PCB Technical - The effect of PCB thixotropy on ink performance

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 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).

In the PCB industry, the most commonly used is the t-type net. 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 meet 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 required for electrical performance, 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. 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 ratio of the screen: it is the ratio of the area of the screen to the area of the screen, expressed as a percentage. The larger the value, the larger the opening of the screen.

3. The ink penetration of the screen: here 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, and the thixotropy of the ink. The hardness of the board, printing pressure, printing speed, touch screen distance and other factors affect it.

2. 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) Composition of ink:

1. Resin: Resin is an important component that forms the ink film and determines the performance of the ink. Well-known PCB ink brands have their own patented formulations. 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:

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.

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 and aging resistance, PCB inks are mostly such inks.

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.

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: short for dynamic viscosity. 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 PCB production, it refers to the fluidity of ink produced by external forces.

(2) Plasticity: After the ink is deformed by external force, it still retains its properties before deformation. The plasticity of the ink is conducive to improving the printing accuracy;

(3) Thixotropic: (thixotropic) The ink is gelatinous when it is left standing, and the viscosity changes when it is touched. It is also called thixotropic and sag resistance;

(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. With low fluidity, it is prone to network formation, resulting in ink formation, which is also known as reticulation;

(5) Viscoelasticity: refers to the ability of the ink that is sheared and broken after the ink is scraped by the squeegee to rebound quickly. 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 the faster the better after the ink is transferred to the substrate;

(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 there are many filaments on the ink surface and the printing surface, making the substrate and the printing plate dirty, or 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, thermal shock resistance, mechanical peel resistance, and meet various strict electrical performance requirements;

(12) Safety and environmental protection of the ink: PCB ink is required to be low-toxic, odorless, safe and environmentally friendly.

Four. Thixotropy

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.

5. Reaction mechanism and control of ink thixotropy

To determine whether the ink has the necessary conditions for thixotropy, first is the resin with viscosity, and then is filled with a certain volume ratio of filler and pigment particles.

Six. Practical application of thixotropy

In practical applications, it is not that the greater the thixotropy, the better, nor the smaller the better. It's just enough. Due to its thixotropic properties, the ink is very suitable for the process of screen printing. Makes the screen printing operation easy and free. During the ink screen printing, the ink on the net is pushed by the squeegee, rolling and squeezing occur, and the viscosity of the ink becomes lower, which is conducive to ink penetration. After the ink is screen printed on the PCB substrate, because the viscosity cannot be recovered quickly, there is a proper leveling space to make the ink flow slowly, and when the balance is restored, the edges of the screen printed graphics will be satisfactorily flat.