Thixotropic Effect on Ink Properties and Exploration

Foreword: 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 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, thixotropy as an ink is often overlooked. And it plays a very important role in the effect of screen printing.

In order to more clearly explain the effect of thixotropy on the screen printing effect, we must also start from the basic principles of ink and screen printing. Then introduce the concept of thixotropy.

First, the screen

Wire mesh is one of the indispensable materials in the screen printing process. The lack of silk screens cannot be called screen printing. Screens are the soul of screen printing. The screens are almost silk fabrics (of course, non-silk fabrics).
The material is generally divided into: nylon, polyester, stainless steel weaving methods can be divided into: flat weaving, weaving to silk structure can be divided into: single, multiple strands of the thickness of the network can be divided into: S (thin), T (medium), HD (heavy)
The number of meshes can be roughly divided into: low mesh number, medium mesh number, and high mesh number. In the PCB industry, the most commonly used is the T-net. S and HD nets are generally not used except for certain special needs. This is due to the fact that the PCB industry is a highly technical and highly specialized industry. It is different from the production of pure handicrafts and artistic skills. The manual and artistic skills required are not as high as the technical level. It can only meet or achieve Human visual effects are fine. On the PCB, it is not just neat and beautiful. The PCB must be used for current conduction and signal transmission. It must reach the fine geometrical dimensions of the electrical performance, and this geometric dimension needs to be strictly measured. of. Therefore, we have to understand several important screen-related technical parameters.

1 the thickness of the screen:
The thickness refers to the measured value of the thickness of the screen when it is left standing without tension. This measurement is a statistical average obtained from the measurement data and is expressed in μm. The thickness is determined by the diameter of the filaments that make up the screen, and is related to the amount of ink transmitted through the screen.
The screen thickness is shown in Figure 4-1
2 open area area of ​​wire mesh:
The ratio of the mesh area to the screen area, expressed as a percentage of %, the larger the value, the larger the opening of the mesh.
Open area ratio is shown in Figure 4-2
3 screen through the ink:
This refers to the theoretical value. In the actual screen printing production, the amount of ink through the screen will be affected by the screen material, the performance, the viscosity of the ink, the fineness of the pigment, the thixotropy of the ink, the hardness of the scraper, the pressure of the printing, Factors such as the speed of printing, the contact distance, and the like are affected.
Ink penetration of ink = mesh thickness Ftx mesh opening area ratio x10000.
= Ft. W2/(W+d)2. 10000 (Unit: cm3 / m2)
See Figure 4-3
The above three technical indicators have a very close relationship with the content discussed below.

two. Ink

Refers to a colored colloidal substance used in printed boards. Often consists of synthetic resins, volatile solvents, oils and fillers, desiccants, pigments, and thinners. Often called ink.

(a) Ink composition:

1. Resin: Resin is an important component that forms the ink film and determines ink performance. The well-known PCB ink brand has its own proprietary formula. What we usually say about the brand's ink performance is excellent. In fact, the resin plays a very important role in the inside, is it determines the ink handling, gloss, adhesion, hardness, water resistance, solvent resistance, acid and alkali resistance, temperature resistance and so on.

2. Solvent: The function of the solvent:

1 Dissolve the resin. Make it a good link material;
2 Dissolving Pigments and Additives;
3 adjust the ink viscosity;
4 adjust the ink drying speed;
5 Solubility infiltration of the substrate, enhance adhesion.

The solvent for the ink is not a single species but a set of components. Due to the problems of dissolution force and drying speed, solvents are mostly mixed materials. Different inks, different uses, have different solvents. So ink manufacturers will be equipped with several different types of solvents. Only one or two types of solvents can be used for an ink. In general, for the same ink manufacturer, since the resin system used in its research and development is basically determined, the solvent provided by the same ink manufacturer can be universally used. For example, the ink produced by British COATES can be basically used as long as it is a solvent produced by British COATES. However, we still insist that an ink should be equipped with a matching dedicated thinner.

(1) COATES Diluent Series

(2) COATES products with corresponding thinner

The use of diluents at random may cause problems such as gelling (caking) of the ink, peeling, uneven color, chemical resistance, thermal shock resistance, and adhesion degradation. No matter what type of ink is used, it is not correct to use "white water" "783" as a universal solvent for Riga.

3. Pigment: The main use of pigments. Pigments are divided into categories and can be divided into organic pigments and inorganic pigments. Organic pigments refer to non-minerals, which have beautiful 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., their hiding power, light resistance and aging resistance, PCB ink is mostly such ink.

In addition to pigments, pigments also play a role in the lightfastness, heat resistance, flux resistance, and chemical resistance of inks.

4. Additives: The purpose of adding additives in the ink is to improve the physical properties of the ink, enhance the printability of the ink, and improve the printing effect. Additives include: defoamers, dispersants, diluents, homogenizers, solidifiers, color separation inhibitors, precipitation inhibitors, plasticizers, coupling agents, ultraviolet absorbers, catalysts, thickeners, and the like.
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three. PCB ink several important technical performance:

Whether or not the PCB ink quality is excellent is in principle impossible to separate from the combination of the above major components. The excellent ink quality is a comprehensive manifestation of the formula's scientificity, advanced nature and environmental protection. It is reflected in:
1 Viscosity: Abbreviation for dynamic viscosity. Viscosity is generally expressed as the shear stress of the fluid flow divided by the velocity gradient in the flow layer direction, and the international unit is Pa/s (Pa.s) or milli-Pascal/sec. (mPa.S). In PCB production, the fluidity of the ink is driven by external forces.

