PCB Blog - Analysis of the effect of thixotropy on ink properties in PCB board

The success or failure of ink use directly affects the overall technical requirements and quality indicators of printed circuit boards shipments. To this end, PCB board manufacturers attach great importance to the performance of the ink. In addition to the well-known ink viscosity, thixotropy as ink is often overlooked. And it plays a very important role in the effect of screen printing. In order to explain the effect of thixotropy on screen printing more clearly, we must also start from the basic ink and the principle of screen printing. Then introduce the concept of thixotropy. In the entire production process of modern PCB boards, ink has become one of the indispensable auxiliary materials in the PCB board production process. It occupies a very important position in the materials used in the PCB board manufacturing process. The success or failure of ink use directly affects the overall technical requirements and quality indicators of PCB board shipments. To this end, PCB board manufacturers attach great importance to the performance of the ink. In addition to the well-known ink viscosity, thixotropy as ink is often overlooked. And it plays a very important role in the effect of screen printing. In order to explain the effect of thixotropy on screen printing more clearly, we must also start from the basic ink and the principle of screen printing. Next, we analyze and explore the influence of thixotropy on the performance of ink in PCB board making:

1. Wire mesh
Screen is one of the indispensable materials in screen printing process. Without silk screen, it cannot be called screen printing. Screen printing is the soul of screen printing craftsmanship. Wire mesh is almost all silk fabrics (of course there are also non-silk fabrics). The material is generally divided into: nylon, polyester, stainless steel, and the weaving method can be divided into: plain weave, woven. According to the structure of silk, it can be divided into: single strand and multiple strands. According to the thickness of the net, it can be divided into: s (thin), t (medium), hd (heavy). The mesh of the net can be roughly divided into: low mesh, medium mesh, and high mesh. In the PCB board industry, the most commonly used is the t-net. s and hd-type networks are generally not used except for individual special needs. This is because the PCB board industry is a highly high-tech industry. It is different from pure manual and artistic skills. Generally, the technical level of manual and artistic skills is not high. As long as it can meet or achieve Human visual effects are fine. For the PCB board, it is not just neat and beautiful. The PCB board is used for current conduction and signal transmission. It must meet the fine geometric size of electrical performance requirements, and this geometric size needs to be strictly metric. Therefore, we need to understand several important technical parameters related to wire mesh.
1) Thickness of the wire mesh: Thickness refers to the measured value of the thickness of the wire mesh when it is standing still under no tension. This measured value is a statistical mean value derived from the measurement data and is expressed in μm. The thickness is determined by the diameter of the wires that make up the screen, which is related to the amount of ink that the screen sees through.
2) The opening area ratio of the mesh: it is the ratio of the mesh area to the mesh area, expressed as a percentage. The larger the value, the greater the opening of the mesh.
3) The amount of ink penetration of the screen: This refers to the theoretical value. In actual screen printing production, the amount of ink penetration will be affected by the material and performance of the screen, the viscosity of the specification ink, the fineness of the pigment, and the thixotropy of the ink. The hardness of the squeegee, the printing pressure, the printing speed, the contact distance and other factors are related.

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.

