PCB Technical - What effect does the thixotropy of PCB factory make on ink performance

In the entire production process of modern PCBs, ink has become one of the indispensable auxiliary materials in the PCB manufacturing process of PCB factories. It occupies a very important position in the PCB process materials. The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB shipments. For this reason, PCB manufacturers attach great importance to the performance of inks. In addition to the well-known ink viscosity, the thixotropy as an ink is often overlooked by people. But it plays a very important role in the effect of screen printing.

Below we analyze and explore the influence of thixotropy in the PCB system on the ink performance:
1. Screen
Silk screen is one of the indispensable materials in the screen printing process. Without screen, it cannot be called screen printing. Screen printing is the soul of screen printing technology. The screens are almost all silk fabrics (of course there are also non-silk fabrics).
In the PCB industry, the most commonly used is the t-type net. s and hd type networks are generally not used except for individual special needs.
2. Ink
Refers to the colored gelatinous substance used for printed boards. It is often composed of synthetic resins, volatile solvents, oils and fillers, desiccants, pigments and diluents. Often called ink.
Three. Several important technical properties of PCB ink
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 quality of the ink is a comprehensive manifestation of the scientificity, advancement and environmental protection of the formula. It is reflected in:
(1) Viscosity: short for dynamic viscosity. 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 milliPascal/sec (mpa.s). In PCB production, it refers to the fluidity of ink produced by external forces.
(2) Plasticity: After the ink is deformed by external force, it still retains its properties before deformation. The plasticity of the ink is conducive to improving the printing accuracy;
(3) Thixotropic: (thixotropic) The ink is gelatinous when it is left standing, and the viscosity changes when it is touched. It is also called thixotropic and sag resistance;
(4) 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. With low fluidity, it is prone to network formation, resulting in ink formation, which is also known as reticulation;
(5) Viscoelasticity: 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;
(6) Dryness: the slower the drying of the ink on the screen, the better, and the faster the better after the ink is transferred to the substrate;
(7) 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;
(8) Stringiness: When the ink is picked up with an ink shovel, the degree to which the silk-like ink does not break when stretched is called stringiness. The ink filament is long, and there are many filaments on the ink surface and the printing surface, making the substrate and the printing plate dirty, or even unable to print;
(9) Transparency and hiding power of ink: For PCB inks, various requirements are put forward for the transparency and hiding power of the ink according to different uses and requirements. Generally speaking, circuit inks, conductive inks and character inks all require high hiding power. The solder resist is more flexible.
(10) Chemical resistance of ink: PCB ink has strict standards for acid, alkali, salt and solvent according to different purposes;
(11) Physical resistance of the ink: PCB ink must meet external scratch resistance, thermal shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements;
(12) Safety and environmental protection of the ink: PCB ink is required to be low-toxic, odorless, safe and environmentally friendly.
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 rather important factor: thixotropy. It is also affecting the printing accuracy.
Four. Thixotropy
Viscosity and thixotropy are two different physical concepts. It can be understood that thixotropy is a sign of changes in ink viscosity.
When the ink is at a certain constant temperature, assuming that the solvent in the ink does not evaporate quickly, the viscosity of the ink will not change at this time. The viscosity has nothing to do with time. The viscosity is not a variable, but a constant.
When the ink is subjected to external force (stirring), the viscosity changes. As the force continues, the viscosity will continue to decrease, but it will not drop indefinitely, and stop when it reaches a certain limit. When the external force disappears, after a certain period of standing, the ink can automatically gradually return to the original state. We call this kind of reversible physical property that the ink viscosity decreases with the extension of time under the action of external force, but after the external force disappears, it can return to the original viscosity as thixotropy. Thixotropy is a time-related variable under the action of an external force.
Under the action of external force, the shorter the duration of the force, and the obvious decrease in viscosity, we call this ink the thixotropy is large; on the contrary, if the viscosity decrease is not obvious, it is said that the thixotropy is small.
5. Reaction mechanism and control of ink thixotropy
What exactly is thixotropy? Why is the viscosity of the ink reduced under the action of external force, but the external force disappears, after a certain period of time, the original viscosity can be restored?
To determine whether the ink has the necessary conditions for thixotropy, first is the resin with viscosity, and then is filled with a certain volume ratio of filler and pigment particles. After the resin, fillers, pigments, additives, etc. are ground and processed, they are very uniformly mixed together. They are a mixture. In the absence of external heat or ultraviolet light energy, they exist as an irregular ion group. Under normal conditions, they are arranged in an orderly manner due to mutual attraction, showing a state of high viscosity, but no chemical reaction occurs. And once it is subjected to external mechanical force, the original orderly arrangement is disrupted, the mutual attraction chain is cut off, and it becomes a disordered state, showing that the viscosity becomes lower. This is the phenomenon that we usually see ink from thick to thin. We can use the following closed loop reversible process diagram to vividly express the whole process of thixotropy.
It is not difficult to find that the amount of solids in the ink and the shape and size of the solids 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 into a thixotropic 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. Practical application of thixotropy
In practical applications, it is not that the greater the thixotropy, the better, nor the smaller the better. It's just enough. Due to its thixotropic properties, the ink is very suitable for the process of screen printing. Makes the screen printing operation easy and free. During the ink screen printing, the ink on the net is pushed by the squeegee, rolling and squeezing occur, and the viscosity of the ink becomes lower, which is conducive to ink penetration. After the ink is screen printed on the PCB substrate, because the viscosity cannot be recovered quickly, there is a proper leveling space to make the ink flow slowly, and when the balance is restored, the edges of the screen printed graphics will be satisfactorily flat.
Seven. Concluding remarks
I hope that through the discussion of ink thixotropy in this article, we can deepen our understanding of the screen printing process in the actual application process, when we often encounter, but can not accurately propose a solution, thixotropy has become a topic worthy of our research.