Solder Mask vs. Solder Paste: Understanding the Difference

The difference between solder mask vs. solder paste goes beyond function. It includes material composition, application areas, and more. Join us as we unmask the two layers, explaining how they differ and their roles in PCB making.

What is Solder Mask?

Solder mask (solder resist) is the protective layer that PCB manufacturers apply to bare circuit boards. It’s usually a resin or lacquer-like material and often green, but it can also be various other colors.

The mask can be a removable or a non-removable type. It can also be a flexible kind for flex printed circuit boards or specific sections; it depends on the PCB’s application and other needs.

In addition to environmental protection, the layer helps prevent electrical incidents such as short-circuits. It also has various other functions, as we find out later when comparing it to solder paste.

What is Solder Paste?

Solder paste is a semi-solid material applied to the conductive surfaces of a bare circuit board, specifically where components attach to the board. It contains solder and thick flux.

The solder is a low-melting alloy that easily liquefies when heated but hardens quickly to mount the parts. On the other hand, the flux cleans the metal surfaces while also enhancing component mounting.

A magnified image of solder paste on PCB pads

Resource: https://youtu.be/7ekHQzLW6r8?si=jYptanm2gMuaF1I8

Solder Mask vs. Solder Paste

So, what is the difference between solder mask and solder paste? For a better overview of the variations, let’s compare the two layers side by side and on various fronts.

Composition

The base material of a solder mask is a resin, such as epoxy. Other major contents include solvents, hardeners, fillers, and dyes or pigments. The resin is the structural component, while the solvent controls viscosity.

Fillers introduce desired characteristics, such as robustness and thermal stability. The pigment determines the color. Sometimes, the mask is a dry film that gets laminated to the circuit board.

The paste is a mixture of tiny solder spheres and thick, sticky flux. The solder is usually a blend of two metals (alloy) and is typically tin/lead or tin/silver-copper. The flux is a mixture of a solvent, resin, and various additives.

Most often, the flux resin is rosin, an organic compound, or it can be a synthetic type. Additives include activators and rheology modifiers. There are also surfactants to help the paste spread.

Purpose

Solder mask covers the PCB surface, offering environmental protection and preventing corrosion. Being non-conductive, it also guards against short circuits. Additionally, the insulation increases the board’s breakdown voltage.

During assembly, the layer helps prevent bridges. It ensures solder remains within the designated areas, which are the SMD component pads.

Solder paste is the glue that holds components to the PCB before assembly. It temporarily attaches the components, allowing time to heat and melt the solder to create permanent joints.

The flux cleans the pads, removing oxides and preventing re-oxidation. It also aids the assembly process by improving flow and promoting adhesion. The suspended spheres melt when exposed to heat, creating component joints.

Appearance

There are two types of solder masks: liquid-based and dry film. The resin can also be any color the PCB manufacturer or customer chooses. The most popular choice is green. Other options include black, blue, purple, and clear.

Customers can select specific colors for branding or aesthetics. The manufacturer may also use color-coding to differentiate prototyping and production boards. The layer’s finish can be gloss or matte.

Solder paste is a grayish material, sticky and highly viscous. The grey color is due to the tin content. It’s also usually smooth with a uniform, creamy texture.

Under a microscope, it appears as solder balls suspended in the flux. High-quality paste will usually have a consistent texture, without lumps. It also spreads easily but is not too runny.

Physical Location

Solder mask covers the entire circuit board except the component pads and other conductive surfaces, such as test points. It’s typically a negative or inverse image of the solder paste areas.

Solder paste covers component attachment surfaces. These are areas where surface-mount components are mounted. They are typically exposed copper surfaces and are usually indicated in the PCB’s circuit pattern.

Application Method

The techniques used to apply the two layers also vary. Solder mask has various application methods. One of them is using a photo-imageable liquid resin and hardening some parts to introduce the desired pattern.

Another method is by directly patterning a dry film that has been laminated to the PCB. The laser exposes and cures the required pattern on the board based on the circuitry.

On the other hand, you can apply the paste manually using a brush or syringe. Industrial machines use a stencil to apply the paste. The stencil contains openings for surface-mount pads.

Another technique uses jet printing. The printing head moves above the board and automatically deposits the paste on the pads.

How Solder Mask and Solder Paste Work Together

The two materials are crucial when manufacturing printed circuit boards. One protects the board while the other helps attach components.

We typically apply solder mask when fabricating circuit boards. The paste is usually necessary when assembling the bare circuit boards.

Solder mask offers long-term protection throughout a PCB’s lifespan. In comparison, the paste is a temporary layer that melts during PCB assembly.

Mistakes to Avoid When Using Solder Mask and Paste

A successful application of the two layers is crucial for PCB quality, reliability, and durability. The following are mistakes to avoid, whether using a manual or an automated method.

Solder Mask Mistakes

Incorrect Thickness: The layer is too thin or too thick, deviating from its protection function or interfering with assembly. The ideal thickness is PCB specific, but typically ranges from 0.02 mm to 0.03 mm.

Inappropriate Spacing: Inadequate clearance causes the mask to cover pads and affect component mounting, while excess spacing exposes the uncoated parts to corrosion. It can also lead to solder bridging.

Standard practice involves allowing at least 0.1 mm clearance from the SMT pads. PCB designers must implement this rule when laying out the board, in addition to performing design rule checks.

Misalignment: The layer doesn’t correctly align with the desired surfaces. It usually results from design errors and can lead to exposed traces. Designers must confirm alignment before generating a Gerber file and sending it for fabrication.

Inappropriate Color Choice: Color has a profound impact. Unless the purpose is well defined and necessary, it’s advisable to stick to high-contrast options like green.

Solder Paste Mistakes

Wrong Paste: The product is available in different formulations and qualities. Ensure you’re using the right type for your PCB assembly–and a high-quality option.

Improper Application: Paste application is prone to errors, especially when dealing with fine-pitch pads. Ensure the deposition tool is an appropriate type. Automated applicators must be correctly calibrated to avoid issues.

Incorrect Amounts: Using too little or too much paste can impact the assembly process, causing wetting and other problems.

Excessive Heat: Too much heat affects the material’s properties, leading to weak and unreliable joints. Use the correct heat level and temperature settings throughout the component-mounting steps.

Conclusion

Solder mask and solder paste are essential materials when manufacturing PCB assemblies or PCBs. They serve different functions, but both are crucial to ensuring board protection and proper component assembly. Their correctness begins at the design stage. It also continues through their application, board inspection, and testing.