Metal contact switch
A metal contact switch is the most common understanding of the term "mechanical", used by manufacturers such as MEI and RAFI to describe their products. For example, RAFI describe RS 76 C as "Solid-state contact system" (albeit a contradiction in terms) and RS 74 M as "Mechanical contact system" in their Electromechanical Components 2015 catalogue.
- 1 Overview
- 2 Design principles
- 3 Design characteristics
- 4 Advantages
- 5 Disadvantages
- 6 References
Metal contact switches are simple conductive arrangements where electrical current is momentarily switched on during keystrokes, or held on when a latching switch is toggled.
Simple in principle, there are in fact many designs of contact arrangement, from simple to elaborate.
Metal contact switches are chiefly found in discrete modules, although arrangements exist such as the Hi-Tek High Profile and MEI Sabrecoil switch grids that place all the switches into a unified frame that is manufactured and shipped to the customer as a single unit.
Negative and positive action
At rest (when the switch is open) the contacts are held apart. As the switch is closed, the contacts are brought together. The contacts can either close of their own accord, or they can be pressed together under operator action. The majority of contact designs isolate the actions of the operator from the contacts: operating the switch removes whatever object was holding the contacts apart ("negative action"), and they close under their own power. Negative action was chosen by Cherry, MEI, SMK and various other manufacturers. Seldom mentioned, this approach is specifically referenced by MEI in their catalogue description of the T-5 series (name and date of publication lost):
- Switching action is accomplished by movement of one gold plated bar against another at right angles (classic cross bar switching). The bars strike with a velocity determined only by spring forces and not subject to the speed at which the switch plunger is struck. This type of design provides a much more constant and lower contact bounce than is found in mechanical contact switches of more conventional design. The effects of operator differences are substantially eliminated.
Likewise, Hi-Tek Dovetail Series switches "[incorporate] cantilevered contacts which are isolated from the switch dynamics, thus assuring that the contact pressure is independent of switch operations."
Some switch types, such as Alps SKCC series and SKCL/SKCM series, and switches inspired by them such as Omron B3G-S series, use the slider to press the contacts closed ("positive action"). A heavy-handed operator is going to be causing the switch contacts to close with more energy than an operator with a light touch.
- Contacts are normally held apart for greatest shock resistance.
No microphonics or bounce during turn-off or at rest.
The patent for the PED keyswitch notes that the contacts are held apart to prevent inadvertent actuations due to impact. Unlike the Hi-Tek High Profile switch that may have inspired them, the contacts don't actually close under their own power and are pressed together via a funnel-shaped section of the slider. This switch combines aspects of both approaches: the contacts are both held apart at rest and closed by direct operator action.
Cherry miniature open "mousetrap" switches have flat contacts with a large surface area. This permits a high level of current to pass through them. When these switches were adapted for use in keyboards, the flat contacts were replaced with gold crosspoint contacts suitable for low-energy circuits. Subsequently, many keyboard switch types used some form of high-pressure contact arrangement where only a very small part of each contact touches the opposing contact. These can take the form of "crossbar" or "crosspoint" contacts where a pair of cylinders or prisms are arranged at right angles, such as in Cherry MX and MEI T-5 series switches. They can also take the form of curved and raised surfaces, such as found in many four-tab clone switches.
Contact form affects bounce time. Non-illuminated RAFI RS 76 M switches have a 5 ms bounce time using gold crosspoint contacts, while the illuminated version uses flat metal contacts that, while still made of gold alloy, have double that bounce time at 10 ms.
Gold is frequently chosen as the contact material. One would assume that this is to prevent switch failure through oxidisation, as it would only take a tiny amount of oxidisation to disrupt the low current that passes through switches. Typically, the gold is applied as a plating, as noted in the specifications for GRI KBM, MEI T-5 series and Datanetics DC-50 series switches.
Switch contacts can also be solid gold alloy. Cherry noted in 1979 that Cherry gold crosspoint switches used a solid prism of 69% gold, 25% silver and 6% platinum for the contacts. Cherry M8 offered a choice of contact materials from AuAg26Ni3 (gold, silver and nickel), AgPd30 (silver and palladium, without gold) and AuAg10 (gold and silver).
It has been noted that the use of gold for contacts reduces bounce. MEI Sabrecoil used silver contacts instead of gold, and this was noted as increasing the maximum bounce time. 'More expensive keyboards often use gold contacts to reduce bounce. Sabrecoil uses a specially coated silver plate contact that is "almost as good as gold" but allows a reduction in cost.'
Some switch designs feature contacts that slide against each other during actuation, allowing the contacts to wipe themselves clean of dirt and oxidisation, albeit only with use. This is cited as a feature of GRI KBM-LP switches. Infrequently-used key positions will not benefit from this property.
