The role of PCB solder mask and flux

Solder masking is an important part of today's printed circuit board technology. The use of PCB solder masks has become so common that it is most unusual to see that printed circuit boards without any solder resist coverage, except for some home built circuits today, and even many prototype boards have solder resists, so they use commercial production. Printed circuit boards can be said to be universal.


The purpose of soldering a printed circuit board

As its name suggests, solder masks cover the field of printed circuit boards used on circuit boards to protect printed circuit boards from taking solder. In this way, only a region where solder is covered is actually required, i.e., where the component is the region to be soldered, the solder resist is free, and soldering is possible, which provides many advantages. The main thing is that only whereit has to be soldered, and reaching a resist by some areas of the solder prevents a small short circuit of the solder bridge can be significantly reduced. This is becoming more and more important because the very fine pitch of many printed circuit boards today means that small solder tracks during the soldering process are prone to bridges and short circuits. The areas where the solder joints are limited to this problem will be soldered, and these areas can be designed accordingly.

In addition to its function of preventing solder from causing a small bridge, the solder of the printed circuit board also acts as a substrate for the protective layer. Solder mask provides electrical insulation and protection against oxidation and corrosion. This can increase the overall reliability of the printed circuit board over time, especially if it is exposed to harmful reagents.


The role of PCB solder mask and flux

What is a PCB flux?

Solder resists for printed circuit boards are permanent resin-based coatings applied to printed circuit boards during the manufacture of bare boards. Solder mask is a permanent coating of a resin formulation, usually green in color, which encapsulates and protects the surface features of all printed circuit boards unless it is necessary to form a specific area of the solder joint.

Although green is the most widely used color for solder resist, almost any color can be used. Although it can be difficult to maintain precise colors, it is possible to make them almost any color. However, from green, other popular colors are red and blue.

Application PCB solder mask

In order to make the solder mask of the printed circuit board meet the very precise requirements of today's surface mounting technology, SMT printed circuit boards, liquid photosensitive (LPI) solder resists are used. Previously, printed circuit board soldering was stencil printing using the silk screen resist application.

The LPI process for solder masks is very different from the previously used stencil printing. The LPI separates the coating and imaging operations to achieve the highest level of precision. The material used in the PCB solder mask by the bare printed circuit board manufacturer is in the form of a liquid photopolymer, and it is coated with the material using the epoxy or epoxy-acrylate resin technology and the entire board. The thickness of the material is typically copper from about 30 microns to 20 microns above the bare board. Once the flux coated resist material is dried, it is exposed to the desired image pattern and then unrolled to obtain the desired solder resist pattern. Then develop a post-weld cure to ensure it provides a tough, long-lasting finish.