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.
