A brief analysis of the influence of thixotropy on ink performance in PCB board making
A brief analysis of the influence of thixotropy on ink performance in PCB board making
Core summary: The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB shipment. For this reason, PCB manufacturers attach great importance to the performance of ink. In addition to the well-known viscosity of ink, the thixotropy of ink is often ignored by people. However, it plays a significant role in the effect of screen printing. In order to more clearly explain the influence of thixotropy on screen printing, we must also start with the most basic ink and the principle of screen printing. Then introduce the concept of thixotropy.
In the entire production process of modern PCB, ink has become one of the indispensable auxiliary materials in the PCB manufacturing process. It occupies a very important position in the materials used in the PCB process. The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB shipment. For this reason, PCB manufacturers attach great importance to the performance of ink. In addition to the well-known viscosity of ink, the thixotropy of ink is often ignored by people. However, it plays a significant role in the effect of screen printing.
In order to more clearly explain the effect of thixotropy on screen printing, we must also start with the most basic ink and the principle of screen printing. Then introduce the concept of thixotropy. Below we analyze and explore the effect of thixotropy on ink performance in PCB board making:
1.Screen
Screen is one of the indispensable materials in screen printing process. Without screen, it cannot be called screen printing. Screen is the soul of screen printing process. Screens are almost all silk fabrics (of course there are non-silk fabrics).
Generally divided by materials: nylon, polyester, stainless steel
Based on weaving method, it can be divided into: plain weave, silk weave
Based on the structure of silk, it can be divided into: single strand, multi-strand
Based on the thickness of the mesh, it can be divided into: S (thin), T (medium), HD (heavy)
Based on the mesh number of the mesh, it can be roughly divided into: low mesh, medium mesh, high mesh
In the PCB industry, the most commonly used is T-type mesh. S and HD-type meshes are generally not used except for some special needs. This is because the PCB industry is a high-tech and highly professional industry. It is different from the production of simple manual and artistic skills. Generally, the technical level required for manual and artistic skills is not high. It only needs to meet or achieve the visual effect of people. For PCB, it is not just about looking neat and beautiful. PCB is used for current conduction and signal transmission. It must meet the fine geometric dimensions required by electrical performance, and this geometric dimension needs to be strictly measured. Therefore, we need to understand several important technical parameters related to the screen.
① Screen thickness:
Thickness refers to the measured value of the thickness of the screen when it is stationary without tension. This measured value is the statistical average value obtained from the measurement data, expressed in μm. The thickness is determined by the diameter of the wire that constitutes the screen and is related to the amount of ink permeability of the screen.
② Screen opening area ratio:
It is the ratio of the mesh area to the screen area, expressed as a percentage. The larger this value is, the larger the mesh opening is.
③Ink penetration of screen:
This refers to the theoretical value. In actual screen printing production, the ink penetration will be affected by factors such as the screen material, performance, specifications, ink viscosity, pigment fineness, ink thixotropy, scraper hardness, printing pressure, printing speed, screen contact distance, etc.
Ink penetration = screen thickness ftx screen opening area ratio x 10000.
=ft. w2/(w+d)2.10000 (unit: cm3/m2)
The above three technical indicators are closely related to the content discussed below.
2.Ink
It refers to the colored colloidal substance used for printed boards. It is often composed of synthetic resins, volatile solvents, oils and fillers, desiccants, pigments and diluents. It is often called ink.
I. Composition of ink:
- Resin: Resin is an important component that forms ink film and determines the performance of ink. Famous PCB ink brands all have their own patented formulas. Usually we say that the performance of this brand of ink is excellent. In fact, the resin plays a very important role in it. It determines the operability, glossiness, adhesion, hardness, water resistance, solvent resistance, acid and alkali resistance, temperature resistance, etc. of the ink.
