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Ergonomic Concepts > Comparing Mechanical, Membrane and Scissor-Switch Membrane Keyboards
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Comparing Mechanical, Membrane and Scissor-Switch Membrane Keyboards - Ergonomic Considerations of Keyswitch Type

What is a Membrane Keyboard (i.e. a Keyboard with Dome Membrane Keyswitches)?

The majority of computer keyboards currently in production use a rubber or silicon dome membrane keyswitch. Keystrokes are generated on dome membrane keyboards by depressing keycaps which include a plastic stem (or plunger) which extend downwards into the keyboard. These plungers are directly positioned above a membrane layer (with domes), which when compressed will apply pressure to a 3-layer plastic that spreads over the entire keyboard. When the user presses the keycap the full key travel distance, a contact point at the top of the dome pushes the top layer through a hole in the middle layer to contact the bottom layer. This closes the circuit which actuates the keystroke, and the keyboard will then send the character information to the computer. The middle layer of the membrane keeps the top and bottom layers from contacting each other except when a switch is depressed completely. Differences in the shape and thickness of the domes determine the travel, resistance, and tactile feedback of the switch, however the keystrokes are only generated when the key is fully depressed.

What are the Characteristics of Dome Membrane Keyboards?

Key Travel Distance: Membrane keyswitches are usually 'full-travel' i.e. the key will move down between 3.5 - 4.0 mm before 'bottoming out', and the elasticity of the membrane returns the keycap and associated plunger to their default 'up' position. However, the key travel can be as short as 2.5 mm.

Noise Level: Membrane keyswitches are the quietest switches due to the lack of any hard objects striking one another without the cushioning of rubber or silicon.

Durability: Most standard dome keyswitches are rated at 1 million keystrokes, however some manufacturers use superior materials and have ratings of as high as 10 million keystrokes. Regardless of the rating, over time some domes will become inelastic and others will become overly elastic due to debris buildup, rubber/silicon fatigue, manufacturing imperfections and even ultraviolet radiation (which can 'vulcanize' rubber). This results in a variance in the amount of force to type on different keys on a single keyboard. As such even though a keyboard may have a rating of 10 million keystrokes, the force curve and tactility of the domes can be affected within the first year of use.

Key Activation Force: The 'factory' actuation force varies widely, and can be as low as 25 grams or as high as 150 grams. Most membrane keyswitches are rated between 60 and 80 grams. 

Tactility ('Feel'): Most dome membrane keyswitches are not tactile and tend to have a 'mushy' feel due to the cushioning that is a inherent in their design. However, they can be engineered to provide a degree of tactility, but it is not possible to have the crisp distinct key action or feel which scissor-switch or mechanical keyswitches can provide.

NOTE: Dome membrane keyswitches should be distinguished from flat-panel membrane keyswitches which have no dome at all (like what is often encountered on a microwave) and have no significant movement (i.e. key travel) whatsoever. These are occasionally found on some specialty computer keyboards such as travel keyboards and industrial keyboards.

What is a Conductive Rubber Keyboard?

Conductive rubber keyboards are a distinct subset of rubber dome membrane keyboards. While the mechanical portion of the switch can be identical to a membrane keyboard (either simple rubber dome or scissors switch), the electrical portion only uses a single layer. The 'pill' portion of the rubber dome is a specially designed rubber which conducts electricity, so that when the switch bottoms out, the pill directly shorts out two different circuits to cause a switch action. Conductive rubber computer keyboards are rare, in part because they are somewhat more expensive to manufacture than membrane keyboards. However, one advantage of conductive rubber keyboards is they can easily be repaired in the field by cleaning or replacing the rubber and the conductive layer. In contrast, a membrane keyboard must be manufactured and assembled in a clean-room environment, so that it is not generally effective to try to clean or repair such a keyboard.

What is a Scissor-Switch Membrane Keyboard?

Scissor-switch membrane keyboards are a distinct subset of rubber dome switch keyboards, which merit their own category due to their widespread use in laptop and other portable keyboards. This keyboard still utilizes rubber domes, but a special plastic 'scissors' mechanism links the keycap to a plunger that depresses the rubber dome with a much shorter travel than the typical rubber dome keyboard. Typically scissors switch keyboards employ the same standard 3-layer membranes as the electrical component of the dome membrane switch. The smaller, shallower footprint makes scissor-switches popular on laptops and other portable keyboards.

What are the Characteristics of a Scissor-Switch Membrane Keyboard?

Key Travel Distance: Scissor-switch membrane keyswitches are not 'full-travel' and typically have a key travel distance of 1 - 2.5 mm as compared to 2.5 - 4 mm for memrance keyboards. As such, when typing on these keyboards it is almost impossible to prevent 'bottoming out' on every keystroke.

Noise Level: Scissor-switch membrane keyswitches are noisier than regular membrane as the physical characteristics of the switch reduce the amount of rubber or silicon in the switch, reducing the 'cushioning' that is available. In addition the scissor mechanisms help to optimize the elasticity of the rubber, resulting in a distinct noise when keys return to the original 'up' position that is not a factor on dome membrane keyboards without scissor-switches.

