PCB News - The effect of thixotropy on the performance of ink in PCB manufacturing

Core summary: 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, the thixotropy as an ink is often overlooked by people. But it plays a very important role in the effect of screen printing. In order to explain the effect of thixotropy on the screen printing effect more clearly, we must also start with the most basic ink and screen printing principles. 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 production 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, the thixotropy as an ink is often overlooked by people. But it plays a very important role in the effect of screen printing.

In order to explain the effect of thixotropy on the screen printing effect more clearly, we must also start with the most basic ink and screen printing principles. Then introduce the concept of thixotropy. Below we analyze and explore the influence of thixotropy in the PCB system on the ink performance:

1. Screen

Silk screen is one of the indispensable materials in the screen printing process. Without screen, it cannot be called screen printing. Screen printing is the soul of screen printing technology. The screens are almost all silk fabrics (of course there are also non-silk fabrics).

Generally distinguished by material: nylon, polyester, stainless steel

According to the knitting method, it can be divided into: plain weaving, satin weaving

The structure of silk can be divided into: single-strand, multi-strand

According to the thickness of the net, it can be divided into: s (thin), t (medium), hd (heavy)

The mesh number of the net can be roughly divided into: low mesh, medium mesh, and high mesh

In the PCB industry, the most commonly used is the t-type network. s and hd type networks are generally not used except for individual special needs. This is because the PCB industry is a high-tech and highly professional industry. It is different from making simple manual and artistic skills. Generally, manual and artistic skills require a low technical level. As long as it can meet or reach Human visual effects are fine. As for the PCB, it is not only necessary to look neat and beautiful. The PCB must be used for current conduction and signal transmission. It must meet the fine geometrical size required for electrical performance, and this geometrical size needs to be strictly measured. of. Therefore, we need to understand several important technical parameters related to wire mesh.

1. The thickness of the screen:

Thickness refers to the measured value of the thickness of the screen when it is left standing under no tension. The measured value is the statistical average value derived from the measured data, expressed in μm. The thickness is determined by the diameter of the wires that make up the screen, and is related to the ink penetration of the screen.

2. The opening area rate of the screen:

It is the ratio of the area of the mesh to the area of the screen, expressed as a percentage. The larger the value, the greater the opening of the mesh.

3. Ink penetration of the screen:

This refers to the theoretical value. In actual screen printing production, the ink penetration will be affected by the material, performance, viscosity of the specification ink, the fineness of the pigment, the thixotropy of the ink, the hardness of the squeegee, the pressure of printing, The printing speed, touch screen distance and other factors affect it.

Ink penetration = screen thickness ftx screen hole area rate x 10000.

=ft. w2/(w+d)2. 10000 (unit: cm3/m2)

The above three technical indicators have a very close relationship with the content discussed below.

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.

(1) Composition of ink:

1. Resin: Resin is an important component that forms the ink film and determines the performance of the ink. The well-known PCB ink brands have their own patented formulas. We usually talk about how good the performance of this brand ink is. 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. Solvent: The function of solvent:

1. Dissolve resin. Make it a good link material;

2. Dissolve pigments and additives;

3. Adjust the 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 group of components. Considering the problems of dissolving power and drying speed, most solvents are mixed substances. Different inks and different uses have different solvents. Therefore, ink manufacturers will be equipped with 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 inks produced by the same ink manufacturer can provide common solvents. For example, the ink produced by British Coates can basically be used universally as long as it is a solvent produced by British Coates. However, we still insist that an ink should be matched with a matching special thinner.

⑴ Thinner series products provided by Coates

⑵Coates product and its corresponding diluent

Random use of thinner will cause the ink to gel (agglomerate), peel off, uneven color, chemical resistance, thermal shock resistance, adhesion drop and other undesirable problems. It is incorrect to add "anti-white water" and "783" as a universal solvent regardless of the ink.

3. Pigment: mainly use pigments. Pigments are divided into categories, which can be divided into organic pigments and inorganic pigments. Organic pigments refer to non-minerals, with brilliant colors and complete chromatograms, but they usually have poor hiding power. Inorganic pigments refer to minerals. Such as: titanium white, zinc white, iron blue, carbon black, etc., they have good hiding power, strong light resistance and aging resistance. PCB inks are mostly such inks.