Some switch designs have the contacts mounted into removable module. Typically there is a plastic block that holds the contacts. This plastic mounting block can be moulded around the contacts (as with Alps SKFL), or the contacts can be secured onto the block using folded metal lugs, as is the case with Mitsumi standard mechanical. Where the switch has an actuator leaf, this is often included, and may simply clip on.
With Alps SKCC and SKCL/SKCM series switches, this contact assembly is termed a "switchplate", and this term is also extended to Omron B3G-S series switches. Both the Alps and Omron designs have a vertical actuator leaf that is pushed aside by the slider; this works a plastic tongue that in turn presses the contacts closed. Although not assumed to be part of the force curve, removing the actuator leaf from an amber B3G-S switch offered a sharp improvement in softness and smoothness of feel.
Having the contacts in a contained unit can ease switch maintenance. Alps SKCL/SKCM series switches can be opened without being de-soldered and removed from the keyboard, allowing for easily cleaning and part replacement. Alps SKBL/SKBM series switches and Alps clone switches share this advantage, but they also use the lid of the switch to hold the contacts apart, making switch reassembly taxing. SMK switches are much worse: both SMK vintage linear switches, and more so SMK second generation switches are extremely difficult to reassemble due to needing to reinsert loose metal parts into the top shell in a specific way.
NEC oval contact module
More commonly, the switch contacts are placed directly into the switch. The lower shell of Cherry MX switches grips the contacts tightly, allowing them to be opened and reassembled with considerable ease. Alps clone switches use the top shell to hold the contacts, which is not substantially harder to work on, but makes for awkward reassembly after in-situ maintenance. (Cherry MX only supports in-situ maintenance in PCB mount keyboards.)
Cherry MX contacts; these are pressed directly into the shell
alps.tw Type OA2 contacts
Clare-Pendar S84010 H3 contacts; in use, the slider holds them apart until the key is pressed
Hi-Tek Series 725; this functions similarly
A small number of switch types use a metal ball as the movable contact. These include Tokai MM9 series and its clones, Omron ball contact, and non-keyboard switches from Sanwa and Hori sold in arcade buttons. No patent for any of these products has been found, and the exact operation remains a mystery. A metal ball, in some cases gold plated, bridges the gap between a pair of stationary contacts pressed into the base of the switch. The return spring sits above the ball, and on releasing the key, both pushes the slider up and raises the ball off one of the stationary terminals. This seems to defy the laws of physics. Ball contact switches are non-linear; Tokai switches are distinctly progressive rate with a sharp step in force before the force gradient increases; some other brands including Omron provide a much less distinct feel that is more like a wavering, imperfect linear design.
Metal contact switches permit each switch to exist in isolation. There is no need for a specialised controller or circuit tuning, and there are no components such as membranes that span multiple switches. The ability to package switches in discrete modules permits them to be used in prototyping and in custom keyboard projects. Modular switches can also be removed and replaced, although depending on how the switch attaches to the mounting plate, this may be quite difficult, as the release latches may be between the plate and the PCB.
When the switch contacts close, they rebound from the impact; this is referred to as contact bounce. Through operator or spring pressure they are driven to their closed position, but they repeatedly bounce back until they are drained of kinetic energy and settle into a stationary closed position. This bounce is subtle but significant: the keyboard controller will see the key being struck rapidly in succession. Switch manufacturers aim to keep this bounce at a minimum, and most keyboard switches are specified to bounce for no more than 5 milliseconds.
This in itself is not a great disadvantage; keyboard controllers are programmed to ignore the 5 millisecond period after a switch is first detected, and wait until the switch settles before reporting a keystroke.
However, contact-based switches exhibit a failure mode where the bounce lasts longer than expected, causing repeated keystrokes to appear. This is usually only on the order of one or two spurious keystrokes, but this is nonetheless a very visible and persistent problem. The generation of spurious keystrokes due to excessive contact bounce is referred to as chatter. Injecting the switch with a substance such as WD-40 can be all that is required to resolve this problem; chatter in both Cherry MX and Futaba clicky switches has been resolved using this method, without any need for switch disassembly (in the latter case, the switch was injected first with WD-40 and then 99% by weight isopropyl alcohol (IPA), which together with repeatedly working the switch, resolved a persistent chatter defect in a sealed switch). Where dirt is the cause, the current Keyboard Company recommendation is WD-40 Electrical Contact Cleaner Spray (a separate product to regular WD-40), and use of a dedicated contact cleaner substance has also seen mention on the forum along with IPA.
- Correspondence with Herb Palm about a failing Focus FK-8000
- Correspondence with the Keyboard Company, 2016-09-21
- Deskthority — Alps Appreciation (page 34)