Nano ink application technology

Nanotechnology is a far-reaching high technology in this century. Its appearance has spawned a large number of new disciplines. Such as nano-physics, nanochemistry, nanobiology, nanomaterials, and so on. The research object of nanotechnology is a 1-100nm-scale material or structure, including the manufacturing and processing technology, characteristics and application technology, characterization and measurement technology of nanomaterials. The so-called "nano" is actually a measure of length, but this unit is very small. 1nm = 10-9m, which is one billionth of a meter. People have discovered that at the nanometer scale, due to its small size effect, surface and interface effects, and quantum effects, they (nanoparticles, nanowires, and nanofilms) have a lot of acoustic, optical, electrical, magnetic and mechanical properties. Each aspect shows a series of distinctive specific energy. For example, ordinary metals are mostly solid at room temperature. Like gold, silver, copper, and iron, they all have high melting points. The situation in the nano-metal is not the case. If we put gold nanoparticles in the palm of our hand, we can find that the particles will melt like ice and become a paste. Furthermore, some metal nanoparticles spontaneously ignite due to strong oxidation even in normal air.

As we all know, ordinary ink is a complex polymer composition, it has a specific viscosity and excellent printability. The main ingredients are pigments (dye), polymer binders, solvents, and small amounts of additives. When ink is formed, people put the above-mentioned components into a special apparatus according to a certain proportion, and are sufficiently dispersed to obtain a uniform printing ink having a certain viscosity and thixotropy. Nano ink composition and manufacturing methods are no different from ordinary ink. If there is a difference, it is only the "pigment" particles used in the two inks, which vary greatly in particle size. The pigment size of conventional inks is in the micron (μm) grade, while the "pigment" size of the nanoinks is nanoscale. The difference between the two sizes is about 1,000 times. The introduction of nano-specific “pigments” with specific energy will give moderate changes to the ink manufacturing process, which is understandable. Although nano inks and common inks are used for printing products, the former mainly focuses on the application of special functions, while the latter is often used for printing monochrome or color prints.

Recently, although nano inks have just emerged, they have shown excellent performance in the fields of processing and installation of electronic components, decoration and decoration of high-end products, sterilization and detection of pharmaceuticals, and anti-counterfeit printing of special products. And great attraction. This article focuses on metal nano inks and their application technology in the field of electronics. In addition, it also makes a brief discussion on related theoretical issues. First, metal nano-ink As described above, nano-particles are the core components of nano-ink. Nanoparticles used in inks can be either organic or inorganic in view of their properties; they can be metallic or non-metallic; or they can be oxides. According to the different applications of ink, people can choose freely. Nanoparticles for nanoinks have a particle size of several nanometers. It should be noted that the particle size of nanoparticles often refers to the average value, even for the same production batch product, the particle size of each particle is difficult to be exactly the same, but only the size distribution of different aggregates (Figure 1 shows the particle size distribution of gold nanoparticles produced by a company). What needs to be emphasized here is that not all nanoparticles can be used as "pigments" for nanoinks. The reason is simple: ordinary nano-particles have large surface activity and high energy, and particles are prone to "agglomeration" in the population. Once agglomeration of particles occurs, it is difficult to disperse them by ordinary methods. Nanoparticles for nanoinks have special requirements that each particle should have monodispersity, which is the key to the success of nanoinks.

