What is a Solder Mask?

Home » Candor Blog » What is a Solder Mask?

Nov 9, 2020|Manufacturing Excellence, Technological Advancements and Materials

Source: Unsplash

Solder masks, also known as solder resist masks or solder stop masks, have become more popular as the market demand for printed circuit board manufacturing increases. Since surface mount technology (SMT) becomes the easiest way to mass-produce the components, soldering masks become vital.

While hand-soldered assemblies do not require a solder mask, masks are essential for mass-produced PCBs. Here, we will help you differentiate between solder masks and fill you in on the latest industry colors and trends.

What is a Solder Mask Used For?

A solder mask is a thin layer of polymer that protects your PCB from oxidation and prevents solder bridges from forming. Solder bridges are unintended electrical connections that can cause problems if left alone.

The solder mask is typically treated as a PCB layer and referred to as a Gerber file.

4 Main Types of PCB Solder Masks

There are four common types of solder masks. They all generally function the same, but some materials may be more costly. The three primary materials used to make solder masks go through thermal curing to solidify their structure and create a protective barrier.

1. Top Side and Bottom Side Masks

   A thermal pad is located between the copper layers of a PCB to manage heat created by the copper layers of the circuit board. Pins are needed to connect these planes, and top and bottom side masks help make it easier to determine where these pins need to be soldered.

   The top-side solder mask helps to identify the openings in the top copper layer of the PCB colored solder mask. Wearing a mask can make it easier to solder component pins to the board.

   Bottom side masks have a similar function but, instead, show openings for the circuit board’s bottom side.

2. Epoxy Liquid Solder Masks

   Epoxy is the lowest cost solder mask and is silk-screened through a pattern onto the printed circuit board. Despite its name, silk is rarely used in electronics today, but the process remains the same. This printing technique uses woven mesh fibers that create patterns in open areas for the ink to transfer through.
   Once the design is defined, the epoxy is thermal cured. Manufacturers often mix dye into the liquid epoxy to change the color. The result is a custom-color mask.

3. Liquid Photoimageable Solder Masks

   Liquid photoimageable solder masks (LPISM or LPSM or LPI) are made from an electronic material of the alkali class. LPI is delivered as an ink formation that can either be sprayed or silk-screened onto a circuit board.

   The liquid ink of LPIs is most commonly processed through hot air surface leveling (HASL). This process must be done in a clean environment, free of particles and contaminants, because the PCB is dipped into molten solder to bond to any available copper.

   An updated method to this is electroless nickel immersion gold (ENIG), which is a plating process where the PCB is covered in a thin layer of gold to prevent the nickel from oxidation. While this process is more expensive, it is meant to replace other coating processes because of its superior oxidation resistance, surface planarity, and suitability for movable contacts.

   LPI uses dual-curing in its process where it is exposed to UV light to harden any exposed areas. Developers or high-pressure water sprays then remove the PCV. After an LPI is developed, it is cured and receives its final surface coating.

4. Dry Film Photoimageable Solder Masks

   Dry film photoimageable solder masks (DFSM) go through a similar photolithography (UV exposure) process as LPIs. Afterward, it is developed, and openings are created in the PCB pattern, and parts are soldered onto copper pads.

   However, instead of being applied as liquid ink, manufacturers apply dry film in sheets using a vacuum lamination process. This process allows for the unexposed solder mask to adhere to the PCB while also removing air bubbles from underneath the film.

   After the exposure process, workers remove the unexposed parts of the solder mask using a solvent and thermally cure the remaining film.

Colors and Finishes of PCB Solder Masks

There is much more to choose from since the original green finish. PCB solder masks are now available in a range of different colors and glossy or matte finishes. Selecting different colors will help you distinguish between your printed circuit boards. You can also match your colors to your tastes and palette.

If you are looking for a wider selection of color choices, you may want to go with a glossy finish because it gives you more options.

• Green
• Black
• Red
• Yellow
• White
• Purple
• Blue

If you prefer a matte finish, you have fewer options. Currently, only green and black colors are available with the matte finish.

While there is no predominant reason why green is the most common color for solder masks, its popularity could be because it creates the smallest solder dam (space) of all the colors you can choose from.

Green is also shown to be most visible to the human eye in daylight, making green PCBs easier to work on, use, and find.

Importance of Solder Masks on PCBs

The solder mask’s most crucial function is to protect against oxidation and corrosion of the copper PCB.

Equally as important is using the solder mask to prevent solder bridges. Solder bridges are unintentional connections of solder joints on your PCB and can lead to circuit board damage and short circuits. Solder masks create a dam between the solder joints and other conductive parts of the PCB to form additional insulation of the components on your PCB.

Similar to bridges, metal whiskers can also form on your circuit board, causing malfunctions and short circuits. Solder masks prevent the growth of metal whiskers, which are thin filaments that protrude from the PCB and lead to system failure. Metal or tin whiskers are most commonly found on lead-free or tin plating.

Along with these most critical uses, solder bridges have many other benefits.

• Less solder paste is needed during the soldering process
• It may increase the breakdown voltage of the PCB dielectric material
• Protects against contaminants you may introduce when handling your PCB

Solder masks can help your PCB last you a long time. While some choices are stylistic, it is important to understand your needs and applications to choose the best, most reliable option for your application.