2.1 Resin: Resin is an important component that forms ink film and determines the performance of ink. The PCB board ink brands have their own formulas. What we usually call the excellent performance of the brand's ink. In fact, the resin plays a very important role in it. It determines the operability, gloss, adhesion, hardness, water resistance, solvent resistance, acid and alkali resistance, temperature resistance and so on of the ink.
2.2 Solvents: Functions of Solvents:
1) Dissolve the resin. Make it a good linking material;
2) Dissolving pigments and auxiliaries;
3) Adjust ink viscosity;
4) Adjust the drying speed of the ink;
5) Dissolve and penetrate the substrate to enhance adhesion. The solvent of the ink is not a single species and a set of components. In consideration of solubility and drying speed, most of the solvents are mixtures. Different inks, different uses, have different solvents. So ink manufacturers have several different types of solvents. For one ink, only one or two types of solvents can be used. Generally speaking, for the same ink manufacturer, since the resin system used in its research and development is basically determined, the solvent provided by the ink produced by the same ink manufacturer can be used universally.
2.3 Pigment: mainly use pigment. Pigments are classified by category and can be divided into organic pigments and inorganic pigments. Organic pigments refer to non-mineral pigments, which are brilliant in color and have a complete color spectrum, but usually have poor hiding power. Inorganic pigments refer to minerals. Such as: titanium dioxide, zinc white, iron blue, carbon black, etc., they have good covering power, strong light resistance and aging resistance, and PCB board inks are mostly such inks. In addition to being used as a colorant, pigments also play a certain role in the light resistance, heat resistance, flux resistance, and chemical resistance of the ink.
2.4 Auxiliary: The purpose of adding auxiliary to the ink is to improve the physical properties of the ink, enhance the printing suitability of the ink, and improve the printing effect. Auxiliaries include: defoamer, dispersant, diluent, homogenizer, firming agent, color separation inhibitor, precipitation inhibitor, plasticizer, coupling agent, ultraviolet absorber, catalyst, thickener, etc.

3. Several important technical properties of PCB board ink
Whether the ink quality of the PCB board is excellent, in principle, it is impossible to deviate from the combination of the above major components. The excellent ink quality is a comprehensive reflection of the scientific, advanced and environmental protection of the formula. It is reflected in:
1) Viscosity: short for dynamic viscosity. It is generally expressed by viscocity, 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 Pascal/second (pa.s) or millipascal/s (mpa.s). In the production of PCB boards, it refers to the fluidity of the ink driven by external forces.
2) Plasticity: After the ink is deformed by external force, it still maintains its properties before deformation. The plasticity of the ink is beneficial to improve the printing accuracy;
3) thixotropy: (thixotropic) ink is gelatinous when standing, and a property in which viscosity changes when touched, also known as thixotropy and sag resistance;
4) Fluidity: (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. If the plasticity and thixotropy are large, the fluidity will be large; if the fluidity is large, the imprint will easily expand. The fluidity is small, and it is easy to form nets, resulting in the phenomenon of ink formation, also known as netting;
5) Viscoelasticity: refers to the rapid rebound performance of the ink that is sheared and broken after the ink is scratched by the scraper. It is required that the ink deforms quickly and the ink rebounds quickly to be conducive to printing;
6) Drying: It is required that the ink dry on the screen as slowly as possible, and after the ink is transferred to the substrate, the faster the better;
7) Fineness: the size of pigment and solid material particles, PCB board ink is generally less than 10μm, and the size of fineness should be less than one-third of the mesh opening;
8) Drawability: When the ink is lifted with an ink shovel, the degree to which the filamentous ink is stretched without breaking is called drawability. The ink filaments are long, and many filaments appear on the ink surface and printing surface, which makes the substrate and printing plate dirty, and even cannot be printed;
9) Transparency and hiding power of ink: For PCB board ink, according to different uses and requirements, various requirements are also put forward for the transparency and hiding power of ink. In general, line inks, conductive inks and character inks all require high hiding power. Solder resist is more flexible.
10) Chemical resistance of ink: According to different purposes of PCB board ink, the corresponding requirements have strict standards for acids, alkalis, salts and solvents;
11) Physical properties of ink resistance: PCB board ink must meet external scratch resistance, thermal shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements;
12) The use of ink is safe and environmentally friendly: PCB board ink is required to be low-toxic, odorless, safe and environmentally friendly.
We have summarized the basic properties of twelve PCB board inks above, and in the actual operation process of screen printing, the viscosity problem is closely related to the operator. The level of viscosity has a great relationship with the smoothness of silk screen printing. Therefore, in the PCB ink technical documents and QC, 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 viscosity of the ink is too high, it will cause printing difficulties 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 occasions, 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, it was discovered that ink viscosity is an important factor, but it is not. Another rather important factor is thixotropy, which also affects the accuracy of printed circuit boards.