PCB Technical - The influence of ink performance on PCB manufacturing

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 of PCB shipments to lower the quality indicators. For this reason, PCB manufacturers are very serious about the performance of the ink. 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 more clearly explain the influence of thixotropy on the screen printing effect, we must also start with the most basic ink and silk screen principles. Then introduce the concept of thixotropy. Below we analyze and explore the influence of thixotropy in the PCB system on the performance of the ink:
One, silk 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 divided by material: nylon, polyester, stainless steel
can be divided into weaving method: plain weave, silk weave
The structure of silk 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)
According to the mesh of the net, it 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 do not require a high level of technology. As long as it can be content or reach Human visual effects are fine. 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 dimensions that the motor can require, and this geometrical dimension 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 under no tension. This measured value is a statistically uniform 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 open area rate of the screen:
is the ratio of the area of the mesh to the area of the screen, expressed as a percentage. The larger the value, the larger the opening of the mesh.
3. The ink penetration of the screen:
This refers to the theoretical value. In the actual screen printing production, the ink penetration will be affected by the material, function, viscosity of the standard 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 rate = screen thickness ftx screen hole area rate x 10000. (Unit: cm3/m2)
The above three technical indicators have a very close relationship with the content discussed below.

B. Ink
refers to colored gelatinous substances used for printed boards. It is often composed of synthetic resins, volatile solvents, oils and fillers, desiccants, pigments and diluents. Often called ink.
1. Ink composition:
1. Resin: Resin is an important component that forms the ink film and determines the function of the ink. The well-known PCB ink brands have their own patented formulas. We usually talk about how good the brand's ink performance is. In fact, the resin plays a very important role in it. It determines the handling, gloss, adhesion, hardness, water resistance, solvent resistance, acid and alkali resistance, temperature resistance and so on of the ink.

The solvent of the ink is not a single type 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 have several different types of solvents. For one ink, only one or two types of solvents can be used. Generally speaking, for an ink manufacturer, because the resin system used in its research and development is basically determined, the solvents provided by the ink produced by an ink manufacturer can be universal. For example, the inks produced by British Coates can basically be used as long as they are solvents produced by British Coates. However, we still insist that an ink should be matched with a matching special thinner.

Any 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 not accurate to add "anti-white water" and "783" as a universal solvent, no matter what kind of ink.
3. Pigment: mainly use pigments. Pigments 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.
Pigments are not only used as colorants, but also play a certain role in the light resistance, heat resistance, flux resistance, and chemical resistance of the ink.
4. Auxiliaries: The purpose of adding auxiliaries to the ink is to improve the physical function of the ink, enhance the printability of the ink, and improve the printing effect. Additives include: defoamers, dispersants, diluents, averaging agents, firming agents, color separation inhibitors, precipitation inhibitors, plasticizers, coupling agents, ultraviolet absorbers, catalysts, thickeners, etc.
C. Several important technical functions 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 of the formula, the improvement of the predecessor and the environmental protection. It is reflected in:
1. Viscosity: is the abbreviation for dynamic viscosity. It is generally expressed by viscosity, that is, the shear stress of fluid activity 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 mobility of ink produced by external force.

2. Plasticity: refers to the ink being deformed by an external force, it still retains its properties before deformation. The plasticity of the ink is conducive to improving the printing accuracy;
3. thixotropic: (thixotropic) ink is gelatinous when standing, and a property of changing viscosity when touched, also known as thixotropy and sag resistance;
4. Mobility: (leveling) the extent to which the ink spreads to the surrounding under the action of external force. Mobility is the reciprocal of viscosity, and mobility is related to the plasticity and thixotropy of the ink. The greater the plasticity and thixotropy, the greater the mobility; the greater the mobility, the larger the imprint. The activity is small, the netting is easy to appear, resulting in the phenomenon of inking, also known as netting;
5. Viscoelasticity: refers to the ability of the ink that is blocked by shearing to rebound quickly 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 I hope 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. Stringiness: When the ink is picked up with an ink shovel, the degree to which the silk-like ink does not break when stretched is called stringiness. The ink filament is long, and a lot of filaments appear on the ink surface and the printing surface, making the substrate and the printing plate dirty, and even unable to print;
9. Ink transparency and hiding power: For PCB inks, according to different uses and requirements, various requirements are also put forward for the transparency and hiding power of the 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 the ink: PCB inks have strict standards for acid, alkali, salt and solvents according to the purpose of use;
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 requires low toxicity, odorless, safety and environmental protection.
Above we have summarized the basic functions of twelve PCB inks. 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 lectures, the viscosity is clearly marked, indicating under what premise and what type of viscosity measuring instrument to use. In the actual printing process, if the ink viscosity is too high, it will cause the problem of missing printing, 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. 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 is still impossible to achieve. Why? After in-depth research, it was discovered that ink viscosity is an important factor, but it is not unique. There is another commensurate important factor-thixotropy. It is also affecting the printing accuracy.
4. Thixotropic: (thixotropic)
Viscosity and thixotropy are two different physical concepts. It can be understood in this way that thixotropy is a sign of changes in ink viscosity.
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 acted by an external force (stirred), the viscosity changes. With the continuation of the force, the viscosity will continue to decrease, but it will not decrease indefinitely, and it will stop when it reaches a certain limit. After 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 significant decrease in viscosity, we call the thixotropy of this ink large; on the contrary, if the decrease in viscosity is not significant, it is said that the thixotropy is small.
5. The reaction mechanism and control of ink thixotropy
What is thixotropy? Why is the viscosity of the ink reduced under the action of external force, but the external force disappears, after a certain period of time, the original viscosity can be restored?
The necessary prerequisites to determine whether the ink has thixotropy, firstly is the resin with viscosity, and secondly, it is filled with a certain volume ratio of filler and pigment particles. Resins, fillers, pigments, additives, etc. are mixed together very evenly after grinding. They are a mixture. In the absence of external heat or ultraviolet light energy, they exist as an irregular ion group. Under normal conditions, they are arranged in an orderly manner due to mutual attraction, showing a state of high viscosity, but no chemical reaction occurs. And once it is subjected to external mechanical force, the original orderly arrangement is disrupted, the mutual attraction chain is blocked, and it becomes a disordered state, showing that the viscosity becomes lower. This is the phenomenon that we usually see ink from thick to thin. We can use the following reversible process diagram of a closed cycle to visually express the whole process of thixotropy.

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