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Solder Masks: Why They Matter, and Available Types

solder mask

Solder masks have two primary purposes — to prevent oxidation and the creation of solder bridges. Essentially, they protect the copper layer (the copper traces) of the printed circuit board (PCB) from the environment. That’s why they are vital for the longevity and functionality of PCBs.

Much like the PCB design, solder masks differ in type. However, there are four main PCB solder mask types:

  1. Top-side and bottom-side solder masks
  2. Liquid photoimageable solder masks
  3. Epoxy liquid solder masks
  4. Dry film photoimageable solder masks

Although the general purpose is always the same, not every mask will be an excellent fit for every PCB. As one of the leaders in the field of PCB and PCBA, MKTPCB knows just how important it is to choose the right solder mask for your circuit board.

What Are Solder Masks?

Solder masks (stop masks or simply solder resist) protect PCBs from oxidation, which they are highly prone to. Aside from that, these masks stop a solder bridge from forming.

They are actually thin layers of polymer that are similar to lacquer in structure and viscosity. This solder mask layer goes over and around the pads and vias on a PCB. That enhances solderability and diminishes the chances of short-circuiting, solder balling, as well as solder bridging.

During the manufacturing process, we can either spray or silkscreen the solder masks onto the PCB. We develop them to provide openings in the pattern of the PCB for components that lay on it. These are the only openings on the masks as the rest is under a tight seal to prevent solder bridges and balls from forming.

The Purpose of Solder Masks

1. Oxidation

Every metal is prone to rust. When we expose any metal to moisture and oxygen, they do their thing. Sooner rather than later, the properties of said metal change. What’s more, it loses electrons, changes color, or loses electrical properties.

As you can imagine (and probably already know), metal oxidation poses a considerable problem for PCBs. The functionality of the board depends on the electrical properties of its copper layer (or layers). Therefore, when rust compromises those properties, the whole PCB loses a fraction or its entire functionality spectrum.

To avoid that, we cover the boards with solder masks. The masks act as seals that prevent the atmosphere from even reaching the copper layer, thus preventing oxidation.

If you’re wondering what happens to the copper pads that solder PCB components to the board and thus have to be exposed, don’t worry. They usually aren’t left to their own devices. Instead, they have a finishing surface layer over them. Therefore, they are also protected.

2. Solder Bridges

Solder bridges are the unfortunate consequence of miniaturization. Because we keep aiming at making smaller devices, thus creating smaller PCBs, we cause new soldering problems that require innovative solutions.

Solder bridges are a great example of that. They are precisely what the name suggests — a bridge that forms between two (or more than two) pads during soldering. As we all know, those bridges aren’t supposed to be there because they can cause a short circuit or burn up a component (or a few).

Because solder bridges and balls are such huge potential issues, we have to ensure that the PCB and all its components are as protected as possible.

That’s where solder masks come in. We add the coating protection layer during the manufacturing process to protect those components and areas of the boards that aren’t supposed to have any solder. So, if there are pads soldered to the copper on the board, we’ll add a solder mask around and between them to make sure no solder gets anywhere except on the pads, essentially preventing the bridges and balls from ever forming.

Green Solder Masks

If you’ve ever wondered why PCBs are usually green, the answer is, of course, solder masks. Most of the time, the masks are what give the entire board the green finish.

However, that doesn’t mean that they have to be green. Green is just the most popular choice. Because white letters contrast nicely with the deep, dark green background, most developers and manufacturers go for the green color in most cases.

Types of Solder Masks

Now that you know what they are and their primary purpose, let’s see which types of solder masks are at your disposal. As mentioned, not all types will be a good fit for your particular PCB design.

The four main solder mask types are top-side and bottom-side solder masks, epoxy liquid solder masks, liquid photoimageable solder masks, and dry film photoimageable solder masks.

1. Top-Side and Bottom-Side Masks

All solder masks are either top-side or bottom-side. The difference between particular types of solder masks lies in the technique of coating — whether it’s in ink, film, or epoxy.

