The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB shipment. Therefore, PCB manufacturers attach great importance to the performance of ink. Besides the well-known viscosity of ink, the thixotropy of ink is often ignored by people. But it plays an important role in the effect of screen printing. In order to explain more clearly the influence of thixotropy on screen printing effect, we must start with the most basic ink and the principle of screen printing. Then the concept of thixotropy is introduced.
In the whole production process of modern PCB, ink has become one of the indispensable auxiliary materials in PCB manufacturing process. It occupies a very important position in PCB manufacturing materials. The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB shipment. Therefore, PCB manufacturers attach great importance to the performance of ink. Besides the well-known viscosity of ink, the thixotropy of ink is often ignored by people. But it plays an important role in the effect of screen printing.
In order to explain more clearly the influence of thixotropy on screen printing effect, we must start with the most basic ink and the principle of screen printing. Then the concept of thixotropy is introduced. Let’s analyze and explore the influence of thixotropy on ink performance in PCB manufacturing:
First, the silk screen
It is one of the indispensable materials in screen printing process. The lack of screen printing can’t be called screen printing. Screen printing is the soul of screen printing process. Almost all silk screens are silk fabrics (of course, there are non-silk fabrics).
Generally, the materials are nylon, polyester and stainless steel.
Weaving methods can be divided into plain weaving and twill weaving.
Silk can be divided into single strand and multi-strand according to its structure.
According to the thickness of the net, it can be divided into: S (thin), T (medium) and hd (heavy)
The mesh number of nets can be roughly divided into low mesh number, medium mesh number and high mesh number.
In PCB industry, T-net is the most commonly used one. And s hd network, except for some special needs, are generally not used. This is because the PCB industry is a high-tech and highly professional industry, which is different from the pure manual and artistic skills. Generally, the technical level of manual and artistic skills is not high, so long as it can meet or achieve people’s visual effects. For PCB, it’s not just to look neat and beautiful. If PCB is to be used for current conduction and signal transmission, it must meet the precise geometric size required by electrical performance, and this geometric size needs to be strictly measured. Therefore, we need to know several important technical parameters related to screen.
① Thickness of silk screen: The thickness refers to the measured value of the thickness of silk screen when it is standing under no tension. The measured value is the statistical average obtained from the measured data, and is expressed in μ m. The thickness is determined by the diameter of the silk making up the screen, and is related to the ink penetration of the screen.
② Opening area ratio of screen: it is the ratio of mesh area to screen area, expressed as a percentage. The larger the value, the larger the opening of mesh.
③ Ink penetration of screen: This refers to the theoretical value. In actual screen printing production, ink penetration will be affected by factors such as screen material, performance, viscosity of standard ink, fineness of pigment, thixotropy of ink, hardness of scraper, printing pressure, printing speed, touching distance and so on.
II. Ink
Refers to colored colloidal substances used in printed boards. It is often composed of synthetic resin, volatile solvent, oil and filler, desiccant, pigment and diluent. Often called ink.
(1) Composition of ink:
1. Resin: Resin is an important component to form ink film and determine the performance of ink. Famous PCB ink brands all have their own proprietary formulas. What we usually call the excellent performance of this brand ink. Actually, the resin plays a very important role in it, which determines the operability, glossiness, cohesiveness, hardness, water resistance, solvent resistance, acid and alkali resistance, temperature resistance and so on.
2. Solvent:
Function of solvent:
① Dissolve resin. Make it a good binder;
(2) dissolving pigments and additives;
③ Adjust the ink viscosity;
④ Adjust the drying speed of ink;
⑤ Dissolve and penetrate the substrate to enhance the adhesion.
The solvent of ink is not a single variety but a single component. Considering the problem of solubility and drying speed, most solvents are mixed. Different inks have different solvents for different purposes. Therefore, ink manufacturers will be equipped with several different types of solvents. However, only one or two types of solvents can be used for one ink. Generally speaking, for the same ink manufacturer, because the resin system used in its R&D is basically certain, the solvent provided by the same ink manufacturer can be universal. For example, the ink produced by British coates can be generally used as long as it is the solvent produced by British coates. However, we still insist that an ink should be matched with a special diluent.
