- Solid state
The following definitions are explained in terms of major manufacturer literature and advertisements.
Electronic Engineer magazine guide, 1971
Electronic Engineer magazine published “this is your keyboard reference” around 1971 or 1972, compiled in conjunction with Micro Switch as an advertisement for their product range. (Bitsavers scanned it and list it as 1972, but it is dated 1971 in the copyright.) This document divides all keyboard technologies into two types: “mechanical” and “solid state”. Membrane keyboards are listed, but not in a form where the membranes have any kind of surface-printed traces.
The term “mechanical” in this instance was used to refer to anything where the switch contacts physically touched, no matter whether they were solid or liquid, sealed or exposed. Unfortunately, the lack of elastomeric rubber and silver-carbon membrane keyboards this far back means that we cannot know for certain how these would have been classified.
The full list of terms is as follows:
TV Typewriter Cookbook, 1976
Don Lancaster’s 1976 book TV Typewriter Cookbook covers many details of keyboard design and operation. On pages 136–138, he divides keyboard switches into three groups: mechanical, elastomeric, and exotic:
|Mechanical||Mechanical switches are said to give “metal on metal contact”. He also notes that if such switches are properly designed, they “should combine both a wiping action and a cross-point contacting [action]”. The only series specifically named is Mechanical Enterprises T-5, while a switch presumed to be Mechanical Enterprises LFW is depicted as the example type.|
|Elastomeric||Elastomeric switches are described as having “a piece of flexible conductive plastic or foam.” Elastomeric switches are divided into types where the elastomeric material is pressed against a stationary contact (as is the case with Datanetics elastic diaphragm and Mitsumi’s two-membrane “hybrid” types), and pressure-sensitive materials that lower their resistance under pressure. Curiously he notes that “light-duty, short-term keyboards have even been built out of the protective foam shipped with many MOS integrated circuits.” No product examples are cited.|
“Exotic” is used to cover a number of other types:
PC Magazine, 1989
In PC Magazine, Vol. 8, No. 21 from 12th December 1989, an article entitled Wanted: More than just a replacement (by Bruce Brown and Kellyn Betts; pages 225–260) covers a range of keyboards on the market. This article also provides definitions of the different switch types found in commercially-available keyboards (pages 240 and 241). These are illustrated with rather poorly-drawn diagrams. The description of capacitive switches indicates that the authors do not fully understand the technology involved, and thus their descriptions cannot be relied on as being completely accurate.
|Mechanical||Described as “a simple switch relying on contact between two conductive materials”. The conductive materials are not stipulated to be metal, but the article notes that “the cost can be high, depending on the contact material—often gold or gold alloy—it uses.” The article also notes that mechanical switches “are reliable, have a relatively long life expectancy, generate an audible click, and (depending on the spring tension used to return the key) can have a very positive feel.” The term “positive feel” is found extensively in literature and patents, but is seldom if ever defined. The example manufacturers given are Chicony, DataDesk, NMB, Northgate and Zeos.|
|Capacitive||Capacitive switches are said to “detect a change in capacitance as a circuit is opened or closed; they don’t make a mechanical contact between conductive elements.” The description is somewhat confused, as it notes that “a dielectric … cushion is pushed down, forcing the conductive elements farther apart (or closer together, depending on the design) to create a closed circuit.” In reality, the conductive element is always kept separated from the sense pads, typically by the solder mask on the PCB.|
|Conductive rubber dome||This covers typical conductive dome keyboards, such as those by BTC. The manufacturer that they cite is Maxi-Switch, and their review model is the ME 101, a 2186 type with conductive domes.|
|Membrane||The term “membrane” here is applied solely to flat keyboards with short travel: “Keyboards that use [membrane switches] are quiet, have little or no feel and are hard to repair, yet they are considered a good choice for harsh environments because the continuous rubber sheet that overlays the full key-switch set helps to keep out harmful dust, crumbs, and liquids.” The article does not review any membrane types. Even though full-travel membrane did exist by 1989, it seems that from the perspective of the journalists, it had not become recognised.|
The term “mechanical” is not precisely defined. So far, there seem to be two separate trends for how the industry defined it. In early 1970s usage, “mechanical” seems to have been used for any switch type that involves electrical conduction. This was used in contrast to “solid state” types to cover any type that relied on electromagnetic detection, as suggested in the table above.
