Printed circuit boards usually have very closed space traces of copper and component pads. To protect these copper traces and component pads with very little space between them are used. It is primarily used for protection from oxidization and contact with the external environment and electrical isolation on the printed circuit board. In addition to the benefits mentioned above, solder mask material also protects from scratching effects of movable electronics parts and thermal aging, as shown in the figure below.
Other names of solder mask material include solder’s resistance and solder stop masking. Photography is a technique to remove the solder mask material from the traces, pads, and terminals to allow soldering of components on the printed circuit board. The solder mask material is dependent on applications for which particular PCB is manufactured, and also it includes physical dimensions of PCB, size of holes, and components.
Standards should also be taken into account for which PCB is made, whether for aerospace or medical or general electronics or telecom industry. The most commonly used material is epoxy liquid material. It had minimal cost and was made of polymer-type material, which is thermal resistant, and it often requires curing thermally.
The liquid photo-imageable solder mask materials are also sprayed or silk-screened like epoxy screens. Other methods include vacuum lamination on the printed circuit board so that the trap of bubbles is avoided. This technique is known as the dry film solder mask.
The thickness of copper traces and the complexity of the PCB design guide for the solder mask material thickness. The general rule is to place 0.5 mils of material over PCB.
Gerber files and PCB designing software have the provision of adding the solder mask materials over PCB.
Solder mask materials come in different colors, but the most popular material's color is green color due to its easily distinguishable color among other electronics and mechanical components and less irritation to the human eyes. Different colors include black, white, yellow, red, and orange.