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, medium mesh, high mesh

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.