As the popularity of solid state keyboards declined, and new cheaper technologies emerged—conductive rubber and silver-carbon traces on polyester membranes in particular—the term seems to have been repurposed to indicate metal contact switches, also known as “hard contact”. Whether or not reed switches were classified as “mechanical” also varied.
Many switch types are defined as mechanical by the manufacturer, including the following:
- Forward Electronics SKBL/SKBM series was described as “High-reliability mechanical contact” on their website in 2004.
- Fujitsu FES-360 Series discrete leaf spring switches are also decribed as “メカニカルスイッチ” (“mechanical switch”) in the datasheet.
- Omron B3G Series was described as “メカキースイッチ” (“Mechanical key switch”) in the 1984 catalogue (curiously, the later B3G-S series is not described as mechanical).
- The SMK J-M9031 keyboard specification datasheet gives the switches as “SMK J-M0404 series, mechanical contacts”.
- Tokai MM9 Series was described on their website as “接点が回転するボールコンタクトスイッチ！” (“Ball contact switch with rotating contacts!”) as well as being classified as “メカニカルスイッチ” (“mechanical switch”).
Monterey International also used “mechanical tactile click keyswitches” in their 1992 catalogue under the K110 keyboard and KP110 keypad listings.
The definitions above however are not found in context against other offerings from the same manufacturer and era. The following are examples where “mechanical” is contrasted directly by the manufacturer against other contact types:
- RAFI’s RS 74 and 76 series keyswitches—which date back to the 1970s—are subdivided into M and C types. In the 2001 Electromechanical Components catalogue, M denotes “mechanical”, and comprises the metal contact types, while C denotes “contactless” and comprises the Hall effect versions. The Electromechanical Components catalogue from 2015 uses the same terminology.
- Clare-Pendar Series S950 was described as a “mechanical keyswitch” in their 1986 catalogue, under the Reed and Hard Contact Switches section.
- ITW’s second generation switch design, as shown in US patent 4227163 (filed in 1979), depicts and describes that this shell style can be “either of the mechanical contact type or of the analog contactless type.”
- Cherry gold crosspoint, in the KB79-2 and KB79-2R catalogues; they are referred to primarily as “hard contact” (compared to capacitive) but the “keyboard designers’ work sheet” from KB79-2 offers a choice between “Capacitive” and “Mechanical”. KB79-2R revised this to “Mechanical (Hard Contact)” and “Solid State (Capacitive)”.
- Alps advertisement from 1983: this is not as clear as it could be, but it indicates that Alps offer a choice of “mechanical or conductive rubber contacts” (KFL and Mem-Tact are the only product names given and the switch technology used in KFL is not stated).
- Similarly, Alps advertised also in 1983 that they offered a choice of “mechanical, conductive rubber or tactile feedback”: this is before SKCM was introduced, and the advertisement only depicts KCC and only mentions KFL, so “tactile” here may mean TACT miniature switches.
- Sejin America’s desktop keyboards, 1997, offering mechanical (which would have been Futaba MA series, based on the models depicted, such as the EAT-1010MB) and membrane options. (The Sejin models in question use -MB, SKM- and SWM- for mechanical (not membrane) and -RB, -SKR and -SWR for rubber dome over membrane.)
- In 1999, NMB described their Series 725-based Right Touch Model 8200W keyboards as “mechanical”, compared to their “membrane” offerings. Their datasheets also depict “Membrane Keyboard Keyswitch” and “Mechanical Keyswitch” types.
In ITW’s case, they describe mechanical contacts as follows in the patent:
The mechanical type of keyswitch has the advantage of being relatively low in cost, and for many applications this factor makes it desirable to employ such a mechanical keyswitch. However, mechanical keyswitches have a number of disadvantages that make them undesirable for use in applications where high reliability is required and the added cost of a analog switch is, therefore, considered to be warranted. These disadvantages include contact bounce, the possibility of arcing, lower life times due to pitting and corrosion and possible deformation of the contact members.
Thus far, mechanical has been contrasted against solid state, membrane and conductive rubber. The above description also strongly implies that the switch contacts are metal.
For my purposes, “mechanical” will be taken to mean directly-operated metal contact switches, per 80s and 90s convention. This also includes ball contact switches, on the basis that Tokai classified MM9 series as mechanical, but excludes reed switches.
