Conductive ink talk
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One of the biggest advances in RFID electronic label printing is the emergence of a new type of printing method, the conductive ink printing method, which can replace the traditional etching method and plating method.
The printing process of the method is to directly print a conductive line on an insulating substrate (film, paper, etc.) with a conductive ink to form an antenna and a circuit.
Conductive ink printing can replace the traditional etching and electroplating methods to make electronic labels, mainly because it has the following three advantages:
●The traditional etching method and electroplating method make the metal antenna, the process is complicated, the finished product is produced for a long time, and the direct printing method is to make the antenna by using the high-speed printing method, which is efficient and fast. Nowadays, direct printing has begun to replace etched antennas in various frequency bands, such as ultra-high frequency band (860MHz ~ 950MHz) and microwave frequency band (2450MHz), and the quality of the antenna can be compared with the antenna made by traditional technology.
● The metal antenna made by the traditional etching method and the electroplating method consumes metal materials, and the cost is high, and the raw material cost of the direct printing method is lower than that of the conventional metal antenna, which has great significance for reducing the manufacturing cost of the electronic label.
●The process of traditional etching or electroplating process produces a large amount of waste liquid containing metal and chemical substances, which causes great pollution to the environment. The direct printing method uses conductive ink to directly print on the substrate without corrosive materials. The use of chemical reagents is less or no, and has the advantage of "green" environmental protection.
Therefore, the development of conductive ink has become a hot spot in the development of RFID printing technology. Conductive inks have been around for a long time. In the past, some companies used to produce conductive inks with silver, silver-plated copper, carbon and graphite for RFID smart card products; today, conductive ink manufacturers are working hard to develop new formulations. Flint recently invested heavily in research and development of new conductive inks; not long ago, a company called Parelec developed an ink that is said to be three times more conductive than traditional conductive inks.
First, the formulation and composition of conductive ink
Conductive ink is a special ink that can be used to add conductive energy to UV inks, flexo aqueous inks or special offset inks to make the ink conductive. Conductive inks are mainly composed of pigments (conductive materials), binders (binders and solvents), and additives for inks.
Conductive materials can be divided into conductors and semiconductors according to their electrical conductivity; they can also be classified into inorganic and organic systems. The inorganic conductive material is a common metal material such as Ag, Cu, etc., and the organic system can be classified into a π-electron conjugated system and an intermolecular compound.
The binder is the main film-forming material that makes up the conductive ink. The solvent is used to dissolve the binder resin. Therefore, the solvent must have the ability to dissolve the binder resin, and on the other hand, the solvent cannot destabilize the conductivity of the conductive material and lower the physicochemical properties of the ink film.
The additives of the ink are mainly the dispersing agent for guiding the electric ink, the leveling agent, the metal conductive anti-oxidant and the like.
Second, the type of conductive ink
The conductive ink is a functional ink, and there are mainly carbon paste, silver paste and the like in printing.
Carbon paste ink is a liquid type thermosetting ink. It has the function of protecting copper foil and conducting current after film formation curing. It has good conductivity and low impedance, is not easy to oxidize, has stable performance, and is resistant to acid, alkali and chemical solvents. It has the characteristics of strong wear resistance and good thermal shock resistance.
Silver ink is a liquid ink composed of ultra-fine silver powder and thermoplastic resin. It can be used on PET, PT and PVC sheets. It has strong adhesion and hiding power, low temperature curing, controllable conductivity and Very low resistance value.
In addition, the conductive nano-carbon ink is added to the conductive ink made of ink, and the metal powder (such as silver powder) in the conductive ink can also be made into nano-scale silver powder to manufacture conductive ink, and the conductive ink not only prints the film layer. Thin and even smooth, excellent performance, and can save a lot of materials.
Third, the performance requirements of conductive ink
Although conductive inks are printed on RFID electronic tags, the cost of RFID tags can be saved in two ways. First, in terms of material cost, ink is less expensive than stamping or etching metal coils. Secondly, in terms of material consumption, stamping or etching consumes a large amount of metal, while conductive inks print antennas or circuits with high speed and low cost. efficient. The two most important parts of RFID are the chip part and the antenna part. Although the chip part is not currently printed, if the antenna part can be printed with conductive ink, the cost of RFID can be reduced.
However, it has to be acknowledged that the conductive ink also has the disadvantage that the electrical resistance of the conductive ink is larger than that of the conventional electric wire material, which means that it cannot effectively conduct a large current, and in addition, the printing resolution, the registration accuracy, The necessary isolation layer has yet to be improved and improved.
