Membrane keyboards use one or more flexible plastic sheets as part of the switching arrangement. This page covers full-travel membrane keyboards, rather than flat keyboards.
There is a conflict in terminology between flat membrane keyboards—such as those found on microwave ovens and on some home computers—and membrane keyboards with distinct keycaps. Although there have been occasional complaints about how the moniker of “membrane” should not be used for anything other than flat membrane keyboards, this has not been followed up with a proposal for a more proper term. Such a split would contradict the widespread keyboard industry use of the term “membrane” to refer to keyboards with discrete keycaps over a membrane-based sensing assembly, most notably Oak Full-Travel Membrane. Mitsumi also used the term “membrane contact type” to describe their single-layer membrane actuated using conductive rubber feet. While the use of “membrane” may be imprecise, one cannot argue that it is not how the industry as a whole refers to such keyboards.
In the early days of membrane keyboards, the term “diaphragm” was often used to describe the membrane sheet flexed by the plunger or the operator’s finger. This term fell out of use. Membrane keyboards can also be described as “elastic” (as in Datanetics “elastic diaphragm”) or “elastomeric”, also terms that seem to lose favour with time.
The vast majority of membrane-based keyboards are conductive. Typical membrane keyboards have a three-layer membrane assembly, with circuit tracks printed on the inward-facing surfaces of the top and bottom membrane sheets. Contact pads formed from these tracks conduct current between the layers when a key (or a finger, in flat keyboards) presses the membrane sheets together. A centre spacer layer ensures that only one pair of contact pads touches when a single key is pressed. The circuit tracks are typically screen printed from a carbon–silver ink. Early Datanetics keyboards used gold-plated membrane tracks for reliable switching.
Far more rarely encountered, is the capacitive membrane system. The notion of a capacitive arrangement is curious, because a typical spacer sheet is only around 0.1 mm (around 0.003–0.004″) thick. The difference in capacitance resulting from such a small change in distance seems like it would be difficult to measure accurately, but Micro Switch offered a choice of CT Series contact membrane and SC Series capacitive membrane. Micro Switch illustrated their design in Computer Design magazine in July 1981, and they indicated a spacer layer thickness of 50 thou, or 1.27 mm, at least ten times thicker than the spacer layer of a conductive membrane keyboard. ST Series replaced the snap action spring of CT and SC with a rubber dome to provide a “silent tactile” design, and this series superseded both CT and SC, providing both conductive membrane and capacitive membrane options.
Membrane keyboards use one, two or three sheets of thin, flexible plastic as part of the switching assembly. These sheets are typically referred to as “membranes”, but some manufacturers refer to them as “flexible printed circuits” or “FPCs”. The flexible membranes were historically described by some manufacturers as a “diaphragm”, including IBM and Datanetics.
The purpose of the membrane sheets varies depending on the keyboard. For conductive elastomer keyboards that traditionally bridged pads on a PCB, a membrane sheet replaces the matrix PCB. In the majority of the cases, the membrane assembly is solely responsible for switching. Conductive areas of the membrane sheets connect the membrane to a PCB that supports the keyboard controller, ancillary components and cable header and, in most cases, the status LEDs (a few designs connect the LEDs directly to the membrane without solder, such as in the Fujitsu FKB4700). In modern keyboards, the controller PCB is small and only occupies the LED area of the keyboard, but in some keyboards, such as the Apple Keyboard II, the controller PCB is still quite large.
A full three-layer membrane assembly uses the membranes to both carry the matrix and handle the switching. Circuit pads on the outer layers are brought together through holes in a centre spacer layer when keys are pressed. This is the form used in the majority of keyboards made today.
Switching with PCB
Some designs, including Mitsumi KSD Type and Datanetics elastic diaphragm array put one half of the circuitry onto a printed circuit board. These hybrid keyboards use both the membrane and the PCB tracks to form the switching.
Mitsumi KPQ Type and related designs combine conductive elements with a single flexible printed circuit (FPC). Conductive-element keyboards could also be found with a regular printed circuit board, as seen in the Silitek SK-4100R-1U and BTC 51 series as well as Mitsumi’s own alternatives. Mitsumi generally use conductive rubber feet to bridge the circuit pathways (as is typical with their keyboards), instead of the more conventional rubber dome with conductive pad.
While it could be argued that these are not really membrane keyboards, Mitsumi themselves described KPQ type in their catalogue as “Unique membrane contact type”.
Membrane keyboard patents were filed in earnest in the early 80s, as manufacturers all recognised the cost benefits to moving to membrane keyboards. The New York Times article The Membrane Keyboard from the 5th of March 1981 writes about new new entrants into the market, specifically Oak Switch Systems and Chomerics (with curiously no mention of Micro Switch). At the time of writing, Oak had only received one volume order, while Chomerics had received none. Paul Nelson from DEC is however quoted as saying “We don’t know how long they’re going to last”, indicating their scepticism to adopt the technology without further investigation. The article indicates that membrane keyboards (as in keypads) began appearing on calculators in the early 1970s, but that their application to computer and word processor keyboards was considered new. Although various designs existed prior to 1981 (from IBM and Datanetics in particular), membrane keyboards had not yet become commonplace, with solid state being the keyboard implementation of choice.
Chomerics claimed to have invented membrane switching in the late 1960s, which is the same time that Datanetics invented a complete full-size membrane keyboard. Chomerics would not apply their technology to full-size keyboards for more than ten years afterwards.
The following types are listed in the order that their patents were filed. This list is far from exhaustive.
