Introduction of the hottest film switch materials

2022-08-22
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Introduction to film switch materials and silk printing technology

film switch is a kind of contact switch made of printed contact circuit with conductive paste on flexible polycarbonate or polyester film

membrane switch has the advantages of flat overall, thin body, small mass, good dust resistance and excellent water resistance. It is a semi closed structure. In addition, there is a large degree of freedom in panel design and color selection. Its application scope is expanding to desktop computers, copiers, fax machines, toys, square computers, word processors and other fields

the manufacturing process of membrane switch is generally:

electrode plate treatment → making silk screen printing plate → printing silver paste → drying → printing carbon paste → drying → stamping processing → printing panel → assembly

the following is a brief introduction of membrane switch materials and printing process

1. Substrate

there are several substrates suitable for making membrane switch panels:

(1) polycarbonate film

the use temperature range of polycarbonate film is very large (-135 ~ 135 ℃). When the temperature or humidity changes, the performance of this material changes little, which ensures that the panel can be used in various instruments 1 in various harsh environments

(2) PVC film

has poor temperature resistance, especially under high temperature conditions, its softening temperature is 59 ℃, and its elasticity is not as good as polycarbonate film. However, it has a high elongation (130%), and because of its low price, it is suitable for VOC use in the whole process of design and manufacturing under general conditions

(3) polyester film

has good elasticity and temperature resistance, and the price is between polycarbonate film and PVC film. However, because the surface of the film is difficult to be processed into matte texture, it is mostly transparent sheet. At the same time, the surface of the polyester film can not obtain satisfactory printing effect for most plastic inks, so special inks are generally required

(4) polyethylene film

polyethylene film has good physical and chemical properties, and can withstand a variety of chemicals, such as alcohols, aliphatic hydrocarbons, aromatics, esters, ketones and alcohol ethers. But the price of polyethylene film is higher

2. Printing process

(1) production of silk screen plate

① wire selection: generally, 180 ~ 240 mesh wires are selected for the printing panel. When the characters and key frames of the printing panel have contour requirements, slightly higher mesh wires should be selected, but it is better not to exceed 300 ~ 500 mesh

for printing silver paste contacts, 180 ~ 240 mesh wires are generally selected. Conductive silver paste is made of metal silver paste with good conductivity. The size and shape of metal silver particles and the content of solid components of metal silver are different. Metallic silver particles are the main factor determining the resistance of switch circuits. When printing conductor circuits, try to make the micro cone of conductor silver smoothly pass through the wire, so the mesh of the wire should not be too small. Although the lines printed by too thin wire are straight and can print circuit graphics with good appearance, in the microscopic state, it is only a layer of loose structure, and the solid content of metal silver particles is very small. This is because most of the silver particles fail to pass through the wire. Such graphics not only have large electrical resistance, but also affect the conductivity and service life of the circuit, which cannot meet the practical application

② plate making method. Direct method, indirect method and direct method can be used. Most printing panels adopt legal edition; Printing contacts and circuits can adopt No. 3 photosensitive film plate making by indirect method. During plate making, the exposure should be sufficient to reduce the loss of film and improve the edge sharpness. The film layer is thick, while the printing ink layer is thick. A thick conductive ink layer is a prerequisite for improving conductivity and reducing resistance

(2) printing

① panel printing. Panel printing is similar to general plastic film printing, but it is printed on the back of the panel, that is, on the non touch surface of the panel, so as to ensure that the graphics and texts on the film switch panel are permanently clear and free from wear. There are many kinds of plastic inks, but the technical requirements and performance standards of Jinan gold assay torsion tester are different. The drying schedule of ink is about 0.5 hours, and it takes more than 5 hours to dry. As the membrane switch panel needs to undergo multiple chromatic printing, attention should be paid to the printing interval of various colors of ink. In order to ensure the printability of the ink, the printing process is best carried out under the condition of constant temperature and humidity

in printing, due to its unique plasticity and elastic quality, the principle of dark color first and light color second should be followed. For large panel background colors or color patches, especially light color patches, it is better to print twice, so that the ink layer is thickened, the ink color is full, and the ink coverage is improved. But avoid color overlap and shadow

after all overprinting, it is also necessary to print a layer of back color on the panel except the transparent window hole, which is usually called the back cover. The back cover layer can prevent the abrasion of the ink layer, protect the graphics and colors of the panel, and also comprehensively clean the printing ink on the back to improve the covering ability, so as to prevent the circuit on the back of the panel from being exposed. It is better to use silver ink for the back cover. On the one hand, it has the characteristics of strong hiding power. On the other hand, it will not change the hue of the ink and can set off the front ink more brightly

transparent windows on the panel usually need to be printed with transparent colors in order to enhance the indication or display effect and mask the outline of components behind the transparent windows. Special transparent color ink shall be used for printing colored transparent windows, and the color of the windows shall be the same as that of the light-emitting elements, so that the clarity of the light-emitting elements can be improved due to wavelength matching. For printing transparent windows, thin wires and thin plate films should be used, and hard scrapers should be used to make the ink layer thin and uniform

in addition, the panel of the membrane switch needs to be repeatedly overprinted, and it also needs to be assembled into a whole with the circuit and isolation layer, so the calibration benchmark of the panel is particularly important. When printing the panel, we should pay full attention to the unity of the overprint benchmark, and the selection of this benchmark should pay special attention to making the design benchmark of the machining drawing completely consistent with the process benchmark, measurement benchmark and assembly registration, so as to ensure the accuracy of the membrane switch

② switch circuit print. The most commonly used printing material for printing switch circuits is silver paste, which is also called silver paste because it is a sticky paste. Processing electronic components with silver paste through silk printing has a long history, but most of them belong to high-temperature sintering prototype. The silver paste used in the membrane switch circuit must be low-temperature curing silver paste due to the limitation of the temperature resistance of the substrate. At present, there are many kinds of silver paste for printing film switch circuits, and the performance and quality are also very different. The performance of silver paste should be evaluated from resistivity, solid content, particle size of metal particles, bonding strength, curing temperature and other aspects, and then selected. Because the silver paste is expensive, and its quality is related to the conductivity of the film. The clamping part at the end of the sample and the shape switch of the chuck are designed respectively, so it should be selected carefully

when scraping, the principle of re scraping and light scraping should be followed. This scraping method is adopted because the viscosity of silver paste is thicker than that of ordinary ink, and it also contains metal solids. The so-called re scraping means that there is a certain squeeze on the silver paste when re inking, forcing it to fill the inked part on the plate film; Scraping is actually the process of cleaning the silver paste and cutting off the printing material. The fullness of the figure has been determined by "rewinding". Excessive printing force will deform the figure

3. Drying

the existence of various solvents and additives in conductive silver paste has a certain impact on the resistivity. In order to release them from the conductive circuit, it mainly depends on hot air drying to volatilize them. Generally, the drying temperature should be controlled at 85-90 ℃ and kept constant for 40 minutes under hot air. It has been proved by practice that the resistance value of the conductive figure with incomplete drying is often 10 times higher than that of the figure with complete drying

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