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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.
What is solder mask?
Solder mask is used to protect metal elements on a PCB 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 since those techniques don't have as much control to ensure no solder bits connect where they shouldn’t. 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.
How is solder mask applied to my board?
Solder mask consists a polymer-layer that gets applied over the metal traces on a PCB. 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 PCB. This is like airbrushing face paint on with a stencil.
Fancier solder masks use photoimaging with either a dry film or a liquid solder mask. Liquid photoimageable solder mask (LPSM) can be silkscreened like the epoxy, or sprayed over the surface, which is often a cheaper application method. The dry film solder mask (DFSM) has to be vacuum laminated onto the board to avoid creating defects with air bubbles. Both photoimaging approaches get developed to remove the sections in the mask where pads will be soldered to components, and cured with a baking process or UV light exposure.
Solder masks are applied as an epoxy or a photoimageable polymer.
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 PCB 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 PCB 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 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 thick should my solder mask be?
The thickness of your solder mask primarily depends on how thick the copper traces on your board. In general, you’ll want about 0.5 mils of solder mask over your traces. If you use a liquid mask, you’ll necessarily have a varying thickness over other features. Over empty laminate areas, you can expect a thickness of 0.8-1.2 mils, and over complex features, like the knee of a circuit, it might be as thin as 0.3 mils.
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.
How do I include the solder mask in my design?
When you design your PCB, 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 two 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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