Durability: Most standard scissor-switch keyswitches are rated at 5 million keystrokes, however some manufacturers use superior materials and have ratings of as high as 10 million keystrokes. They are harder to clean than dome membrane keyboards (due to the limited movement of the keys) but also less likely to get debris in them as the gaps between the key cap tops are often less. This is because there is no need for extra room to allow for the 'wiggle' in the key as you would find on a membrane keyboard.

Key Activation Force: The 'factory' actuation force varies widely, and can be as low as 65 grams or as high as 100 grams. Most scissor-switch membrane keyswitches are rated between 65 and 85 grams. 

Tactility ('Feel'): Most scissor-switch membrane keyswitches have a crisper more tactile feel than regular dome membrane keyboards. They also feel more solid as they are stabilized by the scissor-switches which prevent side to side or twisting movement during key travel.

What is a Mechanical Keyboard?

Mechanical keyswitches are more intricate and of higher quality than other keyswitch types. Each key has its own independent keyswitch mechanism that will register when a key is pressed. For example on the mechanical keyswitch at right the keycap rests on top of the blue plunger mechanism which depresses into the unit. In most cases the key is actuated (that is the keystroke is generated and sent to the computer) halfway through the key travel distance. For example, the key may be capable of traveling 4 mm before hitting the bottom of the keywell, but the keystroke is generated after 2 mm. This means that when typing, there is no requirement to travel the full key travel distance. This affords touch typists the luxury of not pressing keys fully down, reducing the constant jarring action on fingertips when 'bottoming out' and associated unnecessary muscle action. Most non-linear mechanical keyswitches offer increasing resistance after the keystroke is generated, encouraging the user to stop pressing down the keycap and instead move on to the next keystroke. Finally, keycaps snap back to the starting position (i.e. up) more quickly than other keyswitch types, facilitating faster typing speeds.

All these features culminate in multiple types of feedback while typing. There are typically both audible (clicks) and an increased resistance (feel) when a keystroke is successfully actuated. While this will greatly benefit an experienced touch typist, even those learning to touch type will find their speed and accuracy improved. Of course, the time that is wasted looking at the screen to ensure that the correct characters are displaying will be regained. About the only person that doesn't benefit from a mechanical keyboard is a hunt and peck typist (a person who hovers their fingers several inches above the keyboard and uses typically only the index finger on each hand to type).

IMPORTANT NOTE: As everyone is far more familiar with dome membrane keyboards, they will press the keys down too far on a mechanical keyswitch keyboards, 'bottoming out' on every keystroke, resulting in a loud clack in addition to the light click of the keyswitch which is generated half-way through the key travel distance. Once a user learns to not press the keys completely down with every keystroke, the level of noise generated when typing on a mechanical keyswitch keyboard is substantially reduced.

What are the Different Types of Mechanical Keyswitches?

Linear Keyswitches: This type of mechanical keyswitch provides no indication of when the key is actuated (i.e. the keystroke generated) and provides constant force through the entire key travel distance. An example of a linear keyswitch is Cherry MX Red Stems.

Light Tactile Keyswitches: This type of mechanical keyswitch provides a small amount of click feedback (both audible and increased resistance) when the keystroke is generated. This tactility is often so slight that some may mistake the keyswitch for a linear keyswitch. These light tactile keyswitches are considered by many to be more ergonomic as they provide tactile feedback without generating a sensation that one has to 'break through' when generating a keystroke. An example of a light tactile keyswitch is Cherry MX Brown Stems.

Quieter Tactile Keyswitches: This type of mechanical keyswitch provides a small amount of click feedback (minimal audible and increased resistance) when the keystroke is generated.  However, the design incorporates innovative sound dampening on both the downstroke and the upstroke and a click 'leaf' to provide tactile feedback. These tactile keyswitches are very popular and a good alternative to the light tactile keyswitch as they provide tactile feedback and reduced audio feedback. An example of a quieter tactile keyswitch is the Matias Quiet Click Switch.

High Audible Tactile Keyswitches: This type of mechanical keyswitch provides a significant amount of click feedback (significantly higher audible and increased resistance) when the keystroke is generated.  This tactility is apparent to any user but is not significantly harder to press; however the sound can create a sense of increased force.  An example of a tactile keyswitch is the Cherry MX Blue Stems.

High Force / Audible Tactile Keyswitches: This type of mechanical keyswitch is for the most part no longer available in keyboards, but were popular in the early days of computing. IBM Model 'M' keyboards and some early Macintosh keyboards often weighed as much as 5 lbs and featured these type of keyswitches (a buckling spring design).  While some individuals still look for these dynamics, all the tactile benefits are present in the more modern keyswitches without the accompanying muscle fatigue that was often associated with these older style keyboards (similar in feel to the old IBM Selectric typewriter).

What are the Characteristics of a Mechanical Keyswitch?

Key Travel Distance: Most mechanical keyswitches can be described as 'full-travel' and typically have a key travel distance of 3.0 mm - 4.0 mm. Given this travel distance, as long as there is an indication of when the keystroke is generated (audible, tactile, or both), it is usually possible to prevent regular 'bottoming out' when typing.