An awkward case is Datanetics DC-50 series. Meryl Miller of Datanetics affirms that these are “diaphragm switches”, putting them into a classification of their own. He notes that the Mylar membrane facing the actuator has the responsibility of separating the metal switch contacts, which are glued to the membranes. Although these switches are metal contact internally, they also use a three-layer membrane. Although it might seem reasonable to class them as mechanical due to being metal contact (the switch contacts are the same pieces of metal as the terminals), from manufacturer standpoint they were never classified as mechanical.
Alps SKCL/SKCM and related types use metal foil contacts. Alps specifically refer to their foil contact system as “mechanical” in their catalogues.
In Modern Data, April 1970, Mechanical Enterprises Mercutronic mercury tube switches were described as using a “mechanical switching approach based on the movement of mercury in a sealed flexible tube.” 1970 was around the time that Licon Series 550 ferrite core switches came out, and less than two years after the introduction of Micro Switch SW Series Hall effect switches. As such, having a moving tube filled with mercury that is pinched by an actuator to separate the mecury when the switch is opened, was classed by MEI as “mechanical”, in comparison to solid-state designs. Just as with DC-50, these switches are hard to classify, as they used an extremely unusual design.
Clare-Pendar advertised Series S950 as a “mechanical keyswitch” type and S820 and S880 as “reed keyswitch” types in their 1986 Switchlight and Pushbutton Switches catalogue. All three types fall under the general classification of “Reed and Hard Contact Switches”.
Reed switches are still metal contact, but the switch contacts are closed magnetically, allowing the contacts to be sealed against moisture and debris ingress. However, US patent US4370533 for Fujitsu FES-360 switches (filed in December 1980) notes the following:
Switches are divided into two types, that is, switches having a mechanical contact element, such as reed switches, and switches having a non-contact switch element such as hall IC. The present invention is directed to a keyboard comprising the former type, i.e., switches having a mechanical contact structure.
Fujitsu would later separate the types out in the May 1985 edition of their magazine, with Fig. 5 “Development of keyboard switches” (page 429) giving the following descriptions of their switch product lines:
- “リードスイッチ” (“reed switch”), covering FES-5, FES-9 and FES-4 reed switches
- “メカニカルスイッチ” (“mechanical switch”), covering FES-300 and FES-301 leaf spring types
- “メンブレンスイッチ” (“membrane switch”), covering membrane leaf spring
As reed keyboard switches are less common, and documentation on them even less common, terminology regarding sealed versus non-sealed metal contacts is still scarce. For my purposes, however, reed switches will be excluded from the term “mechanical”, as the switch contacts are not mechanically operated and at least two manufacturers classified them separately from mechanical types.
See How reed switches work (magnetically operated switches) for a clear explanation of reed switch operation, including normally-closed contact types.
The normal definition of “solid state” is the use of semiconductor circuitry. Thermionic valves (vacuum tubes) are excluded from this: even though electronic computers using valves replaced electromechanical computers, computers did not become solid state until the introduction of transistors. Solid state storage uses semiconductor memory to hold the data. A broader use of the term “solid state” covers anything without moving parts, which for keyboards excludes everything except capacitive touch screens, as even resistive touch screens have a flexible plastic cover as part of the sensor mechanism. Even so, a number of keyboard manufacturers marketed their full travel switch technology as “solid state”, from as far back as the 1960s.
For keyboards, “solid state” refers to switch types where there are no switch contacts of any kind, be that metal (as in mechanical switches), conductive rubber (as in some Mechanical Enterprises T-15 variants) or silver–carbon ink (as in most membrane keyboards). Keystroke sensing is achieved using capacitance or by electronically detecting the presence of a magnet. Such keyboards may still require one or more additional parts as part of the sensor.
“Solid state” is an awkward choice of term in the context of full-travel keyboards. All such keyboards require moving parts, with at minimum a plunger and some kind of spring. However, the oldest switch type so far discovered that was marketed as “solid state”—Micro Switch SW Series—did indeed have actual semiconductor sensors. Introduced in 1968, the Micro Switch SSK—the original brand name of SW Series—was advertised to be the “first of its kind”, using an integrated circuit within each switch to detect a pair of magnets within the plunger via Hall effect.
