While most people focus on the components and copper traces that make up PCBs, there’s an unsung hero that often goes unnoticed: the PCB solder mask. Dans ce blog, we take a look at the role of the PCB soldermask, the types of solder mask, different PCB solder mask colors, and how they are applied to the board.
What Is PCB Solder Mask?
A printed circuit board solder mask is the protective coating applied to the surface of a PCB during the Processus de fabrication de PCB. The color we see on PCBs is, En réalité, this solder mask layer. The main function of this layer is to protect the PCB from external contaminants, such as dust, humidité, et oxydation, while also preventing ponts à souder during the soldering process. The solder mask acts as a barrier, ensuring that solder only adheres to the designated areas, ensuring proper component placement and electrical connections.
The Purpose of a Solder Mask
- It protects PCB from moisture as well as the contamination that can cause oxidation and corrosion of copper traces on the board that may affect the board’s electrical performance.
- It helps protect the board from external forces as well as to prevent the short circuits by isolating over copper traces.
- Helps avoid accidental solder connections during component soldering, preventing short circuits.
- Minimizes the wear of pad surface finishes (par exemple., ENIG and HASL) in areas that don’t require soldering.
- This gives the PCB an aesthetic, which is available in different colors as per your design.
Different Types of PCB Solder Mask
Dans la fabrication de PCB, there are four primary types of solder masks, each with its distinct properties and benefits:
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Liquid Epoxy Solder Mask
A traditional and cost-effective choice, the Liquid Epoxy Solder Mask is applied via screen printing. After spreading the liquid mask across a PCB, we cure it by heat, achieving strong adhesion to the surface. This kind of masking provides very good protection from chemicals and heat for P.C.B. against harsh environments.
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Liquid Photoimageable (LPI) Masque de soudure
LPI solder masks are provided in the form of a liquid epoxy resin that reacts to ultraviolet (UV) light. Once a physics mask has been applied to the PCB and exposed to UV light via a photomask, the mask would be hardened. And when the unexposed portions are washed away, a precise and even layer is left.
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Dry Film Solder Mask (DFSM)
The Dry Film Solder Mask is a sheet used to cover the PCB surface by means of a lamination. This method provides excellent uniformity in thickness, making it a good choice for circuits souples and other special applications. Though the material is more expensive than liquid masks, DFSM is more environmentally responsible, as it does not require liquid solvents.
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Top and Bottom-side Masks
This kind of solder mask refers to apply masks to each side of the PCB, which is not a special material type. It’s suitable for complex multilayer PCBs that may have specific requirements for each side.
Solder Mask Material Used
Époxy, polyimide, or acrylic are generally used to form the solder masks. The most common epoxy-based solder masks are strong, chemically and thermally resistant, and bond well —making them suitable for general applications. In high temperature environments, polyimide masks are superior, and they are a go-to choice for aerospace and military applications. Acrylic masks are less durable but cheaper. The material of choice is dependent on the particular circuit requirements and anticipated manufacturing and operational conditions.
5 Popular Solder Mask Colors
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Green Soldermask
Today green solder mask is still widely used as it has good visibility characteristics and good contrast with white silkscreen text. Green masks are available in matte or gloss and as particularly helpful for inspection processes and viewing quality workmanship through clear trace visibility.
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Blue Soldermask
Blue solder masks excel with silkscreen labeling but may require specific lighting for trace inspection. They are often chosen because they look so good, especially together with an LCD display, but they come to show dirt more easily.
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Black Soldermask
Black solder mask is aesthetically good, featured with moderate visibility. We can see the components and labels on it without difficulty though shadow can cover traces. D'autre part, it may face the thermal management issues.
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White Soldermask
White masks create visually striking boards but can complicate inspection by obscuring traces, especially under direct lighting. pourtant, they provide exceptional contrast with black silkscreen markings.
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Red Soldermask
Great visibility and contrast make the red solder mask a really popular choice. They are less in contrast than green boards but offer a fresher more modern look. Red boards also lend themselves well to silkscreen markings.
How Is Solder Mask Applied on a PCB?
Étape 1: Préparation de surface
The PCB is cleaned thoroughly to remove dirt and contaminates, and dried to ensure proper solder mask adhesion.
Étape 2: Solder mask coating
Load the board into a vertical coater, then apply the solder mask ink to it. The coating thickness depends primarily on application and reliability requirements.
Étape 3: Initial hardening
Dans ce processus, a partial curing of the solder mask takes place to keep the coating solid. Donc, unwanted mask areas during the development stage can be removed easily.
Étape 4: Imaging and hardening
The mask areas are defined by a photo film, etched with a laser. The film lies at the same angle as the board, which has been coated and partially dried. UV light is used to harden the ink of the masked areas in the film. This step is done in LDI imaging without photo film; UV light is directly applied to the solder mask.
Étape 5: Developing and washing
Immerse PCB in a developer solution to wash away the excess solder mask, and then real the copper areas that need to remain exposed.
Étape 6: Final hardening and cleaning
The PCB is cleaned thoroughly to remove the residues, the Solder mask is fully hardened to its final state and ready for the surface finishing process.
Solder Mask vs. Paste Mask: Key Differences Explained
Solder mask is applied across the PCB except for areas to be soldered, which is mainly used for PCB protection as well as for avoiding the formation of solder bridges. While for paste mask, it is used in the process of PCB assembly. They assist in determining where solder paste will be used within the board. Usually it only covers regions where components will be placed.
Key differences include:
- Solder mask is used during the fabrication of PCB while paste mask is used duringSMD
- Solder maskprotects the PCB from soldering mistake while paste mask helps in paste deposition.
- Soldering mask comes in many colors while the paste mask only comes in gray color.
Conclusion
The PCB solder mask is an important part of printed circuit board design and production. Also it acts as a shield to prevent short circuiting and also increase durability. While the traditional green solder mask remains popular, manufacturers now have options ranging from blue to white to red, each offering specific benefits. But no matter what color of soldermask you choose, or what type of solder mask you use, you have to concentrate on solder mask quality to ensure high performance PCBs are produced.