(1) Diluent series products provided by ⑴coates
⑵coates products and corresponding diluent.
Arbitrary use of diluent will lead to gel (caking), peeling off, uneven color, chemical resistance, thermal shock resistance, decreased adhesion and other bad problems of the ink. It is incorrect to add “anti-white water” and “783” as a universal solvent to Riga, no matter what ink.
3. Pigments: Pigments are mainly used. Pigments can be classified into organic pigments and inorganic pigments according to their categories. Organic pigments refer to non-mineral substances, which have brilliant colors and complete chromatograms, but usually have poor hiding power. Inorganic pigments refer to minerals. Such as: titanium white, zinc white, iron blue, carbon black, etc., which have good hiding power, strong light resistance and aging resistance. Most of PCB inks belong to this kind of ink.
Pigments not only serve as pigments, but also play a certain role in the light resistance, heat resistance, flux resistance and chemical resistance of ink.
4. Additives: The purpose of adding additives to the ink is to improve the physical properties of the ink, enhance the printability of the ink and improve the printing effect. Additives include defoamer, dispersant, diluent, homogenizer, firming agent, color separation inhibitor, precipitation inhibitor, plasticizer, coupling agent, ultraviolet absorber, catalyst, thickener, etc.
Three. Several important technical properties of PCB ink
Whether the quality of PCB ink is excellent or not can’t be separated from the combination of the above components in principle. Excellent ink quality is the comprehensive embodiment of the scientific, advanced and environmental protection of the formula. It is embodied in:
(1) viscosity: it is the abbreviation of dynamicviscosity. Generally expressed by viscosity, that is, the shear stress of fluid flow divided by the velocity gradient in the direction of flow layer, and the international unit is pa/s (Pa. S) or millipascals per second (mpa). s)。 In PCB production, it refers to the fluidity of ink driven by external forces.
(2) Plasticity: refers to the fact that after the ink is deformed by external force, it still keeps its properties before deformation. The plasticity of ink is conducive to improving printing accuracy;
(3) thixotropic: A kind of property that the ink is gelatinous when standing, but its viscosity changes when touched, also known as thixotropy and sag resistance;
(4) Fluidity: (leveling) The extent to which the ink spreads out around under the action of external force. Fluidity is the reciprocal of viscosity, which is related to the plasticity and thixotropy of ink. And the greater the plasticity and thixotropy, the greater the fluidity; If the liquidity is large, the imprint is easy to expand. Small fluidity, prone to netting, resulting in inking phenomenon, also known as moire;
(5) Viscoelasticity: refers to the property that the ink that is sheared and broken quickly rebounds after the ink is scraped by a scraper. It is required that the ink deformation speed be fast and the ink rebound quickly to be favorable for printing;
(6) Dryness: it is required that the ink should be dried on the screen as slowly as possible, but it is required that the ink should be transferred to the substrate as soon as possible;
(7) Fineness: the size of pigment and solid particles, PCB ink is generally less than 10μm, and the fineness should be less than one third of the mesh opening;
(8) Drawability: When the ink is picked up with an ink spatula, the degree to which the filamentous ink is stretched without breaking is called drawability. The ink thread is long, and many filaments appear on the ink surface and printing surface, which makes the substrate and printing plate stained with dirt and even unable to be printed;
(9) Transparency and hiding power of ink: For PCB ink, according to different uses and requirements, various requirements are put forward for transparency and hiding power of ink. Generally speaking, line ink, conductive ink and character ink all require high hiding power. While solder resist is more flexible.
(10) Chemical resistance of printing ink: PCB printing ink has strict standards for acid, alkali, salt and solvent according to different purposes.