Hall effect provides a means to to detect the proximity of a magnet entirely electronically. As the magnet is moved near the Hall sensor chip, current flowing through a special conductor area within the chip is diverted by the magnetic field from the permanent magnet, creating a minuscule potential difference across the conductor. Following amplification, this potential difference can be read in an analogue manner to indicate proximity, or the Hall sensor can set its own trigger and release thresholds and provide digital switch output.
RAFI’s Hall effect switches and keyboards are described as “contactless” and “solid state” depending on the document: “contactless” in the Electromechanical Components catalogues, and “solid state” in the Standard Keyboards catalogues.
Magnetoresistive elements increase their electrical resistance in the presence of a magnetic field. The only known instance of magnetoresistance used in computer keyboards is RAFI’s magnetoresistive range introduced in 1970.
Ferrite core is a type of solid state switch introduced at least as far back as 1970, going by an advertisment for Licon Series 550 keyboards and switches. Ferrite core switches use a very simple 1:1 transformer to detect keystrokes; pulses of electricity are fed into the primary side and are read from the secondary side. Unlike “proper” transformers, the wires are not wound around the core; instead, they are simply passed through or around the core. Licon Series 550 was described in the advertisement as “All solid state”.
At least two general forms of ferrite core switch exist. Licon-designed ferrite core switches have a stationary core, and one or two movable magnets in the plunger. In their rest position, the magnet or magnets sit in proximity of the ferrite core and stop pulses of electricity passing from the strobe line to the sense line. The magnet or magnets are moved out of the way when the plunger is depressed, allowing electricty to pass into the sense line.
ADI and IMS ferrite core switches put a narrow ferrite core inside the plunger. Here, the strobe and sense lines are simply PCB tracks that pass around circular holes in the PCB. To couple the lines together, the ferrite core moves downwards with the plunger and into the space between the PCB tracks.
The Electronic Engineer magazine referred to this technology as “saturated core” in their keyboard guide produced in conjunction with Micro Switch.
Foam pad—or foam and foil—switches are where things start to get awkward. This is a form of capacitive keyboard, where the plunger position is read by a variable capacitor. A foam pad is fitted to the bottom of the plunger, affixed onto which is a plastic-coated foil disc. The distance between foil disc and the PCB affects the capacitance registered at that key position. Keystrokes are detected when the foil disc is resting on top of the PCB; the solder mask on the PCB and the plastic coating on the disc prevent a short circuit. The compressible foam pad allows the plunger to continue being pressed past the point that the foil disc reaches the PCB, to provide overtravel. As such, the complete assembly will not operate without the use of an extra moving part (the foam pad) beyond the plunger itself. Worse, the foam pads are a weakness in the design: as foam can stiffen with age and lose its flexibility, some older keyboards fail due to the foam pad failing to re-expand after a key is struck. This is a problem that does not occur with semiconductor and ferrite sensors.
In the 1979 Cherry Electrical Products keyboards catalogue, Cherry use “solid state” to describe their foam pad keyboards. This terminology is not unique to Cherry. Key Tronic also described their keyboards as having “Solid-state capacitive switches”, as in these KB 5151 advertisements. Computer Products United likewise advertised their unbranded BTC 5339, 5160 and 5151 keyboards as having “solid-state capacitance low-profile key switches”. (No brand is cited, but inspection of the keyboards indicates that they are BTC and not Key Tronic due to the LED and key placement, and 5339 is a BTC model number. This is the larger, wedge-shaped variant of the 5339, rather than the slimline version.)
Cortron used the description “Solid-state, capacitance unit” for their CP-4550 keyboards. Although no definitive details have been found, there is good evidence to indicate that these are metal leaf capacitive switches extremely similar to those of Digitran. Here, instead of a foil disc, there is a metal leaf spring that is pushed down onto the PCB. Overtravel is achieved instead by a special prong on the leaf spring. These somewhat fragile pieces of stamped-out metal stretch the “solid state” description even further.
Membrane keyboards use one or more sheets of thin plastic, called “membranes”, as part of the switching mechanism. In most cases, there are three sheets: two are flexible printed circuits, and one is a spacer placed in between the other sheets. This is the system used in virtually all keyboards made today. The use of “membrane” is found in multiple patents for keyboards using these switches as well as the literature from many manufacturers, and is not in dispute when it comes to full three-layer membrane assemblies. The manufacturer descriptions of two-sheet and single-sheet membrane arrangements are not widely known, but Mitsumi’s later single-membrane types (KPQ and KPR types) were documented in their catalogue as membrane also.