- Solvent: Functions of solvent:
① Dissolve resin. Make it a good binder;
② Dissolve pigments and additives;
③ Adjust ink viscosity;
④ Adjust the drying speed of ink;
⑤ Dissolve and penetrate the substrate to enhance adhesion.
The solvent of the ink is not a single variety and a single component. Considering the problems of solubility and drying speed, most solvents are mixed. Different inks have different solvents for different uses. Therefore, ink manufacturers will be equipped with several different types of solvents. Only one or two types of solvents are allowed to be used for one ink. Generally speaking, for the same ink manufacturer, since the resin system used in its research and development is basically determined, the solvents provided by the same ink manufacturer can be used universally. For example, the ink produced by Coates in the UK can basically be used universally as long as the solvent is produced by Coates in the UK. However, we still insist that a kind of ink should be matched with a matching special diluent.
⑴The diluent series products provided by Coates
⑵The diluents corresponding to Coates products
Random use of diluents will cause ink gel (caking), peeling, uneven color, chemical resistance, thermal shock resistance, decreased adhesion and other undesirable problems. It is incorrect to add “anti-white water” and “783” as universal solvents to any ink.
- Colorant: mainly use pigments. Pigments can be divided into organic pigments and inorganic pigments according to their categories. Organic pigments refer to non-mineral pigments, which have gorgeous colors and complete color spectrum, but usually have poor hiding power. Inorganic pigments refer to mineral pigments. Such as: titanium white, zinc white, iron blue, carbon black, etc., they have good hiding power, strong light resistance and aging resistance, and PCB inks mostly belong to this type of ink.
In addition to being a colorant, pigments also play a certain role in the light resistance, heat resistance, flux resistance, chemical resistance and other aspects of inks.
- Additives: The purpose of adding additives to ink is to improve the physical properties of ink, enhance the printing suitability of ink and improve the printing effect. Additives include: defoamers, dispersants, diluents, levelers, firming agents, color separation inhibitors, precipitation inhibitors, plasticizers, coupling agents, ultraviolet absorbers, catalysts, thickeners, etc.
III. Several important technical properties of PCB inks
Whether the quality of PCB ink is excellent or not, in principle, it is impossible to separate from the combination of the above components. The excellent quality of ink is a comprehensive reflection of the scientificity, advancement and environmental protection of the formula. It is reflected in:
①Viscosity: It is the abbreviation of dynamic viscosity. It is generally expressed as viscosity, that is, the shear stress of fluid flow divided by the velocity gradient in the direction of the flow layer, and the international unit is Pascals per second (Pa.s) or millipascals per second (mPa.s). In PCB production, it refers to the fluidity of ink driven by external force.
Conversion relationship of viscosity units:
1Pa.s=10p=1000mPa. s=1000cp=10dpa.s
②Plasticity: refers to the property of ink before deformation after being deformed by external force. The plasticity of ink is conducive to improving printing accuracy;
③Thixotropy: (thixotropic) The ink is colloid when it is still, and the viscosity changes when it is touched. It is also called thixotropy and anti-sagging;
④Flowability: (leveling) The degree 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 greater the plasticity and thixotropy, the greater the fluidity; the greater the fluidity, the easier it is to expand the print. The less fluidity, the easier it is to form a network, which produces ink formation, also known as reticulation;
⑤Viscoelasticity: refers to the property of the ink that rebounds quickly after being sheared and broken by the scraper. The ink deformation speed is fast and the ink rebounds quickly to facilitate printing;
⑥ Drying property: The slower the ink dries on the screen, the better. If the ink is transferred to the substrate, the faster the better;
⑦ Fineness: The size of the pigment and solid material particles. PCB ink is generally less than 10μm, and the fineness should be less than one-third of the mesh opening;
⑧ Wire drawing property: When the ink is picked up with an ink shovel, the degree to which the filamentous ink is stretched without breaking is called wire drawing property. The ink filaments are long, and many filaments appear on the ink surface and the printing surface, making the substrate and the printing plate dirty, and even unable to print;
⑨ The transparency and hiding power of the ink: For PCB ink, various requirements are put forward for the transparency and hiding power of the ink according to different uses and requirements. Generally speaking, circuit ink, conductive ink and character ink are required to have high hiding power. Solder resist is more flexible.
⑩ Chemical resistance of ink: PCB ink has strict requirements for acid, alkali, salt and solvent according to different purposes;
⑾ Physical resistance of ink: PCB ink must meet the requirements of resistance to external scratches, heat shock, mechanical peeling, and various strict electrical performance requirements;
⑿ Safety and environmental protection of 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, and in the actual operation of screen printing, the viscosity problem is closely related to the operator. The viscosity is very important to the smoothness of screen printing. Therefore, in the technical documents and QC reports of PCB inks, the viscosity is clearly marked, indicating under what conditions and what type of viscosity test instrument to use. In the actual printing process, if the viscosity of the ink is too high, it will cause difficulty in printing and serious jagged edges of the graphics. In order to improve the printing effect, diluents 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 always impossible to achieve it. Why? After in-depth research, it was found that ink viscosity is an important factor, but not the only one. There is another very important factor-thixotropy. It is it that also affects the printing accuracy.
IV. Thixotropy: (thixotropic)
Viscosity and thixotropy are two different physical concepts. It can be understood in this way that thixotropy is a representation of the change 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. 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 decrease indefinitely and stop at a certain limit. When the external force disappears, after a certain period of static standing, the ink can automatically and gradually return to its initial state. We call this reversible physical property that the viscosity of the ink decreases over time under the action of external force, but can return to its original viscosity after the external force disappears, thixotropy. Thixotropy is a variable related to time under the action of external force.
Under the action of external force, the shorter the duration of the force, the more obvious the viscosity decreases, and we call this ink thixotropic; on the contrary, if the viscosity decreases insignificantly, it is called thixotropic.
4. Reaction mechanism and control of thixotropy of ink
What exactly is thixotropy? Why does the viscosity of the ink decrease under the action of external force, and the original viscosity can be restored after the external force disappears and it is left to stand for a certain period of time?
The necessary conditions for determining whether the ink has thixotropy are firstly the viscous resin, and secondly the filler and colorant particles filled with a certain volume ratio. After grinding, the resin, filler, colorant, additive, etc. are mixed very evenly, and they are a mixture. In the state without external heat energy 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 high viscosity state, but no chemical reaction occurs. Once subjected to external mechanical force, the original orderly arrangement is disrupted, the mutual attraction chain is cut off, and it becomes a disordered chaotic state, showing a lower viscosity. This is the phenomenon of ink changing from thick to thin that we usually see. 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 solid content in the ink and the shape and size of the solid will determine the thixotropic properties of the ink. Of course, for liquids with very low viscosity, there is no thixotropy. But in order to make it a thixotropic ink, it is technically possible to add an additive to change the viscosity of the ink and make it thixotropic. This additive is called a thixotropic agent. Therefore, the thixotropy of the ink is controllable.
6.Practical application of thixotropy
In practical applications, the greater the thixotropy, the better, nor the smaller the better. It is to stop at the right time. The ink is very suitable for screen printing due to its thixotropic properties. It makes screen printing easy. When the ink is screen printed, the ink on the screen is pushed by the squeegee, rolling and squeezing, and the viscosity of the ink becomes lower, which is conducive to ink penetration. After the ink is screen printed on the PCB substrate, since the viscosity cannot be quickly restored, there is an appropriate leveling space, so that the ink flows slowly. When the recovery balance is reached, the edge of the screen-printed pattern will have a satisfactory flatness.
7.Conclusion
I hope that through the discussion of the thixotropy of ink in this article, we can deepen our understanding of the screen printing process in actual application. When we often encounter problems but cannot accurately propose solutions, thixotropy becomes a topic worth studying.