Conversion of viscosity units:

1Pa. S = 10P = 1000mPa. S =1000 CP=10dpa.s

2 Plasticity: refers to the nature of the ink before it is deformed after it is deformed by external forces. The plasticity of the ink helps to improve the printing accuracy;
3 thixotropic: (thixotropic) ink when standing gel-like, but by the touch of a change in the viscosity of a property, also known as buckling, sag resistance;
4 Fluidity: (leveling) The extent to which the ink spreads around under the action of external forces. Fluidity is the reciprocal of the viscosity, which is related to the plasticity and thixotropy of the ink. Plasticity and thixotropy are large, and fluidity is large; when the fluidity is large, the imprinting is easy to expand. Small liquidity, prone to netting, resulting in ink phenomenon, also known as reticulate;
5 Viscoelasticity: refers to the ink's ability to quickly rebound after being scraped by a scraper. It is required that the ink be deformed quickly, and ink rebounding can be used to facilitate printing.
6 Drying: It is required that the drying of the ink on the screen is as slow as possible, and it is desired that the ink be transferred to the substrate more quickly;
7 Fineness: The size of pigment and solid material particles, PCB ink is generally less than 10μm, the size of fineness should be less than one-third of the opening of the mesh;
8 Wire drawing: When the ink is used to pick up the ink, the degree of thread-like ink stretching does not break is called stringiness. The ink is 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;
9 Ink transparency and hiding power: For PCB inks, various requirements are also made on the transparency and hiding power of the ink according to the use and requirements. In general, line inks, conductive inks, and character inks all require high hiding power. The solder resist is more flexible.
10 Chemical resistance of inks: PCB inks have strict standards for the requirements of acids, alkalis, salts, and solvents, depending on the purpose of use;
(11) Physical properties of inks: PCB inks must meet resistance to external scratches, thermal shocks, mechanical peeling, and various stringent electrical performance requirements;
(12) Safety of use of ink and environmental protection: PCB ink requires low toxicity, odorless, safe and environment-friendly.

Above we have summarized the basic performance of twelve PCB inks, and among them, in the actual operation of screen printing, the viscosity problem is closely related to the operator. The level of viscosity has a great relationship with silk screen printing. Therefore, in the PCB ink technical documentation and QC report, the viscosity is clearly marked, indicating under what conditions, what type of viscosity testing equipment and so on. In the actual printing process, if the viscosity of the ink is too high, it will cause difficulty in printing, serious jagged edges in the pattern, in order to improve the printing effect, it will add thinner to make the viscosity meet the requirements. However, it is not difficult to find that in many situations, in order to obtain the ideal resolution (resolution), no matter what viscosity you use, it will never be achieved. why? After in-depth study, it was discovered that ink viscosity is an important factor, but not the only one. There is another quite important factor --- thixotropy. It is, it also affects the printing accuracy.

four. Thixotropic: (thixotropic)

Viscosity and thixotropy are two different physical concepts and it can be understood that thixotropy is a characterization of the change in ink viscosity.

The ink is at a constant temperature. Assuming that the solvent in the ink does not volatilize very quickly, the viscosity of the ink will not change. The viscosity has no relation with time. The viscosity is not a variable but a constant.

When the ink is affected by external force (stirring), the viscosity changes, and as the force continues, the viscosity will continue to decrease, but it will not decrease without limit, and it will stop at a certain limit. When the external force disappears, the ink can automatically return to its initial state after a certain period of time. We reduce the viscosity of the ink under the action of external force over time, but when the external force disappears, the reversible physical property that can restore the original viscosity is called thixotropy. Thixotropy is a time-dependent variable under the influence of external forces.

Under the action of external force, the shorter the duration of the action, the viscosity drops significantly, we call the thixotropy of this ink large; on the contrary, if the viscosity does not drop significantly, it is called thixotropy.

Fives. Reaction Mechanism and Control of Ink Thixotropy

What is thixotropic in the end? Why does the ink under the action of external force, viscosity will become smaller, and the external force disappears, after standing for a certain period of time, but also to restore the original viscosity?

The prerequisite for determining whether the ink has thixotropy is firstly a viscous resin, followed by a certain volume of filler and colorant particles. Resins, fillers, pigments, auxiliaries, etc., after grinding, are very homogeneously mixed together and they are a mixture. In the absence of external thermal or ultraviolet light energy, they exist as an irregular cluster of ions. In the normal state, they are arranged in an orderly manner due to mutual attraction and exhibit a state of high viscosity, but no chemical reaction occurs. Once it is subjected to external mechanical forces, it disrupts the orderly arrangement of the original, cuts off each other's attraction chain, becomes disorderly disorder, and shows viscosity change.