Thixotropic Effect on Ink Properties and Exploration (2)

three. PCB ink several important technical performance:

Whether or not the PCB ink quality is excellent is in principle impossible to separate from the combination of the above major components. The excellent ink quality is a comprehensive manifestation of the formula's scientificity, advanced nature and environmental protection. It is reflected in:
1 Viscosity: Abbreviation for dynamic viscosity. Viscosity is generally expressed as the shear stress of the fluid flow divided by the velocity gradient in the flow layer direction, and the international unit is Pa/s (Pa.s) or milli-Pascal/sec. (mPa.S). In PCB production, the fluidity of the ink is driven by external forces.

Conversion of viscosity units:

1Pa. S = 10P = 1000mPa. S =1000 CP=10dpa.s

2 Plasticity: refers to the nature of the ink before it is deformed after it is deformed by external forces. The plasticity of the ink helps to improve the printing accuracy;
3 thixotropic: (thixotropic) ink when standing gel-like, but by the touch of a change in the viscosity of a property, also known as buckling, sag resistance;
4 Fluidity: (leveling) The extent to which the ink spreads around under the action of external forces. Fluidity is the reciprocal of the viscosity, which is related to the plasticity and thixotropy of the ink. Plasticity and thixotropy are large, and fluidity is large; when the fluidity is large, the imprinting is easy to expand. Small liquidity, prone to netting, resulting in ink phenomenon, also known as reticulate;
5 Viscoelasticity: refers to the ink's ability to quickly rebound after being scraped by a scraper. It is required that the ink be deformed quickly, and ink rebounding can be used to facilitate printing.
6 Drying: It is required that the drying of the ink on the screen is as slow as possible, and it is desired that the ink be transferred to the substrate more quickly;
7 Fineness: The size of pigment and solid material particles, PCB ink is generally less than 10μm, the size of fineness should be less than one-third of the opening of the mesh;
8 Wire drawing: When the ink is used to pick up the ink, the degree of thread-like ink stretching does not break is called stringiness. The ink 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;
9 Ink transparency and hiding power: For PCB inks, various requirements are also made on the transparency and hiding power of the ink according to the use and requirements. In general, line inks, conductive inks, and character inks all require high hiding power. The solder resist is more flexible.
10 Chemical resistance of inks: PCB inks have strict standards for the requirements of acids, alkalis, salts, and solvents, depending on the purpose of use;
(11) Physical properties of inks: PCB inks must meet resistance to external scratches, thermal shocks, mechanical peeling, and various stringent electrical performance requirements;
(12) Safety of use of ink and environmental protection: PCB ink requires low toxicity, odorless, safe and environment-friendly.

Above we have summarized the basic performance of twelve PCB inks, and among them, in the actual operation of screen printing, the viscosity problem is closely related to the operator. The level of viscosity has a great relationship with silk screen printing. Therefore, in the PCB ink technical documentation and QC report, the viscosity is clearly marked, indicating under what conditions, what type of viscosity testing equipment and so on. In the actual printing process, if the viscosity of the ink is too high, it will cause difficulty in printing, serious jagged edges in the pattern, in order to improve the printing effect, it will add thinner to make the viscosity meet the requirements. However, it is not difficult to find that in many situations, in order to obtain the ideal resolution (resolution), no matter what viscosity you use, it will never be achieved. why? After in-depth study, it was discovered that ink viscosity is an important factor, but not the only one. There is another quite important factor --- thixotropy. It is, it also affects the printing accuracy.

four. Thixotropic: (thixotropic)

Viscosity and thixotropy are two different physical concepts and it can be understood that thixotropy is a characterization of the change in ink viscosity.

The ink is at a constant temperature. Assuming that the solvent in the ink does not volatilize very quickly, the viscosity of the ink will not change. The viscosity has no relation with time. The viscosity is not a variable but a constant.

When the ink is affected by external force (stirring), the viscosity changes, and as the force continues, the viscosity will continue to decrease, but it will not decrease without limit, and it will stop at a certain limit. When the external force disappears, the ink can automatically return to its initial state after a certain period of time. We reduce the viscosity of the ink under the action of external force over time, but when the external force disappears, the reversible physical property that can restore the original viscosity is called thixotropy. Thixotropy is a time-dependent variable under the influence of external forces.

Under the action of external force, the shorter the duration of the action, the viscosity drops significantly, we call the thixotropy of this ink large; on the contrary, if the viscosity does not drop significantly, it is called thixotropy.

Fives. Reaction Mechanism and Control of Ink Thixotropy

What is thixotropic in the end? Why does the ink under the action of external force, viscosity will become smaller, and the external force disappears, after standing for a certain period of time, but also to restore the original viscosity?

The prerequisite for determining whether the ink has thixotropy is firstly a viscous resin, followed by a certain volume of filler and colorant particles. Resins, fillers, pigments, auxiliaries, etc., after grinding, are very homogeneously mixed together and they are a mixture. In the absence of external thermal or ultraviolet light energy, they exist as an irregular cluster of ions. In the normal state, they are arranged in an orderly manner due to mutual attraction and exhibit a state of high viscosity, but no chemical reaction occurs. Once they are subjected to external mechanical forces, they disrupt the orderly arrangement of the original and cut off the attraction chains of each other, becoming chaotic and disorderly, and exhibiting low viscosity. This is the phenomenon that the ink that we usually see is thick and thin. We can use the following closed-loop reversible process diagram to visually represent the entire process of thixotropy.

It is not difficult to find out how much the solid content of the ink, the shape of the solid, will determine the thixotropic properties of the ink. Of course, there is no thixotropy for liquids that are inherently very low in viscosity. However, in order to make it an thixotropy ink, it is technically possible to add an auxiliary agent to increase the viscosity of the ink and make it thixotropic. This additive is called a thixotropic agent. Therefore, the thixotropy of the ink is controllable.

six. The practical application of thixotropy <br> In practical applications, the greater the thixotropy is, the better, and the smaller is the better. It's just enough. Due to its thixotropic properties, inks are ideal for screen printing processes. The screen printing operation becomes easy and easy. When the ink is screen printed, the ink on the net is pushed by the glue and the rolling and squeezing take place. The viscosity of the ink becomes low, which facilitates the penetration of the ink. After the ink screen is printed on the PCB substrate, because the viscosity does not recover quickly, there is a proper leveling space, so that the ink flows slowly, and when the recovery equilibrium is reached, the edge of the screen printed pattern will have a satisfactory flatness.

Seven. Conclusion <br> I hope that through the discussion of ink thixotropy in this article and deepen our experience in screen printing process, we often encounter, but can not accurately put forward solutions, thixotropic becomes worth studying. Question.