2. Composition and Characteristics of Metal Nano Ink
As described 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 ink (silver + silver nanoparticles)
Conventional 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 Ã— 10-6 Î© cm, and when the silver content is increased to 90%, the volume resistance is reduced to the minimum, that is, 6 Ã— 10-6 Î© 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) Electronic circuit printed on the heat-resistant organic substrate;
(2) forming a BVH on a composite substrate;
(3) printing and bonding of micro-electrode parts;
(4) In addition to the above applications, all kinds of electronic components with high conductivity and high reliability that can be sintered at a low temperature (200-250Â°C) can be realized by screen printing using hybrid silver ink.
Source: China Printing Chemicals Network
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