This requires that technology developers actively develop and improve the performance requirements of conductive inks when printing RFID electronic tags.
The requirements for the characteristics of conductive inks are mainly:
1 resistance to bending. The conductive line is printed on the diaphragm. If the flexibility of the ink is poor, it may break at the bend, or the resistance value may increase even if it is not broken.
2 adhesiveness. The ink adhesion strength based on the polyester film is generally determined by a tape test.
3 resistivity. It is required that the lower the specific resistance of the ink itself is, the ink having a low resistivity is advantageous when printing with the same type of mesh (forming the same film thickness).
4 particle size distribution. It refers to the distribution state of the conductive particles. The finer the particle size of the conductive particles, the better the distribution state of the binder and the particles, and the coated area is large because the ink on the printing plate has good elongation.
5 drying conditions. Refers to the drying time and drying temperature necessary for the complete curing of the conductive ink. The low-temperature drying ink can reduce man-hours and increase productivity.
6 printability. At present, the best conductive ink for printing is silver paste conductive ink. After printing, the outer furnace is cured at 120 ° C for 1 min, and then placed in a batch operation oven for 30 min; with silver (ion) migration protective film ink printing, in far infrared The oven was dried at 120 ° C for 2 min.
It is believed that with the continuous efforts of technicians and those concerned with the printing of RFID electronic tags with conductive inks, the performance of conductive inks will be further improved, and the printing performance of RFID electronic tags will become even better.
Fourth, the nature of conductive ink
Conductive ink is a special ink that allows current to flow, mainly by conductive fillers (including metal powders, metal oxides, non-metals and other composite powders), connecting agents (mainly synthetic resins, photosensitive resins, low melting point plexiglass, etc.) ), additives (mainly dispersants, regulators, thickeners, plasticizers, lubricants, inhibitors, etc.), solvents (mainly aromatics, alcohols, ketones, esters, alcohol ethers, etc.) and other components. Printing on flexible or rigid substrates can be made into printed circuits that act as wires, antennas and resistors. After the conductive ink is dried, since the distance between the conductive particles becomes small, the free electrons move along the direction of the applied electric field to form a current, which has good electrical conductivity and can receive the radio frequency signal dedicated to the RFID. The conductive ink determines the impedance of the RFID printed antenna to a large extent as an important parameter of the performance of the RFID antenna.
Conductive ink is also called conductive paste and conductive silver paste. The conductive ink includes a carbon paste ink, a copper paste ink, a silver paste ink, and a gold paste ink according to a composition of a conductive factor contained in the conductive ink. Because of its strong adhesion and hiding power, low temperature curing, controlled conductivity and low resistance, silver paste ink is the best choice for RFID tags.
Let's take the conductive silver paste as an example to introduce the properties and conductive mechanism of conductive ink.
1. Properties of conductive inks
One of the key technologies to improve the printing quality of the tag antenna is to make the conductive ink have the desired rheological properties and obtain good printing performance.
In the printing process of the tag antenna, printing inaccuracies of high-concentration conductive ink are often encountered. The reason for this phenomenon is that the agglomerated structure of the conductive metal particles in the polymer matrix and the unevenness of the surface thereof are highly likely to form a flocculated structure. Only when the applied shear stress exceeds the yield value can the structure be disassembled and the polymer system of the ink flow.
The fluidity of the conductive ink can be judged according to the viscosity of the conductive ink. Viscosity is a property of the conductive ink that prevents its flow inside. The viscosity is too small, the fluidity of the conductive ink is too large, the ink film imprinting is easy to expand during the printing process, the line definition is lowered, and the fine line resolution and the thickness of the ink film are difficult to meet the requirements. However, the viscosity is too large. At a certain shear rate, the conductive ink has good rolling property and poor fluidity, and is not easily transferred to the printing material. There are voids, broken wires and pinholes in the middle of the ink film, especially the printing uniformity of the thin wires is seriously degraded. .
In addition, the conductive ink must also have good thixotropy, because the thixotropy is a measure of the rapid recovery of the viscosity of the conductive ink. The ink film is deposited on the printing material after printing. If the viscosity continues to be small, the edge of the line tends to collapse. Small gaps are blurred.
Therefore, a conductive ink that satisfies high printing quality should have the following properties in the printing process, as shown in FIG.
