Mechanical Keyboard Guide
This guide is a starting point for mechanical keyboard information. It is meant to be introductory, not comprehensive in nature, and should serve as a starting point for the beginner.
What is a mechanical keyboard?
Definitions vary, but generally it's every type of keyboard that's not one of the now-common keyboards based on rubber dome or scissor switch technology. Another serviceable definition is: "Any keyboard that incorporates a metal spring". Most modern mechanical keyboards have keyswitches by Cherry, IBM, Topre, and ALPS. Sometimes, people don't include Topre or some IBM models as "mechanical".
Why mechanical keyboards?
There are various reasons to get a mechanical keyboard. Some will fondly remember using vintage keyboards in their youth. Others use mechanical keyboards for ergonomic reasons. Some enjoy the benefits for gaming. Many people like the great durability and build quality of many mechanical keyboards. There is some evidence that mechanical keyboards are more efficient for typing. Almost everyone prefers the "feel" of a mechanical to rubber dome and scissor switch keyboards.
What makes it a "mechanical" keyswitch?
A keyswitch comprises of a way of making an electrical change when a key is pressed, ans a way of keeping the key in the "up" position. Most keyboards will electrically connect a circuit, allowing current to flow. Some change the capacitance of a matrix.
Most keyboards operate on "rubber dome" technology. The rubber dome provides a spring force, keeping the keycap up and unactivated. Beneath the dome are three sheets of plastic with a conductive layer on them. The outer sheets contain the circuitry, and conductive pads, and the middle separates them, with a gap where the pads are. When the key is pressed, the rubber dome will collapse, and when it's at the very bottom of its travel, (or "bottomed out") it will press the pads on the outer two sheets together creating a circuit. This is detected by the controller, and keypress information is sent to the computer.
A slight variation on this technology is the Scissor switch keyboard, commonly seen in laptops. It does exist on some keyboards intended for desktops, like the new apple aluminum keyboards. Under each key, there are two stabilizers that only allow it to move up and down on one axis. On a rubber dome keyboard, there is usually a piece of plastic with sliders keeping the keys centered on the domes. These make sure even if the key is pressed a little off axis, it will still only move straight, and the keypress will still be made. The scissors save a lot of vertical space, and the full travel of the keyswitch is greatly reduced (often by about half). On a scissor switch keyboard, the rubber domes are often individual -one per key instead of the full-keyboard-sheet of them often used for rubber dome keyboards. Like the rubber dome keyboard, a scissor switch has to be bottomed out to register.
Most mechanical keyboards are designed to register keypresses without bottoming out the switch. This leads to a number of benefits, and is one of the main reasons to use one. There is a great variation in how keypresses are registered without the switch bottoming out. Most rely on metal contacts that activate about halfway. Some detect a magnet moving using the properties of the "hall effect". Others detect a change in capacitance. Keyboards relying on "buckling spring" technology have a spring that will buckle at a specific point, and press a hammer down, either on a membrane or on capacitive sensors.
There are four commonly available switch types
Cherry MX Cherry is a German maker of keyboards and keyswitches. They have several product lines, and some different keyswitch technologies. Their MX line is very common, especially in modern-mechanical keyboards. The heart of a Cherry MX keyswitch is a spring, and two metal contacts. This specific type of switch is a SPST (Single Pole Single Throw) NO (Normally Open) momentary switch. SPST means there is only one connection to be made, and that one connection is either on or off. NO means that the keyswitch is Normally Open. This means the circuit is kept in the open position, and only closes once the switch has been pressed. A "Momentary" switch is one that must be held down in the closed position. Unlike a light switch, when a momentary switch is released, the circuit will go back to it's normal state. The spring keeps the switch in the NO position.
The spring keeps the switch up in it's normal, open position. When it's pressed, one of the contacts slides on the plastic keystem. As the stem moves down, the contact gets closer to its mate. Eventually, about halfway to the bottom out point, the contact will rest against its mate and no longer move. At this point, the circuit is closed, and the keypress registers. In some cherry MX keyswitches, there is a bit of a "bump" when the circuit is made.
