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Rubber domes

Contents

Overview

The vast majority of keyboards manufactured today use rubber domes. These are domes moulded out of silicone rubber, either discretely or (more typically) in form of a single rubber sheet with a dome at each key position. Each dome serves as the return spring, as well as the device which transfers the keystroke force from the operator’s finger onto the membrane assembly.

Rubber domes have the advantage of a rounded tactile peak, compared to the often sharp and jarring peak of many mechanical designs. This affords rubber dome keyboards a softer and more comfortable tactile feel than that of Cherry or Alps mechanical keyboards. The chief disadvantage however is that the behaviour of silicone rubber changes with age, which limits the lifetime of the keyboard to the amount of time until it becomes too difficult to type on. Typically, the rubber domes stiffen with age, requiring ever increasing effort to press keys. More unusually, the rubber can become limp, to the point of creating sensory deprivation from how much it absorbs the interaction force from your fingers.

Terminology

Rubber dome
A single dome formed from silicone rubber, that acts as a return spring and actuator; this can be a discrete part, or formed as part of a continuous sheet.
Rubber sheet
A sheet of silicone rubber, with a dome-shaped moulding at each key position; this term is used by Cherry in their RS line of keyboards (e.g. RS 3000, also known as G83-3000)
Buckling rubber sleeve
This is a community term for a rubber dome that sits around the plunger; it is not a complete dome, but it operates the same. Buckling rubber sleeves can be found in an upside down orientation, making them closer to a dish than a dome. The term “sleeve” is something of a misnomer considering the conical or hemispherical (rather than cylindrical) form and the lack of contact between the “sleeve” and plunger, but the term has stuck.

Design

The exact feel of the keys depends in part on the profile of the domes. In Electronic Engineers Master 1985–86 Volume 2 (page B·1734), Moxness illustrated five different profiles of rubber key, along with the force curve that each one would produce:

Implementation

Rubber domes can be used in both discrete and non-discrete switch designs. The vast majority of rubber dome keyboards are non-discrete: the rubber domes or rubber sheet is paired with a single-piece moulded housing with guide shafts for the plungers or keycaps, or—with scissor switch keyboards—glued to the top membrane. In modern keyboards, rubber domes are used to apply operating pressure in membrane keyboards, but in some older keyboards, such as various BTC models, the inside of the dome bears a conductive material that bridges exposed areas on a PCB.

At least two manufacturers produced fully self-contained switches, that could be soldered to a PCB, with a conductive rubber dome inside. Alps Alpine (formerly Alps Electric) have a long history of so-called elastic contact switches formed in this manner, including at least five series of keyboard switches (KED, KEH, SKEW, SKPA and an unidentified DIN-compliant PCB-mounted series). SMK produced a keyboard switch of the same design, that was sold rebranded as Maxi-Switch.

History

Although rubber domes are most commonly associated with keyboards made in the 1990s onwards, the idea of using a conical rubber form as a return spring is not a new idea. US patent 3767022 “Return spring key stem boot” filed in April 1970 by Singer specifically refers to its “breakaway feel” and shows the force curve that results from its use. Singer’s design is what would be considered a “buckling rubber sleeve” in that it fits around the plunger, with a hole in the centre for the keystem. Such a keyboard has yet to be seen.

Controls Research Corp filed US patent 3965399 “Pushbutton capacitive transducer” for a capacitive switch design, and this too features a conical rubber form inside the switch. Although this patent includes the term “tactile feel”, the age of the patent is such that it cannot be automatically inferred as to what that term is being used to mean. This design is also yet to be observed.

Further details will emerge with ongoing research.