1. The typical characteristics of monodisperse nanoparticle nanoparticles are that the particle size is extremely small, and the surface area is very large. With a sharp increase in the surface energy of the particles, the melting point decreases drastically. As in the normal state, the melting point of gold is 1063°C. However, when the solid gold becomes a gold nanoparticle with a diameter of 2 nm, the melting point has undergone a significant change, and it has dropped from 1063°C to near room temperature. People use this feature to easily sinter metal conductors even at very low temperatures. The fundamental difference between monodisperse nanoparticles and normal nanoparticles is that the former surface has been coated with a thin layer of a special coating agent, so that the surface activity of the particles and the decrease in the melting point of the particles are temporarily suppressed in a moderate range. Inside. The coated nanoparticles then have a monodisperse function (see Figure 2a). Even in solvent or resin solution, it can always maintain the characteristics of uniform dispersion. In other words, the monodisperse stability of the particle is very good. This is exactly what nano-inks are eagerly looking for in their manufacture or storage. However, ordinary nanoparticles are completely different, and their surface is not protected by any protective film, but is directly exposed to the outside world. The original large surface activity and excessive surface energy make the particles always in an unstable state. Particles and particles can easily agglomerate with each other (see Figure 2b). There are generally two types of methods for producing metal nanoparticles: physical methods and chemical methods. One of the physical methods of evaporation is to add metal to the molten state in a vessel filled with an inert gas, so that the vaporized metal rapidly solidifies into metal nanoparticles. The advantage of this method is that the product is of high purity, with the disadvantage that the continuous production of nanoparticles is poor. Chemically produced metal nanoparticles can be divided into dry and wet methods. Regardless of the former or the latter, the purity of their respective products is not very high, and they often contain small amounts of impurities such as alkalis or sulfides. In this regard, complex purification processes have to be added in order to purify the metal nanoparticles that have just been produced. Although the chemical method can be continuously produced in batches, the total cost of the product after purification is too high. In short, in order to obtain high-purity metal nanoparticles, it is necessary to carefully and carefully select the most reasonable process method.

2. The composition and characteristics of metal nano inks As mentioned above, metal nano inks are uniformly dispersed by a certain proportion of metal nanoparticles (monodisperse particles), resins, solvents, additives, and the like. Table 1 lists the composition and properties of metallic nano inks. Metal nanoparticles are one of the most important components in inks, and they are the only source of conductivity for printed graphics. The good or bad of its conductivity mainly depends on the metal properties of the nanoparticles, that is, the smaller the intrinsic resistance of the metal, the better the conductivity, and vice versa. For example, the conductivity of silver, copper, gold, etc. are all good. In theory, their nano-particles can be used as nano-ink materials. However, it is not always true in actual ink application because we also have to consider the chemical reactivity of the metal used. The chemical reactivity of metals such as copper is relatively large, and the particles, especially nanoparticles, are easily oxidized in the air and lose their metal properties. At present, to solve this problem, from a technical point of view, there are still some difficulties. Therefore, practical examples of copper nanoparticles for nano inks have not been reported so far. In contrast, gold and silver have stable chemical properties and good electrical conductivity, so their nanoparticles have become the primary raw material for metal nanoinks today. As another important component of the metal nano ink, the thermosetting resin has two functions. First, the monodisperse metal nanoparticles are incorporated into the resin system to become a uniform resin composition having a certain viscosity and printability; The ink is given a certain adhesive function so that the ink can be firmly attached to the printed substrate. Therefore, the chemical structure and molecular weight of the resin are critical and cannot be ignored. Commonly used resins include acrylate resins, phenolic resins, and epoxy resins. It is very necessary for people to select the appropriate chemical structure of the polymer resin according to different printing substrates. The main role of the solvent in the ink: First, it is used to dissolve the solid resin, making it a resin solution, so as to match the ink; second, it is used as a diluent to adjust the ink viscosity to meet the printing requirements. When the resin is selected, what kind of solvent is appropriate? Here depends on the solubility parameters of the resin and solvent. If these two parameters are relatively close to each other, it indicates that the solvent is a good solvent for this resin, ie, the solubility is good. On the contrary, it cannot be used as a solvent for the resin. The most commonly used organic solvents are toluene, xylene, cyclohexanone, butanone, diacetone alcohol, glycol ethers and certain higher alkanes. Today, all countries attach great importance to the interrelationship between the environment and people, the environment and society. The toxicity of solvents and their impact on the surrounding environment are major issues that must be considered. In order to improve certain properties of the ink and improve the quality of the ink, a small amount of additives are often used. Such as surfactants, leveling agents, plasticizers and defoamers. Silver nano inks and common conductive silver inks are used for conductive printing, but due to the different conductive particles used, the performance and application of these two inks are also significantly different. Table 2 is a comparison of the characteristics of silver nano inks and ordinary conductive silver inks. From Table 2, it can be seen that metal nano inks have similarities with common conductive silver inks, and there are also different places. When using them, they should be appropriately selected according to the purpose and object.