IBM has a different, and more complicated way of making a keyswitch. They have two types: Buckling spring over membrane (Model M), and Buckling spring capacitive (Model F). In both designs, a round barrel only allows the key to travel up or down. Inside the barrel, a spring connects to the keycap and to a hammer, positioned right over a membrane sheet. This membrane sheet is very similar to ones found in rubber dome keyboards. When the key is pressed, the spring starts to compress, though it's loaded slightly eccentrically (slightly off the center axis). Eventually, the force depressing the spring will cause it to "buckle"or bend sharply outward. When this happens, the spring hits the inside of the barrel, making a noise. Since it's moved so far forward, the hammer it's connected to rotates downward and snaps against the membrane sheet, making a connection. In the similar capacitive version, the hammers are larger, and when they snap down, a change in capacitance is detected.
Alps Electric Co. manufactures an array of electrical items including a variety of mechanical switches for keyboards. They are most commonly found in older "vintage" keyboards. There have been may types, styles, and some major design changes over the years. Alps switches are very complicated, and each contains many parts. Even the so-called simplified ALPS keyswitches are more complicated than most. See the ALPS wiki for more information into the function and favours of ALPS.
Topre Capacitive RD
Rubber Dome Over capacitive These keyswitches, manufactured by Topre, have a normal slider as part of the keycap. They also have a rubber dome that provides the spring force and tactility. These features are very similar to modern rubber dome technology. Under the rubber dome is a spring that is normally cone shaped, but when the dome is compressed, it flattens out into more of a circle shape. As the key is pressed, and the dome collapses, the spring changes the capacitance of the capacitive pads on the PCB beneath it. After it's reached a certain point, there has been enough of a change in capacitance that the keypress is registered. In this design, the dome provides almost all of the force, so some consider these keyboards not to be mechanical.
Vintage and Exotic Switches
Optical and Hall Effect Keyswitches These are somewhat more rare, and are generally only found in very high-reliability applications. Hall effect keyswitches ahve a spring (or sometimes a rubber dome) holding up the keyswitch, which contains a magnet. When it's pressed, a solid state transistor detects the magnets motion by the "Hall Effect" and registers a keypress.
Optical keyswitches have an infared LED and an infared sensor for each keyswitch. As the keyswitch gets depressed, the beam of light shining from the LED to the sensor is broken. One optical keyboard, the Datahnd, uses magnets to keep the switches in the open position, though this is not usually the case. Both of these switch types are considered to be very reliable, and are generally also very expensive.
This is a very important aspect of mechanical keyboards. There are some words that are used to describe some specific properties of how a keyswitch is supposed to sound and feel. "Clicky", "Tactile", and "Linear" are common and generally accepted ones. Tactile refers to a switch that has a "tactile point" which is usually close to its activation point. As the switch is depressed, the force required increases. Afterthe tactile point, there is a sharp drop off in this force. This allows the user otfeel when the switch has activated, and should help to prevet bottoming out. A "clicky" switch produces an audible noise (usually a described as a "click" or "clack") at the tactile and activation point. A linear switch is neither clicky or tactile, it is merely the normal spring force increasing linearly as teh switch is depressed.
Force Diagrams and Key Travel
Key travel is the amount of distance the keyswitch travels duting a full stroke, from the very top resting position to the fully bottomed out position. Rubber Dome keyboards usually ave 3-4 mm of travel. Scissor switches are about 2 mm. Mechanical keyboards vary, but usually are 4-6 mm. Force diagrams show the amount of force involved in pressing a keyswitch. They show both the force necessary to press the key,as well as the force of the return. If a switch has a tactilepoint, it is easy to see on a froce diagram as a hump on the graph. Normally, the switch's activation point is also shown. Generally, a force diagram will show the distance the keyswitch is pressed across the bottom (usually millimeters), and have the activaion force (usually centinewtons) as the vertical axis. Looking at the force diagram, it's relatively easy to see some of the benefots of mechanical keyboards. Looking at a rubberdome force diagram, it's easy to see where the activation point is: at the very end of the stroke, and thusat higher forces. Most mechanical keyboards have the activation point cloer to the middle of the stroke, and at lesser forces. Often, much more force is appied to the rubber dome keyboard at the bottom out point than is necessary. Ths is because it's difficult to know exactly when the key has been pressed. It is not possible to stop applying force exactly at this point. With teh activation point not at the end of thekeystroke, a usr doesn't have to bottom out all the time. The amount of distance after the activation point is shown on the force diagram. For some switches, a "reset point" is also shown. The key has to be released past this point for the mechanism to reset, and another keypress recognized.