Metal contact switch

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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.

Overview

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 linear 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.

Design principles

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.

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.

Operator force is not the only suggested reason for the design. The 1973, 1974 and 1979 Cherry catalogues note the following in relation to Cherry gold crosspoint:

Contacts are normally held apart for greatest shock resistance.
No microphonics or bounce during turn-off or at rest.

The patent for the Philips keyswitch notes that the contacts are held apart to prevent inadvertent actuations due to impact. Unlike the Hi-Tek linear 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.

Advantages and disadvantages

Chatter

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[1]). Where dirt is the cause, the current Keyboard Company recommendation is WD-40 Electrical Contact Cleaner Spray[2] (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.[3]

Modularity

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.

Contact arrangement

Contact module

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. Being not formally defined, the scope of the term is unclear. 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.

Bare contacts

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.)

References

  1. Correspondence with Herb Palm about a failing Focus FK-8000
  2. Correspondence with the Keyboard Company, 2016-09-21
  3. Deskthority — Alps Appreciation (page 34)