requires PCB manufacturers to print a manufacturer’s mark as well as an identifier. The identifier allows for lot tracing should an incident or any issues occur. While there is no requirement in terms of placement for these particular markings, most manufacturers tend to fit them in non-essential areas.

For very small PCBs or ones that are shaped in such a way information is difficult to place, some companies choose not to apply a component outline. In these instances, manufacturers must be careful to prevent asymmetrical placements.

Manufacturers should also take care in considering which information to leave out. If reference indicators, pin 1 indicators, or polarity indicators aren’t present on the PCB, there is a high possibility that the component isn’t oriented properly. In addition, placement could be off. These possibilities could lead to poor functionality of the component itself.

The PCB Silkscreening Process

When silkscreening PCBs, many manufacturers follow a relatively standard process.

First, polyester screens are stretched across aluminum frames. Next, the polyester screens are coated with solvent-resistant emulsions. Once the screens are coated, they and the PCB are precisely aligned to ensure the information is printed exactly where it needs to be. After everything is aligned properly, the ink trace is applied.

While the polyester screens can technically be cleaned and used again, manufacturers will not reuse them on circuit boards that require high-resolution printing.

However, there are a variety of silkscreening techniques that manufacturers follow. These techniques include liquid photo imaging (LPI), direct legend printing (DLP), and manual screen printing. Liquid photo imaging and direct legend printing are the two most popular PCB silkscreening techniques.

Technique One: Liquid Photo Imaging (LPI) 

Liquid photo printing is a PCB silkscreening technique used when character and other line widths exceed four mil. During this process, laminate is coated with a liquid photo-imageable epoxy. Then, the epoxy-coated laminate is exposed to UV light for development and curing. This method is highly accurate, even more so than manual silkscreening.

Only white ink is used during the liquid photo imaging process. In addition, LPI calls for more ink usage than DLP or manual screen printing. Lastly, because of the measures taken during the LPI process, setup time can be lengthy.

Technique Two: Direct Legend Printing (DLP)

Direct legend printing is even more accurate than LPI. In the DLP process, acrylic ink is used with an inkjet projector. The acrylic ink is applied directly onto the raw PCB using CAD software. Once the ink is applied, the PCB is exposed to UV light for curing. It is important to note that this process should not be used for PCBs with silver finishes, as the ink does not cure on the finish.

Additionally, direct legend printing is one of the most expensive PCB screenprinting processes.

Technique Three: Manual Screen Printing

Manual screen printing is a technique that’s used when a PCB’s registration tolerance is 0.005 inches, or when the PCB’s line widths exceed 0.007 inches. A stencil featuring all necessary text is used for this technique. Nylon is used to make the traces. Ink is transferred through the stencil onto the appropriate layer or laminate. Once the ink is applied, it is cured in a baking oven.

Manual screen printing is a simple way to create a silkscreen layer, and it’s especially useful for projects requiring large text.

Companies searching for a qualified PCB manufacturer need look no further than Advanced Assembly. Advanced Assembly offers a range of turnkey PCB services including manufacturing and assembly. From new quick turn printed circuit boards to part removal and replacement, the experts at Advanced Assembly are available for your PCB needs.