When producing a PCB we often encounter a green color layer that goes above the copper. This layer is most commonly known as the “Solder mask”. Solder mask has many use cases, often used as a protective finish to cover the PCB from possible defects.

In this article, we will walk over the core principles of the PCB solder mask: how to apply the paste layer during PCB production and what we can do to improve the quality of our PCB board.

We will explain the differences between solder mask and solder paste and the tests that a single PCB must undergo.

What is a solder mask?

If you are reading this article, you probably know that PCB boards are made of copper. But copper is not enough to create a fully working and quality piece of the printed circuit board.

When PCB leaves the production line, the copper traces are exposed, causing the board to be vulnerable to oxidation, corrosion, and other environmental impacts. To protect from those natural causes, a solder paste mask came into existence.

Solder paste mask vs. solder mask

The solder mask is designed to protect the PCB as an additional protection layer. The mask refers to the coating around the PCB, while the paste refers to the patterns applied to the mask. There are different PCB solder masks, but most share the same principles.

Solder mask expansion

When solder paste is pasted over the PCB board, it’s essential to ensure the paste is at the exact amount needed. Not too much and not too less.

This can be done by using proper solder mask expansion. The recommended increase stands for 2-4 mils. This size stands for the distance from the pads and should not extend the abovementioned size.

By ensuring the holes punched in the solder mask are at that size, we can guarantee a perfect amount of PCB solder paste across the manufactured circuit board.

Solder paste layers

When talking about solder paste layers, we refer to the layers inside the CAD software. Those layers can be exported in a particular format to create a solder paste stencil. The software can generate both the top and bottom layers according to the needs. If the PCB design has only one layer, then the top paste one will be exported by default.

Types of solder masks

There are different ways to create a PCB solder mask and apply a paste mask. Below we collected the most popular ones that PCB manufacturers use:

• Top and bottom paste layers – those layers designed and used by engineers. Engineers use them to recognize particular design openings and differences in the PCB design. The top and bottom layer masks spread using various techniques such as epoxy or film during the PCB production process.
• Epoxy solder mask – epoxy is the most popular due to its low price. This is the most commonly used technique to apply a solder mask to a PCB board. Epoxy as a polymer material is easy to apply and cure using UV lights.
• LPI solder mask – LPI is short for liquid photo imageable. Those liquids are made separately and then combined to create the final PCB finish. The benefit of LPI is the color options such as red, blue, black, and others, which customers commonly order according to the project demands. This PCB solder mask is also considered a low cost as the materials are made of polymers.
• Dry film mask – besides epoxy and polymers, another popular technique is using a dry film mask. Dry liquid photo imageable film is applied using vacuum lights (illumination). By controlling the light intensity (opening and closing), we maintain the printing of solder mask patterns. This technique is considered quite complex and often used by high-density PCB designs as those can benefit the most according to the design’s complex needs.

Solder mask benefits

There are many benefits of PCB solder masks:

• Protects the PCB with an extra layer against corrosion and oxidation.
• Protects the exposed copper on the PCB board, which could cause short circuit board.
• Creates a good bond with solder paste mask. We are reducing the amount of solder paste that needs to be used.
• Creates a longer and better product shelf-life

Normally the solder paste mask is applied by default, but the customer can provide the color, thickness, and other configurations according to the project needs.

Ensuring the pasting quality

It’s essential to ensure that the solder mask is pasted evenly. The typical way to do so is called “solder mask clearance.” This technique will check the clearance of the silkscreen and the solder paste, ensuring no exposed copper wires are left vulnerable. This kind of test can be done through most popular CAD software such as Altium designer, Eagle, etc..… The solder mask clearance is an important rule to ensure the quality and consistency of the PCB solder mask production.

During the production process, there are more ways PCB manufacturers could test the PCB solder mask quality:

• Visual inspection under a magnifier to detect any abnormalities in the production process.
• Resistance test using chemicals and specialized equipment.
• The temperature resistance test ensures the coating is solid and won’t melt under normal operations.

In-house stencil manufacturing

Every PCB project needs protection for long shelf and project life. PCB solder mask and paste mask were designed for this exact purpose.

We create the solder mask stencils in-house if you wonder where and how such coating can be produced at PCBasic. Unlike many manufacturers that outsource the solder stencil to third parties, PCBasic puts quality first.

By offering a one-stop service and manufacturing the solder mask stencils at our pcb factory, we control the quality and the speed of the fabrication process.

Whether your PCB is single-sided or double-layered, we can produce stencils and ensure the solder pasting process goes smoothly. In any need, feel free to submit a solder mask stencil quotation request, and our sales team will get back to you in no time.