PC Hardware in a Nutshell, 3rd Edition

Keyboard Switch Types

Underneath each key is a switch. When the key is pressed, the switch closes. Three types of switches are used in keyboards:

Mechanical

These keyboards use standard spring-loaded momentary-on switches, most of which are made by Alps Electric. Pressing a key compresses a spring and causes a plunger on the bottom of the key to make physical contact to close the connection. When the key is released, the spring forces it back into rest position. These keyboards provide the clacky feel typical of the original IBM Selectric and PC keyboards, are quite durable, and are usually relatively expensive ($75 to $125).

Capacitive

These keyboards are unique in that pressing a key does not make electrical contact to complete the circuit. Instead, movement of the plunger on the bottom of a key alters the state of a capacitive circuit, which the keyboard controller recognizes as a key press. Keyboards with capacitive switches provide clacky feedback, are even more durable than mechanical keyboards, and are quite expensive ($100 to $200). As far as we know, only IBM and Lexmark have produced such keyboards.

Membrane

Most current keyboards use membrane switches. Unlike mechanical and capacitive keyboards, which use discrete physical switches for each key and are correspondingly expensive to produce, a membrane keyboard combines all key switches into one unit comprising three membrane layers. The bottom layer has printed conductive traces that correspond to the individual key switches. ...