IBM pressure ball
IBM filed patent US 3382338 “Pushbutton actuator for elastomeric switch” in April 1966. This is a full-travel design, using a continuous neoprene mat as the return spring. The lower tracks and pads are shown to be on a PCB, while the upper tracks and pads are on a membrane sheet, with a spacer sheet in between. The membrane sheet is described as a “diaphragm”, likely due to how it is deflected under pressure into the spaces below. The patent describes how so-called elastomeric keyboards—already in existence by this time—are limited by the lack of operator feedback from the minuscule amount of travel afforded, and thus goes on to describe an adaptation of this concept to provide full travel.
Pressure is applied to the membrane–PCB arrangement by a small metallic or ceramic ball. The thick mat under the plungers consumes switch travel until the pressure within this mat is sufficient to start transferring force onto the diaphragm via the ball, which focuses the pressure.
Datanetics elastic diaphragm array
Datanetics filed US patent 3594684 “Electrical interconnection system for multilayer circuitry” patent for their batch-fabricated elastic diaphragm array keyboards in May 1969. Just as with IBM’s design, the bottom layer is a PCB, but Datanetics originally opted for a five-layer membrane arrangement to provide sequential switching; this was later simplified down to three laters (flexible printed circuit, spacer and standard PCB). All of the PCB and membrane contact surfaces are gold-plated for reliable connectivity. As with IBM’s patent, Datanetics referred to the flexible printed circuit substrates as “diaphragms”; the spacer sheets were described as “dielectric” sheets or separators.
IBM 1970 designs
Ribbon-controlled membrane hammer
US patent 3662138 “Actuator for momentary closure of an elastic diaphragm switch” was filed in March 1970 by Richard Hunter Harris and George J Laurer. Richard Hunter Harris would soon be involved with the beam spring design, and later revisit membrane keyboards with his second buckling spring design. In this particular patent, the membrane is struck by a hammer moulded from a single piece of plastic, with a solid head on the end of a flexible arm. A “ribbon” is pushed forwards by the plunger, and it lifts and releases the hammer. The membrane system is not detailed, but the dimensions of the drawing suggest a more modern three-layer membrane.
Also in 1970, IBM filed US patent 3676615 “Pushbutton keyboard switch array and associated printed circuit logic cards” in July. This is a fully self-encoding system, using multiple contacts wired separately, with each contact being assigned a position within a four-bit number. The stationary contacts are held on a PCB, while the movable contact pads on the membrane use 0.0005″ to 0.001″ of conductive material (such as Berylco 25, a beryllium copper spring alloy) plated with 0.00015″ of gold.
Oak Industries Full Travel Membrane
US patent 4420744A filed in February 1981 covers achieving N-key rollover with a membrane keyboard. Their idea of N-key rollover is misleading, however. They do not offer true N-key rollover; rather, they describe a means to allow any number of simultaneous keypresses excluding phantom conditions: the controller will output every key pressed except for any set of keys that are involved in a ghost (or “phantom”) situation. The scan rate should be sufficient for all keys to be registered so long as they are pressed and released in sequence (as with fast typing) where the previously-blocked keys will get detected as the ghost situation clears. However, this is not the same as proper N-key rollover where any rollover sequence is valid including those that would trigger phantom keys on a lesser keyboard.
Chomerics were a contemporary of Oak. Presently, little is known about their keyboard products as thus far none are known to have been observed outside of advertisements.
It is not presently clear whether Oak or Micro Switch was first to market with their respective membrane systems; both were introduced in 1980 or 1981. An article in Computer Design in July 1981 covers Micro Switch’s capacitive membrane technology, as used in SC and ST series. It seems that Micro Switch put out both capacitive and conductive membrane keyboards around the same time, both with snap action over-centre actuator modules that offered hysteresis and tactile and audible feedback. Whether alternate action was ever implemented, remains to be discovered, while Oak included alternate action in their original patent.
Digital Equipment Corporation
DEC filed US patent 4467150 in February 1982 for a three-layer membrane system. This was used in the well-known LK201 keyboard. These membranes are actuated by leaf springs in a manner similar to the contemporary Fujitsu design.
The term “membrane” is used, and the conductive material is given as “conductive silver (carbon)”.
Fujitsu membrane leaf spring
This patent covers the two types of membrane leaf spring: integrated membrane and non-discrete. Non-discrete membrane leaf spring is known from the Fujitsu FMR-30BX keyboard. The integrated membrane type may not yet have been sighted. Unlike Datanetics DC-50, which uses metal switch contacts glued to the membranes, Fujitsu integrated membrane uses a completely standard three-layer membrane system one key position in size, embedded inside a switch module. It is possible that this never entered production, with the membrane assembly being replaced with the contact assembly from the standard leaf spring switches.
The patent specifically uses the term “membrane” and describes methods of reducing the full 4 mm keystroke into the 0.1 or 0.125 mm of travel of the membrane system itself.
IBM membrane buckling spring
IBM filed US patent 4528431 for their membrane buckling spring system in October 1983. This would not go into production until 1985.
The history of Cherry’s membrane keyboards is unclear. Their Next Generation Keyboard was advertised in the United States in 1984, around the same time that Cherry in the UK took out a patent for their own membrane design. The UK design formed the basis of the German FTSC (“full-travel sealed contact”) type, Cherry MY, the introduction of which is not known. The Deskthority wiki claims an introduction in 1987 without any evidence, but it seems more likely that full-scale production was well underway by then.