Therefore, this isn’t exactly a stand-alone category of masks, but it’s still worth mentioning. All masks have openings on them that can fit in component pins in the process of manufacturing.

2. Epoxy Liquid Solder Masks

Epoxy is a particular liquid thermosetting polymer. The most affordable choice, epoxy solder masks, fall into the “most popular” category.

We apply the epoxy to the PCB pattern via the silkscreen printing technique that includes a woven mesh. This particular mesh supports not only stencils but also creating patterns via ink-blocking.

Thanks to the mesh, which has openings, the ink transfer is patterned. The mesh can be silk (hence the name silkscreening) or, more commonly, made of synthetic fibers. After the ink printing, the entire solder mask goes through the process of thermal curing.

3. Liquid Photoimageable Solder Masks

LPI masks use liquid photoimageable ink, a mix of several liquids (solvents and polymers) that are usually mixed just before coating the PCB. That way, the entire mask has a longer shelf life.

We can spray or silkscreen the LPI onto the PCB and then expose it to the necessary pattern and develop it. Thanks to their structure, LPI layers are thin and stick well to the surface area. Because of their efficiency, there’s really no need for any plating finish.

Since the LPI is UV-sensitive, the entire board is cured with a UV laser or photolithography. Before that, the board needs to be as clean as possible and without any traces of oxidation.

After the board goes through the UV exposure process, the mask goes through another development cycle that includes removing the mask with high-pressure water sprays. The final stage is thermal curing, as well as organic coating.

4. Dry Film Photoimageable Solder Masks

The manufacturing process for these particular solder masks includes vacuum lamination. The main difference between DFP masks and LPI masks is in the structure of the protective layer. While LPI uses a liquid, DFP uses dry sheets of specific film.

That film goes through the processes of lamination, exposure, and development. The vacuum makes the film sheets adhere to the board and forces any bubbles and imperfections out.

After the development stage, we make openings in the mask to create a specific pattern. This pattern determines where the copper pads will go. Once that’s done, we layer the copper onto the PCB with electrochemical processing.

Which One Is the Best Choice for Me?

There are a few factors that you need to take into consideration while you’re trying to decide which solder mask to use:

  1. the size (and dimensions) of the PCB
  2. the components you need to include
  3. what you’ll ultimately use the board for
  4. particularities of the surface layouts
  5. positons of holes
  6. your budget

When it comes to miniature devices that require tiny PCBs, those usually use one of the two photoimageable masks. The best way to choose between the two is to consider the surface layout. If it’s not uniform, then LPI is your best bet. The uneven layout has many nooks and crannies that are perfect spots to trap gas if the mask is layered with a film technique.

However, that doesn’t automatically mean that LPI is the superior choice. Ink printing can’t guarantee a uniform thickness. That means that there might be the tiniest difference in your board’s thickness from one end to the other. Depending on your design, that might not be acceptable for you.

Usually, a solder mask is 0.5mm thick. Of course, the thickness will vary if you go for the LPI option. When it comes to the thickness of the DFP masks, they are around 0.3mm thick in the thinnest spread and 0.8–1.2mm thick in the board’s empty areas.

A Few Parting Words

Solder masks are integral pieces of the PCB puzzle. They protect the board from oxidation and stop any soldering bridges and balls from forming. That way, they ensure longevity, functionality, and durability of the board.

The four (or rather three) most common types you can choose from all have their own sets of advantages. Depending on what type of PCB you’re designing, its size, as well as what you’re designing it for, you will need either the epoxy liquid, LPI, or the dry film photoimageable solder masks.

If you’re unsure which one is the best fit for your project or simply want more information on the wonders of solder masks, don’t hesitate to contact us

With over ten years of experience in the field, we have all the answers. MKTPCB is the industry’s leader when it comes to manufacturing and design, so we’re not only willing but also able to help you make a perfect choice.

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