Graphic Overlay, also called Panel, is an important part of membrane switch, which can provide control function and beautiful modern appearance. It is made by printing graphics and text description on such materials as PVC, PC and PET. It is composed of multi-layer polyester and adhesive, which can be directly connected with electronic equipment through flexible circuit board connector.
From material to material
The main materials suitable for making membrane panel are: PVC (polyvinyl chloride), PC (polycarbonate) and PET (polyester), with the thickness of 0.125 \ 0.175 \ 0.25, and three surfaces of sand, matte and smooth.
The physical and chemical properties of PC material are good, and it can adapt to many kinds of inks. It is the most common material for membrane switch panel. PC materials can be divided into sand surface and smooth surface. The reason for choosing sand surface materials is that the use of film switches is mostly electronic products, and the panel used as the control system does not want to be interfered by light, while the sand surface is only diffuse reflection, which will not produce bright reflection; at the same time, because the surface is disordered sand like, it has the function of masking scratches. In contrast, smooth materials do not have the above characteristics. But as a smooth material, if combined with the back printing, the color will appear more brilliant. In the case of strong decorative requirements and not frequently touched, the panel is often made of smooth materials, or due to the special needs of some display areas such as LCD, it is more favorable to choose smooth materials.
The price of PVC material is lower, about half of PC material. When producing popular civil products, PVC material can reduce production cost. At the same time, PVC material has good ductility, and can be used to cold press three-dimensional graphics. The thickness of smooth PVC plate is generally more than 0.5mm, and both sides are protected by PE or glue paper, most of which are used to make decorative panel; the thickness of PVC material on sand surface is generally less than 0.3mm, which is generally supplied by fixed length sheet, without coil.
The surface state of PET film is divided into smooth surface and fine sand. At present, the brands of British Autotex and Japan Toray are common in China. In addition, Autotex’s PH scratch resistant PET film is suitable for products with high requirements for scratch resistance, but the price is relatively expensive. Because the general properties of PET film can be replaced by PC film, there are not many PET panels, unless in some special occasions, such as insulation, resilience, solvent resistance and other conditions that other materials can not meet, PET will be used.
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The panel layer is generally made of fine patterns and characters on the colorless transparent sheet such as pet and PC less than 0.25mm. Because the main function of the panel layer is to mark and press keys, the selected materials must have the characteristics of high transparency, high ink adhesion, high elasticity and folding resistance.
The main function of the cushion layer is to closely connect the panel layer and the circuit layer, so as to achieve the effect of sealing and connection. The thickness of this layer is generally required to be between 0.02-0.05mm, with high-strength viscosity and anti-aging; in production, the special membrane switch double-sided adhesive is generally selected, and some are required to be waterproof and high-temperature resistant, so the cushion layer must also use materials of different characteristics as per requirements.
This layer adopts polyester film (PET) with good performance as the carrier of switch circuit figure, and uses silver paste carbon paste or gold paste on it to make it have conductivity. Its thickness is generally within 0.05-0.175mm, and the most common is 0.125mm PET material.
It is between the upper circuit and the lower circuit layer and plays the role of sealing and connection. Generally, PET double-sided adhesive is used, with the thickness ranging from 0.05 to 0.2 mm. When selecting the material of this layer, the overall thickness, insulation, touch and sealing of the circuit key package of the product shall be fully considered.
The use of back glue is closely related to what kind of material the membrane switch is pasted with. The common double-sided adhesive, 3M adhesive, waterproof adhesive, etc. are often used.