Analysis of the effect of thixotropy on the performance of printed circuit board inks in PCB manufacturing

　　In the entire production process of modern PCBs, ink has become one of the indispensable auxiliary materials in the PCB manufacturing process of PCB board 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, circuit board 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.

　　Analyze and explore the influence of thixotropy on ink performance in PCB manufacturing:

　　One, circuit board wire mesh

　　The circuit board 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. Circuit board ink

　　refers to colored glue-like substances used for printed circuit boards. It is often composed of synthetic resins, volatile solvents, oils and fillers, desiccants, pigments and diluents. Often called ink.

　　pcb circuit board ink screen printing room

　　Pcb circuit board ink screen printing room

　　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:

　　⑴ Ink use safety and environmental protection: PCB ink requires low toxicity, odorless, safety and environmental protection.

　　⑵Drying: 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.

　　(3) 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 printing accuracy.

　　⑷Fineness: 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.

　　⑸ Chemical resistance of ink: PCB ink has strict standards for acid, alkali, salt and solvent according to different purposes.

　　⑹ Physical resistance of the ink: PCB ink must meet external scratch resistance, heat shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements.

　　⑺ thixotropic: (thixotropic) ink is gelatinous when standing, but the viscosity changes when touched. It is also called thixotropy and sag resistance.

　　⑻Viscoelasticity: Refers to the rapid rebound performance of the ink that is sheared and broken after the ink is scraped by the squeegee. It is required that the ink deformation speed is fast and the ink rebounds quickly to be beneficial to printing.

　　⑼ Stringiness: When the ink is picked up with an ink shovel, the degree to which the silk-like ink is stretched without breaking is called stringiness. The ink filaments are 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.

　　⑼Ink transparency and hiding power: 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 all require high hiding power. The solder resist is more flexible.

　　⑾Viscosity: is the abbreviation for dynamic viscosity. It is 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 the production process of circuit board manufacturers, it refers to the fluidity generated by the external force of the ink.

　　⑿ 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. The liquidity is small, and it is easy to form netting, resulting in the phenomenon of inking, also known as netting.

　　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 printed circuit board process, if the viscosity of the ink 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 in this way that thixotropy is a sign of changes in ink viscosity.

　　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 acted on (stirred) by an external force, 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, the obvious decrease in viscosity, we call this kind of ink the thixotropy is large; on the contrary, if the viscosity decrease is not obvious, it is said that the thixotropy is small.

　　5. The reaction mechanism and control of ink thixotropy

　　What 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?

　　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 content 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, the 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 circuit board, because the viscosity cannot be recovered quickly, there is a proper leveling space, so that the ink flows slowly, and when the balance is restored, the edges of the screen printed graphics will be satisfactorily straight Spend.

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