The text is described as: high viscosity is required during storage to prevent metal particles from sinking due to gravity; at high speed printing, viscosity is rapidly reduced, fluidity and ductility are greatly increased, and conductive ink can be smoothly transferred to the substrate through the nozzle. After printing, the conductive ink quickly restores high viscosity, making the line boundaries clear and uniform, achieving higher resolution.
2. Conductive mechanism of conductive ink
The conductive mechanism of conductive ink mainly includes macroscopic percolation theory and microscopic quantum mechanical tunneling effect. The volume fraction of metal particles in conductive ink is an important indicator of the electrochemical performance of conductive inks and a key factor affecting the rheological properties of conductive inks.
The conductive mechanism of the conductive ink directly affects the conductive behavior of the conductive ink film printed with the conductive ink. The two conductive mechanisms of conductive inks are described below.
2.1 seepage theory: in the printed ink film of the wire, if the volume fraction fv of the conductive metal particles is high, the particles are in direct contact with each other, or the particle gap is smaller than the normal migration distance of the atom (about 10 nm), the wire ink film is along The direction of the electric field is applied to form a continuous network of conductive channels, and free electrons move directly along the conductive path to form a current. The more metal particles in the ink film, the smaller the particle gap, the more metal particles in contact state, the denser the conductive network, and the stronger the conductivity of the wire, which is the seepage phenomenon of the conductive ink.
2.2 Tunneling effect: When the volume fraction fv of the conductive metal particles in the printing ink film is lowered, the conductive channels in the conductive ink are gradually reduced, a part of the particles are completely separated by the insulating medium, and a part of the metal particles are still connected to each other. When the insulating medium is less than 100 nm, the electrons activated by the thermal vibration can also jump over the barrier formed by the insulating layer to the adjacent conductive particles, forming a large tunnel current, or crossing a very low insulating layer potential. The flow of the barrier generates a large field emission current. At this time, the interface of the insulating layer acts as a capacitance corresponding to the internal distribution, which is a "tunneling" phenomenon of the conductive ink.
With the change of the volume fraction fv of the conductive metal particles in the printing ink film, the seepage phenomenon and the tunneling effect of the conductive ink also exist or disappear, and the conductive behavior of the printed wire ink film also exhibits three different states.
In the presence of only the flow of conductive ink, when the particle gap is less than 10 nm, the insulating polymer matrix is only some isolated islands in the ink film. The conductive mechanism of the ink is similar to that of pure solid metal, keeping the migration of free electrons in the metal. The conductivity is determined by the ohmic conduction of the conductive metal particles. However, the insulating polymer island in the ink prolongs the conductive path of the free electrons, and has a strong scattering effect on the metal conductive carriers, so that the average free path of the electrons is slightly reduced.
When the particle gap is larger than 10 nm and less than 100 nm, the transmission mode of free electrons has two kinds of penetration along the maze structure and electron puncturing between the metal particles. The conduction behavior of the wire ink film is a combination of tunneling effect and percolation effect.
If the volume fraction fv of the metal particles in the conductive ink continues to decrease, and the particle gap is larger than 100 nm, the conductive ink changes from the metal structure to the isolated islands of the metal particles in the middle of the insulating medium, and the density of the permeation channels between the particles decreases. Metal conductance is gradually weakened, and the process of transporting carriers between adjacent isolated metal particles by thermal or field-excited tunneling becomes increasingly important. Until the free electron transition phenomenon in the ink film disappears, the wire ink film of the tag antenna becomes almost an insulator.
V. Conclusion
Conductive inks are considered to be the only technology that has gained worldwide acceptance. Conductive inks are inks that are printed on non-conductive substrates to conduct current and eliminate static charge. They are usually printed on plastic, glass, ceramics. Or non-conductive substrates such as cardboard. There are many printing methods, such as screen printing, letterpress printing, flexographic printing, gravure printing and lithography. Different printing methods can be selected according to the film thickness requirements, and the resistance, solder resistance and abrasion resistance are also different depending on the film thickness.
Without the development of conductive inks, there is no application of printing technology in the manufacture of RFID tags. Although the RFID market has been growing, the long-term growth that has been expected for a long time depends on the reduction in the production cost of RFID tags and their large-scale application on common commodities. Printing conductive inks will be an ideal solution to reduce the cost of RFID tag production. At present, conductive inks for RFID antenna printing account for only a very small portion of the entire conductive ink market. However, the industry believes that in the next 10 years, the share of conductive inks used in printing RFID antennas will continue to grow rapidly in the entire conductive ink market. At the same time, the share of conductive inks in the entire ink market will also increase significantly.