Jump to page content

Membrane keyboards

Contents

Overview

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.

Terminology

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.

Sensing

Conductive

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.

Capacitive

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 ST is documented as offering both conductive membrane and capacitive membrane options. The use of SC parts in capacitive ST keyboards suggests that SC keyboards were also available with capacitive membrane sensing.

Design

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.

Switching

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.

Circuit only

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”.

History

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 Technology; The Membrane Keyboard from the 5th of March 1981 writes about new new entrants into the market, specifically Oak Switch Systems and Chomerics. 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.

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 switch

Datanetics filed US patent 3594684 “Electrical interconnection system for multilayer circuitry” patent for their batch-fabricated elastic diaphragm switch 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.

Self-encoding

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

Oak filed many patents, but US patent 4367380A filed in August 1980 is the one that depicts their well-known Full Travel Membrane system in its original design.

US patent 4420744A filed in February 1981 covers achieving N-key rollover with a membrane keyboard. Their idea of N-key rollover is somewhat deceptive, however. They do not offer N-key rollover; rather they appear to be describing 2-key rollover with blocking, where 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, being able to hold any combination of keys at once is still impossible.

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

In April 1983, Fujitsu filed Japanese patent H0445924. While the contents are not easily available, it is presumed to be at least largely the same as US patent 4529849 filed a year later.

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.

Cherry MY

Cherry MY was a comparatively late entry into the game. The first patents were filed in March 1984 by Cherry, but the membrane production tooling was not set up until around 1987, which is the time that MY is alleged to have gone into production.