Rubber dome

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Membrane rubber domes from the AppleDesign Keyboard

Rubber dome is a type of keyboard switching technology that is most commonly used on modern keyboards, using collapsible rubber domes to provide resistance and tactility to keys.

Description

In rubber dome keyboards, the collapsible rubber domes replace the helical springs found in other switch types. In some cases, metal springs are still used, including the Fujitsu Peerless switch and one Alps integrated dome variant. The domes themselves can be discrete objects, or collectively manufactured as a single sheet of rubber. When discrete, they may be glued down onto the top membrane sheet. A single keyboard may use multiple smaller sheets of domes.

Most rubber dome keyboards use a membrane contact sheet below the dome to provide electrical switching, but certain rubber dome switches, such as Topre switches, use capacitive switching, and a small number of boards used a PCB.

Varieties

Topre switch with the slider visible in black

Plain rubber dome

A stem is moulded into the keycap that pushes down on the dome; this is the most common variety

Domes with sliders

Similar to a microswitch mechanical, each switch contains its own slider that presses on the dome, onto which the keycap is mounted; on some keyboards the slider has a standard Cherry MX mount or Alps mount interface and the board will accept keycaps from mechanical keyboards and donate keycaps for mechanical boards. Dome switches with sliders are now uncommon, but this is the form taken by Topre switches

Scissors

Laptop keyboards introduced the use of scissor-like mechanisms to stabilise low travel switches that lack a shaft for the keycap to slide inside

Contact mechanism

Dome switch keyboards use several contact mechanisms:

Pressure

The "ceiling" of the dome presses two plastic membranes together to connect circuit traces; such keyboards require three membrane layers: upper and lower with exposed conductive traces, and a separator sheet to keep the conductor membranes separated until pairs of contacts are pressed together.

Conductive

The ceiling of the dome is coated with carbon, and this conductive layer connects a pair of circuit traces together; the circuit layer can be a membrane, PCB, or (as with the Alps integrated dome switch), metal contacts.

Capacitive

Topre Realforce keyboards use the rubber dome solely as a spring, and switch actuation is performed using capacitance via a 5 cN spring compressed by the dome.

Gallery

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  Dome and membrane sheets from a Dell L100 office keyboard

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  Pressure domes from the AppleDesign Keyboard, glued down onto the top membrane sheet

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  Loose domes

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  Contiguous sheet of domes

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  Scissor switch with its rubber dome

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  Conductive domes

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  Conductive dome turned inside out to demonstrate the conductive pad

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  Metal contacts in the Alps integrated dome discrete conductive dome switch

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  Conductive dome PCB, from a Key Mouse RB-2001-1 keyboard