Membrane keypads

These are typically built on a polyester base membrane (PETP, 35 pm, copper-laminated) with a polyester spacer membrane, safety chamber and front membrane. The snap disc being gold plated stainless steel. Other designs use electrical contacts made from silver, silver on carbon or carbon only. Keypads may also be made from conductive silicone rubber. Conventional computer keyboards made from ABS, polyester or other polymers, may be covered by a polymer overlay to protect it against dirt, dust, water or other substance present in a hostile environment. Polyester and PVC overlays are used in a wide variety of applications with computer and instrumentation equipment in dental and medical healthcare and other areas. Typical casing materials include PC and polychloroprene. 

Membrane keypads face significant competition from new technologies now being developed. For example, research at the University of Pennsylvania and Yamanashi University, by Hohnholz, Okuzaki and MacDiarmid [5], involves the fabrication of a polymer-dispersed fiquid crystal display using electrodes of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) in conjunction with a push-button keyboard array. The display is produced by the design and printing of a line pattern on a substrate which is then coated with a conductive polymer solution. Subsequently the reverse-printed toner pattern is removed. 

In another development researchers at Fujitsu Laboratories and Fujitsu Component have succeeded in developing conductive polymer touch panels where the manufacturing process involves the layering of a low resistance conductive polymer on to a plastic film. Proposed applications include touch panels for computers and mobile phones. 

Extreme operating conditions call for heavy duty materials, a case in point being the membrane switch material required for the ocean going yacht made by Tiara for which Autotype s Autotex XE (Extreme Environment) hard coated polyester film was used. Unlike traditional substrate materials Autotex XE can withstand high light levels, 85 °C operating temperatures and humidity levels of 85% relative humidity. Conventional films might delaminate, become brittle or flake under extreme conditions. Autotex XE film is polyester-based with specially constructed hardcoat and primer layers. 

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The system can be operated from stand-alone control via an LCD and a membrane keypad screen on the instrument. Seven pre-programmed protocols are provided and a maximum of 36 sequences containing 101 amino acids in length in each of the eight user profiles is allowed. Alternatively, for more flexibility, full protocol editing, chemical editing, and database capability, the system can be controlled from a computer workstation with icon-driven easy-to-use Windows 95-based software. 

The source of the new technology is MacDermid Autotype, where the antimicrobial protection is built into the hard-coat layer, which enables the protection to be built into traditional flat panel displays, membrane keypads, touch screens and fascia panels. The manufacturing process follows traditional lines and comprises a flexible polyester base, primed on the underside with an ink adhesion layer, for high definition screen and ink-jet printing, and coated on the outside with an exceptionally tough hard-coat layer. 

There are several different constrnctions for keypads on flexible circuits. Thick-film circuits on polyester film called membrane switch could be a low-cost solution with many millions of reliable contacts. The basic mechanism of the switching is illustrated in Fig. 64.25. Generally, the membrane switches are connected through FFC connectors to the main circuit board. Polyimide-based circuits are necessary with soldering of components. 

Fig. 3.12. Microcomputer keypad made from a single moulding of a liquid silicone rubber, with its underlying membrane switching circuit. (Sinclair Research, Haffenden and NFI Electronics.)...

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