Understanding Soldering – Part 5: Solder Mask Application Process

Like many other neighborhoods, my street was filled with masked children searching for treats this last Halloween. A mask is often the crowning component of a Halloween costume in order to complete the persona of the character. Not only does it conceal the identity of the child wearing it, but more importantly to their parents, a mask can also provide a measure of protection from the colder fall weather.

Protection is also one of the most important benefits of another kind of mask; the solder mask used on a printed circuit board. Without the solder mask, a circuit board would be exposed to harmful environmental conditions as well as abuse during the soldering process. There are different types of solder mask that can be applied to a circuit board, as well as different methods of application. Let’s take a look at this and how the PCB designer can be better prepared for their board to go through the solder mask application process.

The Solder Mask Application Process: What is Solder Mask?

Solder mask, which is also known as solder resist, is a protective layer of material added to the front and back exterior surfaces of a circuit board for the soldering process. As the name suggests, solder mask prevents unintentional bridges of solder from being formed between metal pads by masking or blocking those areas of the circuit board that are not to get solder on them. This is normally all of the surface areas of the board including both conductive metal and non-conductive areas, except for the metal pads that the electronic components will be soldered to. Since the majority of the circuit board is covered by solder mask, it also protects the board from other problems such as metal oxidation and contamination from corrosion, dirt, or even human handling.

Solder mask is usually made from resins because of its ability to resist humidity, solder, and high temperatures. Although solder mask is available in many colors, it is most commonly applied in green. When other colors are used it is often because of specific needs, such as the color red being used to designate a prototype board. Solder mask is applied to the circuit board through three different media forms, after which it is thermally cured to set and harden it. Next, we will look at what those different media forms are and how they are used.

How is Solder Mask Applied to a Printed Circuit Board?

There are three main types of solder mask application media that are typically used during circuit board manufacturing:

  • Epoxy Liquid: The epoxy liquid is screen printed on the board through a stencil made of a synthetic fiber mesh. This is the lowest cost option for applying solder mask, but it doesn’t have the high level of resolution that comes with using a photoimageable application.
  • Liquid Ink Photoimageable: The liquid ink solder mask is screen printed or sprayed onto the circuit board. Next, the solder mask is exposed to ultraviolet light through a negative image of the solder mask. Those areas exposed to the UV light will harden while the other areas will be cleaned off leaving the metal pads to be soldered exposed.
  • Dry Film Photoimageable: The dry film solder mask is processed the same way that the liquid ink with it’s exposure to UV light followed by stripping off the unhardened areas of solder mask. What is different is that instead of a liquid, the dry film is applied in sheets of solder mask film over the board using a vacuum lamination process.

All three media types require that the boards first be cleaned to remove any dust or tarnish, then they have their solder mask material applied. For those boards that are screen printed with liquid epoxy, their next step is to go into the final hardening and curing process. For the photoimageable applications, they will be exposed to UV light to harden the solder mask that is to remain on the boards, and then will be processed through a cleaning cycle to remove the unhardened material. Afterwards, they too will go through their final hardening and curing process to create a hard and permanent coat of solder mask.

Throughout the entire process, technicians constantly check to make sure that the solder mask material is applied at the correct thickness, and that the temperatures of the different hardening processes are at optimum. It is also essential that the technicians regularly check to make sure that the correct alignment of the solder mask image to the board has been maintained. This way they can make sure that there aren’t any traces of ink on the pads or going through any of the drilled holes in the board.

Solder mask is regulated by industry standards for its materials, shelf life, physical requirements, and other requirements. IPC-SM-840D has been developed specifically to cover these standards.

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What You Should Know About Solder Mask When Designing Your PCB

For the most part, the creation of solder mask in PCB design tools has become entirely automated. Most of the interaction between the designer and the tools is to make sure that unique pads and shapes are correctly configured for correct solder mask relief in the software. There are usually ways to change the size of the solder mask clearances around pads within the tools, but you should first check with your manufacturer on what they need to work with. There are some design issues related to solder mask though that you should be aware of:

  • Solder mask thickness: This can vary depending on the topography of the board. For example, a trace running between the pads of a small passive component may bump up the solder mask that covers it in an effect called drawbridging. This could effectively raise the component or one of its pins up off the board during manufacturing, and some designers will avoid running traces in those areas if at all possible.
  • Solder dams: Making sure that there is sufficient solder mask between the narrow pads of a fine pitched pin part is essential for isolating one pad from another. By using solder dams between the pads, it will keep solder from unintentionally bridging between them.
  • Solder mask color: Solder mask is available in different colors, but those colors can impact the manufacturability of the board. Green solder mask is the most widely used due in part to its superior properties, and it can reliably create the smallest solder dams over any other color. However, other colors may be preferred for specific purposes. Red, for instance, is often used to distinguish prototype boards from regular production runs.
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  • Smooth transition from prototyping to production.

At Tempo Automation, we are ready to work together with you on the solder mask requirements for your PCB design. We can advise you on areas of your design to pay particular attention to such as solder dams so that your board can be manufactured without errors or delays.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the solder mask application process that will be used on your design, contact us.

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