(11) Physical resistance of ink: PCB ink must meet external force scratch resistance, thermal shock resistance, mechanical peeling resistance, and meet various strict electrical performance requirements;
(12) Use safety and environmental protection of ink: PCB ink is required to be low-toxic, odorless, safe and environmentally friendly.
We summarized the basic performance of twelve PCB inks above, and in the actual operation process of screen printing, the viscosity problem is closely related to the operator. The viscosity has a great influence on the smoothness of silk screen printing. Therefore, in PCB ink technical documents and qc reports, the viscosity is clearly marked, indicating under what conditions, what type of viscosity testing instrument is used, etc. In the actual printing process, if the ink viscosity is on the high side, it will cause difficulty in missing printing and serious jagged edges of graphics. In order to improve the printing effect, thinner will be added to make the viscosity meet the requirements. However, it is not difficult to find that in many cases, in order to obtain the ideal resolution (resolution), no matter what viscosity you adopt, it will never be achieved. Why? After in-depth study, it was found that ink viscosity is an important factor, but not the only one. There is another important factor: thixotropy. Yes, it is also affecting the printing accuracy.
IV. Thixotropy
Viscosity and thixotropy are two different physical concepts. It can be understood that thixotropy is a sign of the change of ink viscosity.
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, and the viscosity is not a variable, but a constant.
When the ink is stirred by external force, the viscosity will change. With the continuation of the force, the viscosity will continue to decrease, but it will not decrease indefinitely, and it will stop at a certain limit. When the external force disappears, after a certain period of standing, the ink can automatically and gradually return to the initial state. We call this reversible physical property that under the action of external force, the viscosity of ink decreases with time, but when the external force disappears, it can return to the original viscosity, which is called 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 acting force, and the obvious drop in viscosity, we call this ink thixotropic; On the contrary, if the viscosity decrease is not obvious, it is said that the thixotropy is small.
V. Reaction mechanism and control of ink thixotropy
What is thixotropy all about? Why is the viscosity of ink reduced under the action of external force, but the external force disappears, and after standing for a certain time, the original viscosity can be restored?
The necessary conditions for determining whether the ink has thixotropy are firstly the adhesive resin, and secondly the filler and pigment particles with a certain volume ratio. After grinding, resin, filler, pigment, additives, etc. are mixed together very evenly, and they are a mixture. They exist as an irregular ion cluster when they are not exposed to external heat energy or ultraviolet light energy. Under normal conditions, they are arranged orderly due to mutual attraction, showing a state of high viscosity, but there is no chemical reaction. However, once it is subjected to external mechanical forces, it disrupts the original ordered arrangement, cuts off the mutual attraction chain, and becomes a disorderly and chaotic state, showing a lower viscosity. This is what we usually see as the phenomenon of ink thinning from thick to thin. We can vividly express the whole process of thixotropy with the following reversible process diagram of a closed loop.
It is not difficult to find that the content and shape of solid in ink will determine the thixotropic performance of ink. Of course, for liquids with very low viscosity, there is no thixotropy. However, in order to make it a thixotropic ink, technically, an additive can be added, so that the viscosity of the ink changes and improves, making it thixotropic. This additive is called thixotropicagent. Therefore, the thixotropy of the ink is controllable.
VI. Practical application of thixotropy
In practical application, the bigger the thixotropy, the better, and the smaller the thixotropy. But enough is enough. Because of the thixotropic property of ink, it is very suitable for screen printing. The screen printing operation becomes easy, and when the ink is screen printed, the ink on the screen rolls and squeezes under the push of the squeegee, and the viscosity of the ink becomes low, which is beneficial to ink penetration. After the ink is screen printed on the PCB substrate, because the viscosity can’t recover quickly, there is a proper leveling space, which makes the ink flow slowly. When the recovery balance is reached, the edge of the screen printed graphics will get a satisfactory flatness.
It is hoped that through the discussion of thixotropy of ink in this paper, we can deepen our understanding that thixotropy becomes a worthy research topic when screen printing process is often encountered in practical application, but the solution cannot be put forward accurately.