Noise Level: Mechanical keyswitches are noisier than any other type of keyswitch. This is because there is not only a 'click' at the point of actuation (for tactile keyswitches), but also a clack at the end of the keystroke if the key 'bottoms out'. 
    Linear Keyswitches: There is no click at point of actuation, but there is almost always a clack due to the lack of indication.
    Light Tactile / Tactile Keyswitches: There is a click at point of actuation, but there often no clack due to the tactility.
    Quieter Tactile Keyswitches: The click at the point of actuation is quieter, as is the clack which often doesn't happen due to the tactility.
    High Audible Tactile Keyswitches: The click at the point of actuation is noisier, the clack is 'normal' and often doesn't happen due to the tactility.

Durability: Most mechanical keyswitches are rated at 50 million keystrokes. They are relatively easy to clean when compared to dome membrane keyboards and it is very unlikely that debris can get in the keyswitch as the gaps in the mechanism are quite small. 

Key Activation Force: The rated actuation force varies widely, and can be as low as 45 grams or as high as 350 grams. Most mechanical keyswitches are rated between 45 and 65 grams. For example, Cherry MX Red Stem (45 grams), Cherry MX Brown Stems (55 grams), Matias Quiet Click Switch (60 grams), Cherry MX Blue Stems (60 grams), Buckling Spring Model M (80 grams or higher).  

Tactility ('Feel'): Most mechanical keyswitches have a crisper more tactile feel and action than membrane keyboards. They also feel more solid as the movement of the keys are stabilized by the housing of the keyswitch, preventing twisting of the keycap during movement and associated non-tangential force when keying.

What is Tactility?

Tactility in reference to keyboards refers to feedback that a user receives when typing, specifically associated with the position along the key travel when a keystroke is generated. This feedback can be audible (i.e. a click sound), tactile (i.e. an increase in the resistance during key travel) and visual (seeing the fingers depress the key and spring back up with the keycap).

What makes a keyboard 'light' or 'soft' touch?

The concept of 'light' or 'soft' touch is a common and desirable attribute sought by both users and ergonomic professionals. However, this can mean different things to different people. For some it might mean the lowest total Work (force times distance) required, which would make a higher actuation force for lower travel distance mechanism such as the scissor-switch membrane 'lighter'. For others it may refer solely to actuation force which would make zero-force touch-surface interfaces such as smart phones or tablets 'lighter'. However, studies have shown that tactility is the most significant factor in yielding a sensation of 'light' or 'soft' touch as tactility directly affects the amount of 'force' used by an individual when generating a keystroke.

The actual rating on the force used to actuate a key (i.e. generate a keystroke) is not the only factor in determining the amount of force actually used. The reality is that most people will press keys harder than necessary unless they are given an indication that the key has fired i.e. a tactile sensation where one can feel the 'rollover' effect when the keystroke is actuated. Many membrane keyboards offer little to no tactility as the key must be pressed all the way down to actuate, and few can accurately generate the amount of force and distance needed when the only indication of this is when the keycap 'bottoms out' at the end of the keystroke. This is an even greater problem with scissor-switch membranes due to the reduced key travel distance, although because of the reduced distance there is a perception of less force. All mechanical keyswitches (except for linear keyswitches) provide a tactile indicator when the key fires and a 'stopping distance' after this point, which provides a clear sensation of the amount of force required to cause the key to fire and also a 'deceleration ramp' of sorts for your finger as you are typing, preventing you from the jarring impact of reaching the end of the key travel distance.

The graphs below provide a clear indication of the force dynamics of different types of keyswitches. The yellow dot indicates the peak force, or the highest resistance point as the key moves through the full key travel distance. The black dot indicates the point along key travel when the keystroke is actuated. The ideal attributes for a force curve should provide an increase in resistance (i.e. tactile indicator) PRIOR to the activation point (the black dot) without significantly increasing the peak force required to actuate the key. Following actuation, the resistance should increase gradually (i.e. further tactile feedback) as opposed to declining after activation. There must also be significant key travel distance after activation to provide the opportunity for the user to stop pressing down and avoid 'bottoming out'. As keyswitches move to overall lower force, they increasingly fail to be able to provide tactility as there can be no significant differential between the peak and the trough on the graph. 

A tactile membrane dome keyboard (upper left) offers a very similar force curve to a tactile mechanical switch (upper right). The primary difference is superior tactile demarcation of the peak force
and a more optimal force curve following actuation. On the standard membrane (bottom left) the actuation point is too far into the key travel distance, coming after a significant increase in force. A user would interpret this as signaling them to stop pressing down (as presumably the keystroke has been generated) when in fact it has not. After noticing repeated keystrokes which fail to register, the user would increase their force and perceived travel distance to ensure that the key fires every time. On scissor switch membrane keyboards the user experiences tremendous increase in force immediately after actuation, which makes it impossible to prevent the ‘bottoming out’ at the end of a keystroke. There have been studies which show that while a lower actuation force reduces the amount of force exerted when striking a key, a longer key travel distance reduces the amount force exerted significantly more. 

Key Force Comparison Graphs


Last edited December 9th, 2013

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