Research on Screen Printing Process Parameters of Smart Label Antenna
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First, Introduction
At present, the RFID tag antenna printed by conductive ink is mainly designed and manufactured by screen or gravure printing. The RFID tag antenna has higher and higher requirements on its conductivity, and it is often required that the conductive ink contains a high content of metal particles. In the case of relatively high solid-liquid ratio, the screen printing has a high-quality antenna line with uniform density, smooth surface and high resolution. However, it is rare to study the electrical properties of the label antenna for the printing process parameters. A problem faced by companies that produce tag antennas. In order to determine the key parameters for process improvement, it is necessary to carefully analyze the relationship between process and performance in the process of conductive ink printing. In this paper, the label antenna is printed by silver-containing conductive ink and screen printing, and the influence of the key parameters of conductive ink and screen printing on the process is analyzed.
Second, the choice of conductive ink screen printing method
Screen printing is generally divided into contact type and non-contact type printing. In contact printing, the substrate is directly in contact with the screen. When the squeegee moves on the screen, the screen is not inclined and deformed; non-contact printing, There is a fixed distance between the screen and the substrate. When the scraper pushes the slurry through the screen, the screen is tilted and touched with the substrate to print the pattern. Since the screen is bounced back after printing, the printing will not be printed. The pattern is blurred. In order to obtain good printing quality to avoid smudging, non-contact printing is generally used as a conductive label printing method, and FIG. 1 is a schematic diagram of a process of non-contact printing.
Figure 1 Schematic diagram of the process of non-contact printing
Third, the calculation of the thickness of screen printing ink layer
At present, the thickness of the ink layer of the high-efficiency transponder coil is generally 6 μm or more, and the gap between the coil wires is determined according to the size of the transponder, and the small-sized transponder is a development trend. The screen printing ink layer thickness ranges from 4μm to 70μm, and has a wide adjustable range. It is currently the most suitable technology for printing RFID transponder coils. In screen printing, the control of the thickness of the ink layer plays an important role.
According to the connotation of the amount of ink permeation, the amount of ink permeation can be divided into two types, namely, theoretical ink permeation amount and actual ink permeation amount.
1. Theoretical ink penetration
The theoretical amount of ink permeation refers to the total amount of ink passing through the screen mesh per unit area determined by the technical parameters of the screen. Since the theoretical amount of ink is determined by the technical parameters of the screen, the basic technical parameters of the screen include wire diameter, mesh number and screen thickness. Other parameters can be calculated by these parameters.
Therefore, the theoretical expression of the theoretical amount of ink can be written as follows:
(1)Among them, the theoretical amount of ink permeation, unit cm3/m2, is the number of mesh mesh, unit mesh / cm, is the wire diameter of the wire mesh, the unit is μm, the thickness of the wire mesh, the unit is μm. Figure 2 is a schematic diagram of the technical parameters of the screen.
Figure 2 Schematic diagram of wire mesh technology parameters
It can be seen from the above analysis that the mesh number of the screen is the main factor determining the theoretical ink permeability. If the corresponding relationship between the mesh number and the theoretical ink permeability can be found, the theoretical ink permeability can be basically determined. Approximate range.
The key factor determining the thickness of the screen printing ink layer Thv is the amount of ink G of the screen opening. The amount of ink permeation is proportional to the volume V of the screen mesh. The mesh mesh volume V is mainly determined by the mesh number M, as shown in Figure 3.
Figure 3 Wire mesh technical parameters
The mesh number M refers to the number of meshes per unit length, and the mesh side length L is:
(2)Mesh area
(3)When static, the screen thickness t is approximately:
(4)Therefore, when the screen is static, the amount of ink G of a screen mesh is also the volume V of the screen mesh, and its value is about:
(5)When the ink is transferred to the surface of the printing material, the thickness of the ink layer on the printing material is approximately the amount of ink per channel:
(6)At present, the wire mesh has a minimum wire diameter of 30 μm. In general, the smaller the mesh size M, the larger the wire diameter d of the wire mesh. Therefore, it can be seen from the formula (6) that the theoretically calculated ink layer thickness Thv should decrease as the mesh number of the screen is increased.
