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Connect the Dots: Designing for the Future of Manufacturing Reality—Solder Mask and Legend

PCBs seem to be getting more complex by the day, so it is important for designers to keep educating themselves about the realities of manufacturing increasingly intricate boards.

Last month’s column highlighted design considerations for the strip-etch-strip process and how manufacturers approach that phase of production. Now that we have a functioning board, we must protect the PCB from environmental hazards and document the circuit components. 

This article will detail the solder mask and legend process and offer best practices for designers to ensure a high-quality result.

Board Inspection
Before the process begins, inspect the boards to ensure the plating and etching are effective and that the PCB meets customer functionality requirements. There are two inspection methods to choose from: visual inspection and automated optical inspection (AOI).

A visual inspection under magnification by an inspector is adequate for simpler designs. Inspectors examine board components for various issues and reject those that do not meet quality standards.

For more complex, higher-density boards or PCBs where a customer requests higher than Class 2 IPC-A-600 inspection, we use an AOI process that performs a near pixel-to-pixel comparison between the physical board and the original design data.

How Designers Can Facilitate a More Effective Board Inspection
Designers can help make the inspection process run more smoothly and effectively by providing their manufacturer with clean data files. The size of the file and the memory required can affect the speed and accuracy of the AOI process, so it is important for the designer to ensure the design file is not overloaded with unnecessary data.

Roughening the Copper
Once the board has passed inspection, begin the solder mask process by slightly roughing up the board’s copper. This promotes adhesion and creates a strong bond between the copper surface and the solder mask. 

There are three methods for roughing up copper, each with its own distinct pros and cons:

1. A mechanical scrub is a trusted, commonly used, and cost-effective method for roughening copper. The process can sometimes create gouges, troughs, and rivers that impact performance for boards requiring high-speed signals.
2. Another method involves running the panels through a slurry of a soft pumice material. This process does not have quite as much impact on the trace performance as a mechanical scrub, and it creates great adhesion. This method also carries a risk of creating pits and gouges, but the key downside for manufacturers is that the pumice becomes hazardous waste that requires extra care during disposal.
3. The last method is an adhesion promoter that chemically roughens up the surface. This is the least likely to create performance-impacting abnormalities on the PCBs.

Designers can communicate their preferences about which method they prefer by consulting with their PCB manufacturer and calling it out with good, thorough design notes in the design specifications.

Solder Mask Application
The most common, proven method for applying solder mask involves flood-coating the board with liquid solder mask and removing it in the areas we want to remain solderable.

Hand screening of the solder mask involves placing the boards in a tight mesh screen contraption: a rectangle with a wood or metal frame with the mesh stretched across it. We use a squeegee to apply the solder mask by pushing it through the mesh onto the board’s surface in a uniform thickness.

This same concept is employed with a screen coater, except it is done by coater equipment rather than a person, which achieves a more consistent deposit. They load the board into the loading section, push a button, and the panel slides into the coating chamber where the liquid mask is applied and squeegeed to both sides simultaneously.

With either of these processes, we coat every square inch of the panel that can possibly contain a circuit board, effectively flood-coating the entire side. To ensure we leave holes where the solder goes, we use a liquid photo-imageable solder mask (LPI) that allows us to transfer the solder mask image file onto the board.

After thermally drying the panel to remove some of the solvents used in the process, we transfer the image onto the board. We have two methods: photolithography and laser direct imaging (LDI).

The first method involves creating a silver halide film with all solderable components, such as surface mount pads, test points, through-hole pads, and some via pads. These pads show up in the film as dark black shapes, effectively blocking the UV light during the exposure process from cross-linking the LPI in those spots. We then run the boards through a developing process that removes the solder mask from these solderable areas and leaves the exposed mask everywhere else. LDI achieves the same result by exposing the LPI that needs to remain on the boards, while not exposing the solderable areas.

After running through the chemical development process, the board is the familiar green (or other color of choice) with the copper pads exposed. We have applied the solder mask and exposed it, but before the final cure of the panel, we apply the legend layer on top with an inkjet printer, so there is good adhesion between the solder mask and the legend ink. Then we thermally cure the panel again to remove all the remaining solvents. Once it comes out of the oven and cools, the solder mask and legend are hard and durable, and all solderable pads are exposed.

Solder Mask Design Considerations
As with any phase of production, designers should be aware of what can go wrong during solder mask and know how to prevent issues before they occur.

Avoid Missing or Thin Dams

As HDI designs become more common, solder mask becomes a more critical part of the manufacturing process. In tight pitch areas of the board, designers need to be sure they have enough solder mask web or dam between fine pitch pads to avoid solder shorts while assembling.

Most manufacturers can resolve and maintain a 23 mil-wide web between pads with green solder mask. Other colors are less capable, especially white and black, and may require a slightly larger dam. Keeping as much of the spacing between pads to accommodate a smaller dam or web between features is important. If pad spacing leaves something like a 1-mil web, most manufacturers will struggle to produce boards that meet design requirements. Narrow pad spacing can also create solder bridging during assembly and result in shorting defects.

Mind Your Solder Mask Thickness

Thickness can have an impact on the manufacturing process and so can the type of solder mask. When we get down to an impedance tolerance of ±5%, every little thing can have an impact on impedance, including the thickness and pigmentation of the solder mask. Solder pad shapes, sizes, and locations should provide sufficient room for solder mask webs and dams, accounting for the solder mask’s swell. Knowing your tolerances and ensuring design consistency will be key to a smooth solder mask process.

PCB Legend Design Considerations
The PCB legend is typically printed over the solder mask layer. It puts all the identifying text, component labels, symbols, and patterns onto the surface of the board. The legend layer serves as a visual guide for people working with the board during processes such as assembly and testing.

As with every stage of production, designers can do a lot to ensure their PCB legends are well organized and clear to the manufacturer. Some best practices include:

• Collaborate with your manufacturer about their specific legend guidelines. Their capabilities and processes will inform the legend design rules, covering everything from text size to clearances to text orientation.
• Make your legends readable. Pay attention to contrast, font readability, and overcrowding. An easy-to-read legend will help avoid production delays and ensure smooth manufacturing all the way through assembly.
• Double-check your legend for accuracy and functionality. Spelling errors can happen, so try to catch them before they go to production. Your manufacturer will thank you.

The solder mask and legend process occurs near the end of the manufacturing process, but we still have to apply a surface finish. Keep an eye out for my next article on that subject. If you can’t wait, listen to the Designing for Reality: Surface Finish episode of On the Line With… To hear our in-depth conversation about solder mask and legend, please listen to episode 10.

Read Matt’s book, The Printed Circuit Designer’s Guide to… Designing for Reality, or listen to his podcast here.

This column originally appeared in the March 2026 issue of I-Connect007 Magazine.