Published in Elektronik Industrie 08/2020
Article as .pdf (German) >>
Membrane switches still have their justification
In the food, laboratory and medical sectors, the membrane keypad continues to play an important role at present due to its closed surface, resistance to chemicals and impermeability to liquids and dust. N&H Technology from Willich has been manufacturing membrane keyboards for all industries for years.
Membrane keypads are the preferred choice especially in devices with a small number of functions, which at the same time require good visibility and are in constant user interaction. In the medical field, these include nurse emergency call systems, hospital bed controls, input keypads for diagnostic equipment and defibrillators, and other patient care interfaces. Many of these input devices are also operated by inexperienced users such as elderly patients, so that a complex touch input solution is rather disadvantageous. Membrane keyboards can serve with a user-friendly and intuitive interface due to the wide range of design options. Through the use of tools, the membrane keypad can take on almost any shape. In addition to the common screen printing process, digital printing is particularly suitable for the realization of color gradients and for prototypes and small series. Specially embossed foils can even be used to achieve metal look effects, for example brushed stainless steel or anodized aluminum. Another advantage over display solutions is that the printed surface and optionally integrated LED status displays can also be seen from a 180° rotated viewing angle.
The flat structure of a membrane keypad consists of a foil composite made up of at least four foils: the decorative foil, the top and bottom switching foils and a spacer foil. The decorative foil, as the top layer of the keyboard, is made of polyester or polycarbonate and is printed on the reverse side to ensure the highest possible protection against abrasion. Compared with other flexible materials, polyester in particular impresses with a high life cycle of over one million switching cycles, very high abrasion resistance and very good resistance to chemicals. For applications in the medical sector and in areas with high hygienic requirements, the front film can be equipped with an antibacterial coating. This antimicrobial polyester kills or inhibits bacterial growth and provides constant protection against bacterial contamination.
The functional elements of the membrane keypad are the lower circuit foil with the conductor tracks, LEDs, switching points and the connection lug located on it, and the upper circuit foil with the contact surfaces. The spacer foil acts as an insulator separating the upper and lower switching foils or, in the case of a design with metal snap-on washers, as a fixing foil.
Illuminated membrane switches
Depending on the requirements of the application, the options for backlighting vary. Especially in the often darkened work environments in the medical field, illuminated keypads are necessary to easily find important key functions. LEDs can be integrated very easily into the existing switching foil or, if required, an additional LED switching foil can be added. Special adhesives and sealing materials are used to ensure the function of the LED even in harsh environments with, for example, strong vibrations. The backlighting of a membrane keypad can be achieved by using LGF (Light Guide Film). In this case, the typical flat design and flexibility are retained. With LGF technology, LEDs are inserted at the edge into a very thin (0.1 to 0.2 mm), highly refractive film. Due to the high light refraction, very few LEDs are required for homogeneous illumination, depending on the size of the membrane keypad. The backlighting of each key can be in different colors.
In contrast to EL backlighting, LGF technology does not require a high-frequency AC power source. A special feature is lettering with a disappearing effect. This is labeling that is not visible when the keypad is not illuminated and is only visible when the keypad is illuminated. This allows a clear on/off mode to be signaled.
Another advantage of the membrane keypad is the tactile feedback of the key function to the user. Snap domes provide a noticeable tactile feel when pressed and, depending on the design, an audible clicking sound. This direct feedback can reduce file entry errors. Metal snap domes come in a variety of sizes, shapes and actuation forces and are preferred for applications with a high number of actuations.
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