In addition to being used as colorants, pigments also play a certain role in the light resistance, heat resistance, flux resistance, and chemical resistance of the 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: defoamers, dispersants, diluents, homogenizers, firming agents, color separation inhibitors, precipitation inhibitors, plasticizers, coupling agents, ultraviolet absorbers, catalysts, thickeners, etc.

Three. Several important technical properties of PCB ink

Whether the quality of PCB ink is excellent, in principle, it is impossible to break away from the combination of the above major components. The excellent quality of the ink is a comprehensive manifestation of the scientificity, advancement and environmental protection of the formula. It is reflected in:

1. Viscosity: is the abbreviation for dynamic viscosity (dynamicviscosity). Generally expressed by viscosity, that is, the shear stress of fluid flow divided by the velocity gradient in the direction of the flow layer, the international unit is Pa/sec (pa.s) or milliPascal/sec (mpa.s). In the production of PCB, it refers to the fluidity generated by the external force of the ink.

Conversion relationship of viscosity unit:

1pa. s=10p=1000mpa. s=1000cp=10dpa.s

2. Plasticity: It means that the ink still retains its properties before deformation after being deformed by external force. The plasticity of the ink is conducive to improving the printing accuracy;

3. Thixotropic: (thixotropic) ink is gelatinous when it is standing, and the viscosity changes when touched, also known as thixotropy and anti-sagging;

4. Fluidity: (leveling) the extent 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 plasticity and thixotropy are large, the fluidity is large; the fluidity is large, the imprint is easy to expand. Small liquidity, easy to appear netting, 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 scraped by the squeegee. It is required that the ink deformation speed is fast and the ink rebounds quickly to be beneficial to printing;

6. Dryness: the slower the drying of the ink on the screen, the better, and it is hoped that after the ink is transferred to the substrate, the faster the better;

7. Fineness: the size of pigment and solid material particles, PCB ink is generally less than 10μm, and the size of the fineness should be less than one-third of the mesh opening;

8. Drawnability: When the ink is picked up with an ink shovel, the degree to which the filamentous ink is stretched without breaking is called the drawability. The ink filaments are 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. Transparency and hiding power of ink: For PCB ink, according to different uses and requirements, various requirements are also put forward for the transparency and hiding power of ink. Generally speaking, circuit inks, conductive inks and character inks all require high hiding power. The solder resist is more flexible.

10. Chemical resistance of ink: PCB ink has strict standards for acid, alkali, salt and solvent according to different purposes;

11. Physical resistance of the ink: PCB ink must meet external scratch resistance, heat shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements;

12. Ink use safety and environmental protection: PCB ink is required to have low toxicity, odorless, safety and environmental protection.

Above we have summed up the basic performance of twelve PCB inks, and among them, in the actual operation of screen printing, the problem of viscosity is closely related to the operator. The viscosity is very important to the smoothness of the silk screen. Therefore, in the PCB ink technical documents and qc reports, the viscosity is clearly marked, indicating under what conditions and what type of viscosity testing instrument to use. In the actual printing process, if the ink viscosity is too high, it will be difficult to print out, and the edges of the graphics will be severely jagged. In order to improve the printing effect, a thinner 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 still impossible to achieve. Why? After in-depth research, I discovered that ink viscosity is an important factor, but not the only one. There is another quite important factor-thixotropy. It is also affecting the printing accuracy.

Four. Thixotropic: (thixotropic)

Viscosity and thixotropy are two different physical concepts. It can be understood that thixotropy is a sign of changes 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 at this time. The viscosity has nothing to do with time. The viscosity is not a variable, but a constant.

When the ink is subjected to external force (stirring), the viscosity changes. As the force continues, the viscosity will continue to decrease, but it will not drop indefinitely, and stop when it reaches a certain limit. When the external force disappears, after a certain period of standing, the ink can automatically gradually return to the original state. We call this kind of reversible physical property that the ink viscosity decreases with the extension of time under the action of external force, but after the external force disappears, it can return to the original viscosity as thixotropy. Thixotropy is a time-related variable under the action of an external force.

Under the action of external force, the shorter the duration of the force, and the obvious decrease in viscosity, we call this ink the thixotropy is large; on the contrary, if the viscosity decrease is not obvious, it is said that the thixotropy is small.

The above is an introduction to the influence of thixotropy on ink performance in PCB manufacturing. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology