The RFID tag consists of a chip, an antenna, and a substrate. There are three methods for fabricating an antenna: an etching method, a coil winding method, and a conductive ink printing method. Among them, the conductive ink printing method has been widely used in the printing of RFID antennas because of its advantages of low cost, high precision, high printing speed, various printing materials, and easy change of antenna shape according to requirements.
Conductive inks are divided into organic conductive inks and inorganic conductive inks. At present, organic conductive inks are used less in actual production. In the printing, inorganic conductive inks are commonly used, including carbon conductive inks, silver conductive inks, and the like.
(A) The characteristics of RFID antenna printing
At present, there are four main methods for printing RFID antennas: screen printing, gravure printing, flexo printing, and offset printing . In view of the specificity of the conductive ink composition and the special requirements of the RFID antenna, RFID antenna printing has its own characteristics, such as: accurate print positioning, fine print lines and strict ink adhesion. In addition to the above common features, different printing methods also have their own characteristics.
Screen printing
At present, screen printing is the most widely used method in RFID tag antenna printing. The screen printing ink film thickness is 0.2 ~ 100μm, its highest resolution is low, usually below 50l/cm, and its printing speed is slower. The silk screen with good effect of printing conductive ink is a nickel foil perforated net, which is formed by drilling a nickel foil. The mesh is hexagonal, and it can also be formed into a round hole shape by electrolytic forming. The entire net surface is even and thin, which can greatly improve the stability and accuracy of the imprinting. It can distinguish the antenna spacing of 0.1mm and the positioning accuracy can reach 0.01mm.
There are two issues to be aware of when screen printing RFID antennas.
(1) Determine the thickness of the screen pattern according to the thickness of the conductive layer of the RFID antenna. In the printing, it is necessary to maintain the uniformity of the thickness while maintaining the consistency of each product. Because one of the important parameters of the antenna is the resistivity, the resistivity needs to be measured after the printing is completed, and the resistivity is controlled within the allowable technical range, and screen printing is performed again.
(2) Although the antenna can be produced by screen printing, it can be used for mass production. However, it is often used for single-sided printing. When double-sided printing, the alignment of the two printing positions is difficult, mainly because the winding tension is difficult to control. Only some of the better performance of automatic double-sided screen printing machine alignment more accurate.
Gravure
Gravure printing plate is resistant to high printing force, up to 500,000 printing, and the printing materials are extensive, such as thin paper, cardboard, plastic film, etc., ink viscosity is about 10 ~ 50Pa · s, ink film thickness is 8 ~ 12μm, resolution With the plate-making accuracy, such as the use of laser engraving up to 1000l/cm.
The advantages of the gravure printed RFID antenna are: the pattern is accurate, the thickness is uniform, and the printed pattern can meet the requirements of conductive properties. However, attention should be paid to the drying time of the conductive ink. The drying time of the conductive ink after printing has a certain influence on the quality of the antenna. Too long drying time is not conducive to the stability of the antenna size, and the gravure ink layer is thick, so during the printing process, To minimize
The drying time of the ink layer.
3. Flexo printing
Flexographic printing speed and high efficiency, ink viscosity of 0.01 ~ 0.1Pa · s. But flexographic resolution is low, usually about 60l/cm fine printing up to 80l/cm, ink film thickness 6 ~ 8um, suitable for antenna printing.
With the development of technology, the printing accuracy of flexographic printing has also increased. The disadvantage of its printed RFID antenna is that the imprint appears on the edge of the imprint, which is caused by the printing pressure deforming the plate. The appearance of imprints makes the edge of the antenna line imprinted irregularly, which will affect the resistivity of the line and easily produce waste products.
4. Offset printing
The use of offset printing for RFID antenna printing has the advantages of high efficiency, high precision, and high resolution, but the offset ink film thickness is small and the impedance of the antenna is large. This problem can be solved by multiple printing. However, this has caused new difficulties for overprinting control of fine lines, so it is required that conductive offset inks have better conductive properties, and the number of repeated printings should be minimized.
(B) RFID antenna printing requirements on the performance of conductive ink
Different printing methods have different characteristics, and these characteristics also impose requirements on the performance of the conductive ink.
1. Conductive performance requirements
One of the important parameters of the antenna is the resistivity. The conductive component in the conductive ink is an important factor that affects the resistivity. A method to improve the conductive properties of the following three: increasing the content of the conductive component in the conductive ink; conductive component selected depending on the kind of application area; particle morphology control of the conductive component, particle size and particle size distribution. However, the price of conductive components is relatively high. Antenna manufacturers should reduce the content of conductive components in the conductive ink as much as possible to meet the antenna resistivity and size requirements to reduce costs.