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Membrane keyboards: a general description

Membrane keyboards have for a long time been commonly used as in all kinds of electronic devices. A number of technical, operational and aesthetic properties that standard keyboards fail to offer are conducive to an extensive application of membrane ones. Importantly, a relatively low price and a wide range of conditions in which they can be used, such as high humidity or high temperatures, make membrane keyboards particularly valuable.

The principles of the operation of membrane keyboards are similar to those of standard keyboards: by pressing a key, the film layer comes into contact with the electric circuit layer, thus causing a galvanic short circuit under the key.

Membrane keyboards consist of a number of film layers. The upper layer is called a front panel, whilst electric circuit paths are called membrane switches.

The screen printing process is the basic technology used in the production of membrane keyboards. Front panels are printed on polyester or polycarbonate films, and membrane switches on polyester films. Only the underside of the front panel is printed over, which prevents the varnish from rubbing off during the operation of the keyboard. Circuit layers are printed with special varnish that conducts electricity.

The most important characteristics of membrane keyboards include:
  • small thickness that ranges between 0.8 and 1.5 mm;
  • relatively low price that results from a cost-effective and flexible technology;
  • high resistance to climatic, chemical and mechanical conditions. These properties of membrane keyboards are especially useful in the production of waterproof and dustproof devices, resistant to a number of chemical and organic substances;
  • beauty - each device constructed with membrane keyboards can be individually designed;
  • long lifetime:
    1. life expectancy of flat, tactilely "dumb" keyboards, is 3×107 switches;
    2. life expectancy of "clicking" keys, either embossed or with metal domes, is 5×106 switches.

Types of membrane keyboards:

Flat membrane keyboards

Flat membrane keyboards – not tactile (when a key is pressed, its operation is not perceptible to the touch). Their life expectancy is the longest. Flat membrane keyboards are most reliable.

Membrane keyboards with embossed keys

Membrane keyboards with embossed keys - tactile (when a key is pressed, its operation is perceptible to the touch). Keys are produced with dome embossing, frame embossing or "pillow" embossing. Only dome-embossed keys produce the effect of "clicking" when pressed.

Membrane keyboards with metal domes

Membrane keyboards with metal domes- as a metal dome is used inside the keyboard, the effect of "clicking" is produced when a key is pressed.

Membrane keyboards with 3D keys

Membrane keyboards with 3D keys – the convexity of keys in such keyboards could not be achieved in keyboards with embossed keys. Special optical effects of 3D keys are an additional advantage of such keyboards.

Features of different keys may be combined. Membrane keyboards with embossed keys can, for instance, be combined with metal domes, or membrane keyboards with 3D keys with metal domes.

A number of additional solutions may be used in membrane keyboards:

1.    Display windows (LED, LCD or others) that are integrated into the front panel of the keyboard. As the membrane of the front panel remains intact, it successfully protects the internal part of the device from humidity or pollution. Displays can be translucent or can be covered with coloured filters. So-called laminated windows are used to provide additional protection of the display from mechanical hazards. A plate made from polycarbonate or other transparent material is mounted on the underside of the display.

2.    SMD LEDs, one- or two-colour, mounted inside a membrane keyboard.

3.    The illumination of keys or other graphic elements with EL lamps printed on polyester films and placed under the front panel.

4.    Electromagnetic screens that protect the device from electromagnetic interference. An electromagnetic screen constitutes an additional electronically conductive layer that is connected to, for instance, the tail.

5.    "Pockets" for removable labels with key descriptions that can be modified or updated at any moment during the operation of the device.

6.    High-voltage membrane switches made from elastic copper laminates.

So-called laminated windows are used to provide additional protection of the display from mechanical hazards: