purpose of solder masks in PCBs

Solder masks play an essential role in protecting a circuit board from shorts and other problems. Whether you’re designing PCBs, manufacturing products or troubleshooting boards, understanding the factors that determine solder mask reliability is important to your success.

The primary function of a solder mask is to protect the copper of a pcb from accidental contact with other components during assembly and in use, but they also provide visual contrast between silkscreen text and component traces for inspection and repair. A contrasting color, such as green or red, is usually chosen for this purpose. A solder mask’s durability is determined by the manufacturing process used to apply it, the materials it contains, and the environment in which the board will operate.

A solder mask is made from photo-imageable resist that can be applied as a dry film or liquid. The dry film is imaged on the panel using laser plotting to define the mask area, and then it’s cured with UV light. Liquid mask is typically a liquid photo-imageable solder mask (LPSM) formulation. This can be silkscreened or sprayed onto the panel, then exposed to a pattern and developed. During this process, the PCB is thoroughly cleaned to ensure it’s free of impurities that could interfere with soldering or cause damage to the finished product.

What is the purpose of solder masks in PCBs

Regardless of which type of solder mask is used, the final result is the same: a layer of durable coating that protects the copper traces from contamination by solder, soldering, rework, cleaning chemicals and other environmental hazards. It’s important to keep in mind that the quality of the PCB’s copper and surface finish is equally as important for reliability as the quality of its solder mask.

Like other polymers, the materials used to make a solder mask are susceptible to degradation over time and exposure to the elements. In order to ensure the long-term reliability of a PCB, it’s crucial for designers to understand what factors influence the aging of the solder mask and how to reduce those risks during the design process.

One of the most important factors influencing solder mask reliability is the amount of curing that occurs during and after application. This step is vital for ensuring that the cured solder mask adheres to the substrate and reflects light in a way that’s consistent with the fabrication process.

Another key factor is the amount of thermal cycling that occurs during reflow. Large excursions in the reflow temperature can pre-age the solder mask and reduce its lifetime, so it’s important to keep this in mind when choosing the number of reflow cycles your circuit board will undergo during manufacturing.

Finally, the surface of the substrate that is coated with the solder mask must be clean and dry to ensure proper adhesion. This can be accomplished through a thorough degreasing process that’s performed by either an automated system or manually with chemical solvents. Keeping these factors in mind can help designers ensure that the quality of their boards is consistent from batch to batch, and that they’re ready for manufacturing.