Whether the quality of PCB ink is excellent, in principle, it is impossible to break away from the combination of the above major components. The excellent ink quality is a comprehensive manifestation of the scientific, advanced and environmentally friendly formula. It is reflected in:
Viscosity
It is an abbreviation for dynamic viscosity (dynamicviscosity). Generally expressed by viscosity, that is, the shear stress of fluid flow divided by the velocity gradient in the direction of the flow layer, the international unit is Pa/sec (Pa. S) or milli Pascal/sec (mPa. S). In PCB production, it refers to the fluidity generated by the external force of the ink.
Conversion relationship of viscosity units:
1Pa. S=10P=1000mPa. S=1000CP=10dpa.s
plasticity
It means that after the ink is deformed by external force, it still retains its properties before deformation. The plasticity of the ink is conducive to improving printing accuracy;
Thixotropic
The ink is gelatinous when standing, and the viscosity changes when it is touched, also known as tremor and sag resistance;
fluidity
(Leveling) The extent to which the ink spreads around under the action of external force. Fluidity is the reciprocal of viscosity, and fluidity is related to the plasticity and thixotropy of the ink. The plasticity and thixotropy are large, the fluidity is large; the fluidity is large, the imprint is easy to expand. Small liquidity, easy to appear netting, resulting in ink formation, also known as netting;
Viscoelasticity
It refers to the ability of the ink that is sheared and broken after the ink is scraped by the squeegee to rebound quickly. It is required that the ink deformation speed is fast and the ink rebounds quickly to be beneficial to printing;
Dryness
It is required that the drying of the ink on the screen is as slow as possible, and after the ink is transferred to the substrate, the faster the better;
Fineness
The size of pigment and solid material particles, PCB ink is generally less than 10μm, and the size of the fineness should be less than one-third of the mesh opening;
Wire drawing
When the ink is picked up with an ink shovel, the degree to which the filamentous ink is stretched without breaking is called stringiness. The ink filament is long, and there are many filaments on the ink surface and the printing surface, which make the substrate and the printing plate dirty and even unable to print;
Transparency and hiding power of ink
For PCB inks, according to different uses and requirements, various requirements are also put forward for the transparency and hiding power of the ink. Generally speaking, circuit inks, conductive inks and character inks require high hiding power. The solder resist is more flexible.
Chemical resistance of ink
PCB inks have strict standards for acid, alkali, salt and solvents according to the purpose of use;
Physical resistance of ink
PCB ink must meet external scratch resistance, thermal shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements;
Safety and environmental protection of ink
PCB ink requires low toxicity, odorless, safety and environmental protection.
Above we have summarized the basic properties of twelve PCB inks. Among them, in the actual operation of screen printing, the problem of viscosity is closely related to the operator. The viscosity is very important to the smoothness of the silk screen. Therefore, in the PCB ink technical documents and QC reports, the viscosity is clearly marked, indicating under what conditions and what type of viscosity testing instrument to use. In the actual printing process, if the ink viscosity is too high, it will be difficult to print out, and the edges of the graphics will be severely jagged. In order to improve the printing effect, a thinner will be added to make the viscosity meet the requirements. But it is not difficult to find that in many cases, in order to obtain the ideal resolution (resolution), no matter what viscosity you use, it is still impossible to achieve. Why? After in-depth research, I discovered that ink viscosity is an important factor, but not the only one. There is another quite important factor-thixotropy. It is also affecting the printing accuracy.