Jump to page content

Membrane keyboards

Contents

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.

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 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 following types are listed in the order that their patents were filed. This list is far from exhaustive.

Datanetics elastic diaphragm switch

Batch-fabricated elastic diaphragm switch keyboards are among the oldest, if not the oldest full-travel membrane system, with US patent 3594684 filed for the membrane arrangement in May 1969. While later membrane systems would use three membrane layers and silver–carbon ink for the circuit traces, Datanetics keyboards used a five-layer membrane arrangement over a PCB, and with all contact surfaces being gold-plated.

Datanetics elastic diagphragm keyboards required the high number of layers to provide sequential switching; this was later simplified down to three laters (flexible printed circuit, spacer and standard PCB). Datanetics terminology referred to the flexible printed circuit substrates as “diaphragms”, and to the spacer sheets as “dielectric” sheets or separators.

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.