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Fresh PCB Concepts: What You Should Know About Your Board’s Solder Mask

It’s a weird time we’re all in, and for most of us designers, work has slowed down or even stopped. While work might have slowed, now is a good time to review some of your past PCB projects to see if you can improve functionality and reduce field failures. One of the simplest ways to do that is to start from the top: solder mask.

Solder mask might just be the most overlooked portion of the PCB, but it’s a big part of protecting the circuitry and components. Not all solder masks are the same, and the circuitry of the PCB could affect solder mask performance. Let’s look at a few quick solder mask improvements you can make to improve the robustness of your PCB.

When looking at the solder mask covering the full board, the solder mask thickness is always a consideration. This is pretty minor for typical boards with 1-oz copper, but as you increase copper weight, solder mask thickness can be a concern. The reasoning is that the traces of 2-oz copper is taller than 1 oz when looking at a cross-section. That means the solder mask must cover the taller trace, including the susceptible areas like the corners of the trace when looking at the cross-section (Figure 1).

It’s desired to have solder mask bridges between exposed copper pads. This helps to prevent shorts during the assembly process. As components are being miniaturized, designers have to understand that the component pitch might affect the ability of a factory to create a solder mask bridge between the pads.

It’s not only important to know that a solder mask bridge can be made, but also that the solder mask will hold up to simple wear and tear. One of the ways we test the structural integrity is through the peel strength test. This test confirms the adhesion properties of the solder mask, and it can be as simple as putting a piece of tape on the solder mask then lifting if up to see if any of the solder mask lifted. The peel test is helpful when checking the weakest points of the solder mask, the bridge area. Having a good solder mask adhesion is a characteristic of factory quality. If you constantly have solder mask failures, consider working with the factory for better ways to improve performance. Also, it might be time to consider a higher-quality factory.

Instead of looking at every component and confirming the spacing is enough for proper bridging, you can leverage your design software. This is done by adjusting the design guidelines. Not only will a design guideline check help you for better component placement, but it will also save time on the factory side. Countless times factory will catch a solder mask bridging error, then stop to confirm the customer acknowledges this. Don’t waste unnecessary time and make the fix today (Figure 2). NCAB has provided design guidelines available for download on our website for various technologies; multilayer, HDI, semi-flex, and flex/rigid-flex.

Figure 2: An example of thin bridges and the challenge of solder mask adhesion to the PCB.

The color of the solder mask will affect some of the parameters listed previously. The chemicals that make green solder mask differ from what’s used to make purple and white. This also will affect the adhesion of the board if the factory has not mastered the nuances of the chemistry. While having a cool board color helps give that PCB some personality, be mindful of unintended effects that could be a result of that pink PCB.

Reviewing the solder mask can also mean making improvements to help the functionality of the whole PCB. One way this could be done is through a process of tenting vias. This means covering the annular ring of the via with solder mask. When done on the full PCB, this reduces the number of conductive points that are exposed. If the PCB brushes up against wires or the metal casing, it’s preferred to tent the vias.

Similar to tenting vias, some designers take it a step further and fill the vias with solder mask. This is called solder mask plugging. PCB vias can store moisture and dust, degrading the via connection over time. To reduce this, plugging the via with solder mask is a simple fix (Figure 3). There are other ways to fill the via hole, but that’s outside the scope of this column.

Figure 3: Example of a solder mask via plugging.

Continue to use the current downtime to learn something new and improve in an area. Researching solder mask is about as fun as watching paint dry, but these techniques will help you design robust PCBs that save your company time and money.

Harry Kennedy is a field application engineer at NCAB Group.