Introduction To PCB Solder Mask Coating/Ink

PCB ink in a Printed Circuit Board. The important physical properties are viscosity, thixotropy, and fineness of the ink. These physical properties need to be known in order to improve the ability to use ink.

PCB ink characteristics

1. Viscosity and thixotropy
Screen printing is one of the indispensable processes in the manufacture of printed circuit board. In order to obtain the fidelity of image reproduction, the ink must have good viscosity and suitable thixotropy.
The so-called viscosity is the internal friction of the liquid, which means the friction exerted by the inner liquid under the action of external forces to make one layer of liquid slide over another. Thick liquid inner layer slip encounter mechanical resistance is bigger, thinner liquid resistance is smaller. The unit of measurement of viscosity is berth（Pa。S）. Temperature, in particular, has a marked effect on viscosity.
1Pa。S=10P=1000mPa。S=1000CP=10dpa.s
Thixotropy is a physical property of a liquid, that is, its viscosity decreases in the state of agitation, and soon returns to its original viscosity after standing. By stirring, thixotropy lasts long enough to reconstitute the interior. To achieve high-quality screen printing, ink thixotropy is very important.
In particular, in the scraping process, the ink is agitated, and then make it liquid. This role to speed up the ink through the mesh, promote the original line separated ink evenly connected. Once the scraper stops moving, the ink to a static state, the viscosity will quickly revert to the original required data.

2. Fineness
Pigments and mineral fillers are generally solid, finely ground, with a particle size of less than 4/5 microns and a solid form of homogenized flow. Therefore, it is very important to require ink with fineness.
Types of PCB inks
PCB ink is mainly divided into three lines, soldering resistance, three types of character ink.

wire(or circuit) ink is used as a barrier to prevent corrosion of the line, etching the protection of the line, generally liquid photosensitive ink. There are acid corrosion and alkali corrosion resistance of two types, alkali corrosion resistance price is usually more expensive, this layer of ink in corrosion out of the line to be dissolved with alkali.
Soldering resistance ink is the line after the coating line as a protective line. There are liquid photosensitive also have heat curing, and UV hardening, in the board to retain the welding plate, convenient welding components, play the role of insulation and oxidation.
Character inks are used to mark the surface of the board, such as the component symbol, and are usually white.
In fact, there are other inks, such as: adhesive ink, is to do copper plating or surface treatment of the parts that do not need to be treated to protect, and then can be removed; Silver ink and so on.

Note for PCB ink usage

1. In any case, ink temperature must be kept below 20-25℃, temperature change cannot be too big, otherwise it will affect the ink viscosity and screen quality and effect. Especially when the ink in the outdoor storage or storage at different temperatures, before use must be placed in the ambient temperature to adapt to a few days or make the ink barrel to achieve the appropriate temperature. This is because the use of cold ink will cause screen printing fault, causing unnecessary trouble. Therefore, to maintain the quality of ink, it is best to store or store in normal temperature process conditions.
2. Before use must be fully and carefully on the ink manual or mechanical stir evenly. If the ink into the air, use to stand for a period of time. If dilution is required, the mixture should be thoroughly mixed before viscosity is tested. The ink barrel must be sealed immediately after use. At the same time, never screen ink back into the ink bucket and not used ink mixed.
3. It is best to use mutually adapted cleaning agent for the net, and to be very thorough and clean. When cleaning again, it is better to use a clean solvent.
4. Ink drying, must have a good exhaust system within the device.
5. Screen printing shall be carried out on the operation field where the operating conditions shall meet the technological requirements.

What is the role of PCB inks in the PCB process?

Ink plays a role in protecting copper foil in PCB production, keeping copper skin from being exposed, which will affect subsequent processes. There is photosensitive ink, carbon oil and silver oil, and both carbon oil and silver oil have electrical conductivity. The usual ink colors are white oil, green oil, black oil, blue oil, red oil and butter.