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With Halloween coming up, I’ve been spending so much time thinking about costumes that it probably borders on the irresponsible. This year, I’m going to require fairly extensive face paint, and I’ve been doing a lot of research about how to apply it properly. Different layers, colors, and types of paint require different tools and timing to get it right. I did a practice run without any directions and found sections of my face paint flaking off while other areas remained perfectly intact.
While I’m not likely to have any short circuits occur due to terrible face paint, I’ve found the situation for solder masks is similar, and they now come in multiple colors! Selecting the type and thickness of solder mask is just as important to a successful product as it is to good face paint, and mistakes are a lot more expensive than other types of circuit boards.
What is Solder Mask?
Solder mask is used to protect metal elements on a Circuit Board from oxidation, and to keep "bridges" from forming between solder pads if a little piece of solder attaches somewhere that it shouldn't. It's a critical step in PCB manufacturing if reflow or solder bath is used as those techniques don't provide as much control over where solder bits land on the board. Solder mask is sometimes called “solder resist,” which I think is a better term since I used to think that solder mask was a full layer of solder applied to the board.
PCB Solder Mask Types
All solder masks consist of a polymer layer that is applied over the metal traces on a Printed Circuit Board. There are different types of mask material, and the best option for your board depends on cost and your application. The most basic solder mask option is to use a silkscreen to print a liquid epoxy over the Printed Circuit. This is like airbrushing face paint on with a stencil. Solder mask can be applied in nearly any color.
How is Solder Mask Applied?
Liquid Epoxy Solder Mask
The most basic solder mask option is to use a silkscreen to print a liquid epoxy over the PCB. This is like airbrushing face paint on with a stencil. This involves using a woven mesh to support ink-blocking patterns.
Liquid photoimageable solder mask (LPSM)
Fancier solder masks use a photolithography process with either a dry film or a liquid solder mask, similar to that used for photoresist exposure in semiconductor fabrication. LPSM can be silkscreened like an epoxy, or it can be sprayed over the surface, which is often a cheaper application method.
In this process, a photolithography mask that matches your desired solder mask is made from your Gerber files. Your panelized board is then thoroughly cleaned to ensure no dust particles are trapped under the hardened solder mask. The panels are completely covered on both sides with the liquid LPSM.
One thing you will notice with LPSM is that the black portions of the photolithography mask define areas where you want conductors to be exposed, while areas of the board that you want covered in solder mask will be clear.
Solder masks are applied as an epoxy or a photoimageable polymer.
Once the boards are covered with LPSM, the boards are dried in an oven and placed into a UV developer. The photolithography mask is carefully aligned over the dried board and the board is illuminated with UV light. The exposed areas of LPSM material are cured by the UV light, while the unexposed areas are washed off with a solvent, leaving behind a hard layer of solder mask.
Dry Film Solder Mask (DFSM)
DFSM is applied in a similar process as LPSM. Both are exposed in a photolithography-type process, and the unexposed areas are removed with a solvent. The difference with DFSM is that the unexposed solder mask material must be vacuum laminated onto the board to avoid creating defects with air bubbles.
No matter which type of solder mask material you use, the resulting solder mask will leave exposed areas of copper on the board. These exposed areas must be plated with a surface finish to prevent oxidation. The most common surface finish is hot air solder leveling (HASL), although other popular surface finishes are electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium Immersion gold (ENEPIG).
ENIG plating on exposed copper through a solder mask.
What is the Standard Solder Mask Thickness?
The thickness of your solder mask primarily depends on how thick the copper traces on your board. LPSM and DPSM solder mask thickness in empty areas of the board will generally vary with location. The typical solder mask thickness (perpendicular to the board) is at least 0.8 mils. Solder mask will be thinner near the edges of traces and can reach as thin as 0.3 mils or less. In general, you’ll want about 0.5 mils of solder mask over your traces. Sprayed epoxy solder mask can take a more uniform thickness throughout your Printed Circuit Board.
The lateral thickness of solder mask is quite important when working with fine pitch components, such as high pin count BGAs. In addition to preventing corrosion on copper traces, solder mask is used to place a dam between neighboring pads on a component.
Note the solder mask thickness between pads on your components.
The color of your solder mask is determined by the dye used in the solder mask material, and the chemical properties of the dye will influence the cured solder mask thickness. One reason that green solder mask is extensively used is that it can be used to create thin solder mask dams (~0.1 mm). The dyes used in solder masks with different colors tend to form thicker solder mask dams. No matter which dye you choose to use, solder mask thickness on PCBs for use in certain industries or applications is defined in IPC-SM-840D.
Like any other fabrication parameter or process, you should consider how sensitive your final application will be, and plan your design accordingly. It's always important to discuss the fabrication options with your manufacturer. They may even be able to suggest better options based on their capabilities.
What Solder Mask Should I Use?
Deciding on an appropriate solder mask depends on the physical dimensions of your board, holes, components, and conductors, the surface layout, and the final application for your product.
First, if you have a Circuit Board solder mask that will be used in aerospace, telecom, medical, or other “high reliability” industries, check on industry standards around solder mask, and your intended application in general. There are specific requirements that supersede whatever else you learn on the internet.
For most modern printed board designs, you’ll want a photoimageable solder resist. The surface topography will dictate whether to use a liquid or dry application. A dry application lays down a uniform soldermask thickness across the entire surface. However, dry mask adheres best if your board surface is very flat. If you have complex surface features, then you’re probably better off with a liquid (LPISM) option for better contact with the copper of your traces and the laminate. The downside to a liquid application is that the thickness isn’t perfectly uniform across the board.
You can also get different finishes on the mask layer. Talk to your manufacturer about what they have available and how it will affect production. For example, a matte finish reduces solder balls if you are using a solder reflow process.
PCBs manufactured using a solder reflow process need a solder mask. The finish of the mask can affect the quality of the reflow.
How Do I Include Solder Mask in my Design?
When you design your printed circuit boar, the solder mask should be its own layer in the Gerber files. Check the design rules for a solder mask layer. Usually, you'll want a 2 mil border around your feature in case the solder mask isn't perfectly centered. You'll also have a minimum distance between pads, often 8 mils, to ensure the mask is enough to prevent solder bridges from forming.
If you are producing more complicated PCB designs, it’s important to choose PCB design software that lets you adjust these design rules as necessary. Altium Designer® is a very flexible option. You can even remove the solder mask layer entirely if you have particularly unusual design requirements.
Want to learn more about Altium's capabilities? Contact an expert at Altium here.
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