3. Mixed silver inks (silver powder + silver nanoparticles) Common conductive silver inks are often used for conductive printing of thin film switches and thin film circuits. However, in the face of the printing of electronic products with higher transmission speeds and higher performance requirements, common silver inks have large defects in both conductivity and reliability and cannot meet the higher requirements for inks of these electronic products. Of course, we can choose silver nano ink to complete this task, but if the product does not have ultra-fine printing requirements, rush to use silver nano ink to print, from the perspective of business management is inappropriate. The reason is that this means that the use of raw materials with excess performance, which is often referred to as "big horse carts," is an unnecessary waste. In response, a new product has been developed that uses a small amount of silver nanoparticles to modify metallic silver powder (flakes or spheres) and produces a so-called hybrid silver ink. This product is suitable for the printing of electronic products with high conductivity, high reliability and no special fine requirements. Table 3 lists the composition of the ink and its characteristics. In a mixed silver ink, the total silver content has a large influence on the conductivity of the ink, and experiments have shown that the silver content is preferably 80% to 95% in the solid component of the ink. If the content of silver is too small, the conductivity of the ink is insufficient; conversely, if the amount of silver is too large, the conductivity of the ink may be impaired for other reasons (see FIG. 3).

As can be seen from Figure 3, when the silver content is 85%, the ink volume resistance is within 10 × 10-6 Ω cm, and when the silver content is increased to 90%, the volume resistance is reduced to the minimum, that is, 6 × 10-6 Ω cm. After this, as the amount of silver in the ink increases, its volume resistance not only does not decrease, but gradually increases. The reason for this can be explained as follows: When the increase in the amount of silver in the ink reaches a maximum, in other words, the percentage of resin binder (binder) has reached a minimum. At this time, the further reduction of the resin content (silver content continues to increase), the uniformity of the ink film during the drying of the ink film will be a problem, that is, the internal of the ink film gradually produces some small cracks, which hinders the electrons inside the ink film. successfully passed. In mixed silver inks, the proportion of silver nanoparticles in the total silver content is good, and practice has shown that it accounts for a ratio of 10% to 20%. Too little modification effect is not obvious, too much will cause unnecessary waste. It should be noted that when we formulate mixed silver inks, in addition to taking into account the percentage of silver, what type of resin structure is used, what the molecular weight is, and what additives and mixed solvents are used will affect the ink. Printability (viscosity, thixotropy, adhesion, drying and curing properties, etc.). Figure 4 is a test sample (electronic part) printed with a hybrid silver ink screen. The data obtained after the sample was subjected to the reliability test is shown in Table 4. From the comparison data in Table 4, it can be seen that the reliability of hybrid silver ink is significantly higher than that of ordinary silver ink. If there is no special fineness requirement for printed graphics, it is undoubtedly the best choice for the printing of highly conductive and highly reliable electronic components. Another point to be reminded is that the mixed silver ink has a relatively low cost, but the conductivity and reliability have been greatly improved (compared with ordinary silver ink), and the ink can be applied in many fields through the screen printing method. : (1) Printed electronic circuitry on a heat-resistant organic substrate; (2) BVH formed on a composite substrate; (3) Printing and bonding of micro-electrode components; (4) In addition to the above uses, any one can be sintered at a low temperature (200-250 °C). A variety of uses for high conductivity, high reliability electronic components can be achieved using screen printing using a hybrid silver ink.