To produce the best possible unit, it is essential that the options and limitations of the differing materials and processes are fully understood. IGT Industries' design and applications engineers are always available to give advice on specific instances. In order to achieve the optimum design for the application, it is strongly recommended that IGT are involved as early in the project as possible, ideally at the concept stage.

RFI screening in a membrane keyboard can be achieved in five ways.

• Printing silver conductive ground layers between the keyboard's tracking layers.
• Incorporating an indium/tin oxide (ITO) layer over windows. ITO is both conductive and transparent.
• RFI protection over display/clear areas by incorporating blackened copper or blackened stainless steel mesh windows.
• Providing a ground plane layer throughout the keyboard to continue earth continuity into the rest of the equipment.
• In a PCB membrane, tracking the signal lines on the inner layers of a multilayer PCB to reduce susceptibility to electrical noise.

Textured, hard coat polyester is the best material. It is mechanically strong and dimensionally stable, resistant to chemicals and accidental damage. It is thermally stable, operating over a wide ambient temperature range. It has excellent light transmission properties, enabling subtle colours and special effects to be used to create attractive and sophisticated graphic effects. Graphic overlays can be finished with gloss, matt or textured UV-stable finishes to cater for individual needs and graphic requirements.

Almost any colour can be printed onto the overlay, either as a solid or a half-tone, giving the graphic designer complete freedom in creating attractive graphic effects. Colours can be matched to company's custom colours or to any standard colour matching library. UV curing inks are solvent free, minimising the environmental impact during the manufacturing process.

A large range of adhesives are available. The choice of adhesive depends very much on the particular application; IGT's experienced design engineers are able to advise on the best material for the application.

High gloss or anti-glare lacquers are printed onto the overlay's surface to achieve highly transparent windows with wide viewing angles. Windows may be either clear or coloured, and since there is no physical aperture in the overlay, the environmental sealing of the overlay is unbroken. RFI screening can also be incorporated if required.

Tactile feedback can be achieved in three ways: by thermally embossing a dome into the graphic overlay layer, by creating a dome into one of the internal polyester layers or by inserting a metal dome below the graphics layer.

Both ridge embossing, where only the periphery of the area is raised, and full key embossing, where the entire key area is raised, can be combined with any of the three methods of providing tactile feedback.