Solder mask

The solder mask is the green part of the printed circuit board(PCB) we often see. It is also called a solder resist or solder-top mask, which is a thin layer of polymer lacquer to protect copper traces from oxidation and prevent short circuits. In fact, besides the green cover, there are other colors of the solder mask such as white, blue, black, red, yellow, etc. The choice of different colors depends upon the varies demands of the application.

Solder mask exists on the top side and bottom side of the board, they are usually shown as the GTS layer and the GBS layer in the Gerber file. Solder mask layer means to leave pad positions on the whole solder resist ink area to allow welding. Green ink is applied to areas without pads by default, and the pads also call solder mask opening. The ink part between two pads or IC pins is called a solder bridge.

The functions of the solder mask

1. Preventing short circuit and non-solder parts from being contaminated with solder paste

The direct effect of the solder mask is to prevent welding of areas that should not be welded and prevent solder paste from leakage and causing short circuits. The solder mask is bonded to the board firmly, which has high heat resistance during the soldering process. What’s more, the solder paste may form “tin heads” during the soldering process or cause a solder bridge between conductors on the PCB surface due to print adhesion, which will lower the reliability of welding. Therefore, it is necessary to solder all the parts of the PCB that are not required to connect to the components pins. High insulation of the solder mask shows a significant position in high-density PCB manufacturing.

2. Anti-corrosion and anti-pollution

First, a solder mask can protect the PCB surface from chemical corrosion and pollution during the PCB manufacturing process. For this reason, the solder mask itself should have chemical corrosion resistance and penetration(plating) function, and have superior binding force with the PCB surface. Secondly, the non-connection parts on the PCB are easy to be oxidized and polluted by air and other corrosive dust if without the solder mask. Many major accidents and breakdowns have occurred on this account before the solder mask appeared.

3. Anti-moisture and anti-fungal

PCB parts in the whole machine are easily affected by a wet atmosphere without the protection of the solder mask. The exposed PCB surface will absorb moisture when the relative humidity of the atmospheric and ambient temperature change, and accelerate the corrosion and pollution, even ionic migration(CAF), which lead the damage of machine and accident. Besides, there is a large number of bacteria in the air, and the metal wire will be gradually etched and form a lot of mold between leads if the bacteria are absorbed on the bare PCB surface and mildewed. Then short circuits are inevitable.

The capabilities of the solder mask NextPCB can provide

As an experienced PCB prototype and PCB assembly manufacturer, NextPCB has a mature solder mask process, excellent technical level, and shorter lead time. What you need to know is as below:

About the solder bridge, the minimum space between IC pins should be 10 mils (0.25mm). Please feel free to contact online service or send E-mail if the solder bridge is too narrow, NextPCB will provide the right solutions to help you in the PCB prototype and PCB assembly.

NextPCB is mainly using liquid photo-imageable ink for the material of the solder mask. Green ink is the most commonly applied, but the color of red, blue, white, black, and yellow is also available at the same cost.