Membrane Keyboards construction
The graphic layer (typically 0.125mm – 0.180mm thick) has printing inks on the reverse to prevent keypad wear.
There are options of fine and coarse surface texture with clear (unstructured finish) in particular areas to either enhance the look or provide a window through the material for Displays or changeable insert legends.
Specific areas on the graphic layer can also be embossed to help the user to locate the switch quickly. The embossing styles can be rim, pillow, or dome with braille if required.
The upper circuit is typically 0.050mm – 0.130mm thick material and acts as a seal for the circuit and can also have a shield printed on the top surface in conductive inks to act as an RFI shield. This can be connected to the tail, or have tabs to enable a conductive connection to the customer’s assembly.
An adhesive spacer is used to separate the upper and lower circuit or as a means of restraining the metal domes in place.
The lower circuit is made from either Polyester which is surface printed with conductive ink, a flexible Polyamide FPC plated and chemically etched double-layer circuit with gold plated connections or a multi-layer PCB. All types of the lower circuit can have LEDs strategically placed to provide a lighting effect. FPC / PCB circuits have the added advantage of being able to have components soldered to either side such as ribbon cable connectors etc.
Finally, the assembly then has a 3M adhesive specifically chosen to suit the customers housing material.
Developments such as the introduction of UV resilient and anti-microbial coated graphic layers and also thin-film light conductors known as “LGF” (Light Guide Film) can be integrated within the circuit. Rather than simple spot illumination, it is now possible to light whole areas of the overlay such as the switch area or even customers logo, to enhance the look of the device whilst still providing a thin membrane assembly.