2. Actual amount of ink
Since the factors affecting the amount of ink permeation are complicated, in addition to the technical parameters of the screen, there are other factors in the printing which will have a great influence on the amount of ink. Therefore, the printing is usually obtained under certain printing conditions. The amount of ink is called the actual amount of ink. The actual ink layer thickness of screen printing is determined by the combined influence of the structural parameters of the screen and the actual printing conditions. Therefore, the thickness of the screen printed ink layer is a function of the screen technical parameters and the printing processing conditions. which is:
(7)Where: thickness of ink layer printed by Thv-screen; M-mesh mesh; m-screen material; δ-mesh shape; θ-scraper angle; ξ-scraper hardness; τ-ink viscosity; Ink loading; ρ-printing pressure; ν-printing speed; ε-printing environment. As shown in Fig. 4, when the printing pressure is increased, the screen screen is compressed, and the circular wire becomes elliptical.
Figure 4 Schematic diagram of pressure deformation of screen screen
The hardness of the squeegee also affects the amount of ink on the screen. Generally, the hardness of the scraper material should be selected to be about 60 degrees Shore hardness. The greater the squeegee angle of the squeegee on the plate, the less the amount of ink permeable, the smaller the squeegee angle, and the greater the amount of ink permeable. The angle between the squeegee and the printing plate is generally about 70 degrees at the time of flat printing, and the squeegee angle is preferably about 50 degrees when the surface is printed. The moving speed of the squeegee is also the screen printing speed, and the moving speed of the squeegee has a certain influence on the amount of ink transferred and the uniformity of ink transfer. The faster the squeegee moves, the less the amount of ink permeable; conversely, the more ink permeable. Generally speaking, when the mesh screen is tightened, the volume of the mesh is reduced by 15% compared with the static state, and the mesh volume after the deformation of the screen is reduced by about 15% under normal printing conditions. Therefore, the actual ink layer on the screen is reduced. The thickness is reduced by about 25% from the theoretical ink layer thickness.
3. Demonstrate the relationship between the number of screen meshes and the thickness of the ink layer
According to the conclusions of the above analysis, this experiment uses a flat screen printing machine, polyester mesh screen, the hardness of the scraper is 60 degrees Shore, the scraper angle is 650, the moving speed of the scraper is 4 m / min, conductive The viscosity of the ink is 90pa.s. Under the same conditions of other printing and processing conditions, only change the mesh number of the screen, measure the thickness of the ink layer, analyze the relationship between the thickness H of the ink layer and the mesh number n, and find out The theoretical ink permeability corresponding to different mesh mesh numbers (from 10T mesh/inch to 700T mesh/inch) is shown in Table 1. It is found that there is a good correlation between the two, and the relationship curve is drawn, as shown in Figure 5. Show.
Table 1 Printing manufacturing process parameters of RFID tag antenna
Figure 5: The most relevant curve between mesh number and theoretical ink permeability
Analysis of this curve reveals the following rules:
3.1 As the number of meshes increases, the theoretical amount of ink transmitted decreases accordingly.
3.2 When the mesh number is low mesh (10~50T mesh/inch), that is, 1~2 segments of the curve, the theoretical ink permeability decreases sharply with the increase of the mesh number, and the theoretical ink permeability is from 40μm dropped to 20μm, indicating that the mesh size has a great influence on the theoretical ink throughput. In this case, you should be especially careful when selecting the mesh size.
3.3 When the mesh number is medium to low mesh (50T ~ 200T mesh / inch), that is, 2 to 3 segments of the curve, the theoretical ink permeability decreases correspondingly with the increase of the mesh number, and the theoretical ink permeability is from 20μm. Drop to 12μm, indicating that the effect of the mesh size on the theoretical ink throughput has been alleviated.
3.4 When the screen is medium to high mesh (200T ~ 450T mesh / inch), that is, 3 to 4 segments of the curve, as the mesh number increases, the theoretical ink permeability decreases slightly, and the theoretical ink permeability is from 12μm. Drop to 8μm.
3.5 When the mesh number is 450 to 700 T mesh/inch, that is, the curve 4 to 5, the change in the thickness of the ink layer Thv is small, and it is almost unchanged from 8 μm to 7 μm. It shows that the influence of the mesh number on the theoretical ink permeability is more gradual. Therefore, when selecting a screen within this range, try to select a screen with a lower mesh number.
Due to the high uniformity of the RFID transponder coil over the entire wire range, the author used a polyester screen screen to carry out a large number of experimental printing, and found that the coils obtained by screen screen printing below 200 mesh/inch were rough. Can not meet the working requirements of RFID. With a 250 mesh/inch screen screen and high-quality conductive ink, the thickness of the coil ink layer obtained by printing can meet the working requirements of RFID.