Switch mount

From Deskthority wiki
Jump to: navigation, search
Plate-mounted vs. PCB-mounted Cherry MX switch

Switch mount refers to the method by which discrete switches are mounted into the keyboard. Typically, each switch is soldered onto a printed circuit board (PCB). Switches held in place by the PCB alone are said to be PCB-mounted.

It is more common in contemporary keyboards however for switches to also be secured into holes in a metal plate. In this arrangement, the switches are said to be plate-mounted.

Most switch types are either made for PCB-mounting or plate-mounting, but the Cherry MX switch can be either. Versions intended for PCB-mounting have extra prongs in the circuit board and often a jumper or diode in the switch soldered to the PCB to increase the key's stability. In many keyboards such as those of the Cherry G80 series and the KBC Poker, this is considered to be adequate.

Methods

Plate mount

In a plate-mount keyboard, the switches are secured into a plate. Retention clips on each switch hold the switches in place. In most examples, the plate is made from sheet steel, but there are exceptions. Most BBC Microcomputer keyboards used aluminium plates, with the exception of the "Type 2" SMK keyboards, which had steel plates. Some keyboards used plastic plates, including the NTC KB-6251 series and the Strong Man SMK-8112JU.

Japanese websites are more likely to refer to steel plates as "iron" plates.

The steel plate is generally painted to protect against rust; the most common paint colour used is black, but white is also used in keyboards such as the Dell AT101 series and Matias Tactile Pro family. Rosewill RK-9000 is a commercial keyboard with a red plate; red is more commonly used in enthusiast keyboards.

PCB mount

PCB mount switches are secured solely by the solder joints between the terminals and the PCB. PCB mount switches typically have one or more fixing pins (small pegs protruding from the bottom of the switch) to help align the switch (since the terminals are often soft and easy to bend) and possibly also prevent rotation of the switch in use. The return spring shaft may also protrude below the base of the switch, and this also helps to locate the switch in the PCB.

Some switch types (e.g. the PCB mount variants of the Cherry MX family, and SMK inverse cross mount) have both fixing pins and plate clips and can be used either with or without a mounting plate.

Fixing screw

A small number of switch types, such as Datanetics DC-50 series, are secured to the PCB by a screw.

Case mount

Acer switches are a hybrid design which combine discrete switches with a membrane layer. In some Acer keyboards, the discrete switches are mounted directly into the upper shell of the keyboard itself; other Acer keyboards use a plastic mounting plate.

Integrated mount

The switches are not discrete, and are typically formed from a single moulding, either as a discrete plate, or out of the case itself. Rubber dome over membrane keyboards are typically integrated mount.

Comparison

Rigidity
Plate-mounted keyboards are more rigid and sturdy than PCB-mounted keyboards, as well as being heavier.
Keyfeel
The steel plate keeps the switches perfectly rigid, and this leads to Cherry MX-based boards having a particularly sharp landing; the subtle flex of the PCB when typing on a PCB-mounted board helps alleviate this problem for typists who bottom out the switches.
Noise level
The type of switch mount affects the noise level of the keyboard according to the switch type: the steel plate increases the volume of Cherry MX-based boards, while it is reported to reduce the volume of Topre switch boards.
Switch access
The Cherry MX switch in particular can normally not be opened up while it is mounted into the steel plate. The switch must be desoldered and removed from the board in order to open up the switch for maintenance and modification. The mounting plates of some enthusiast keyboards, notably the Phantom, have specially shaped mounting holes that allow the switch to be opened while still soldered in place.
Alps SKCL/SKCM series switches are not restricted in this way: the upper shell of the switch can be released while the switch is still fully mounted.

References