Membrane Keyboards - (i.e. Keyboards with
Membrane Keyswitches)
What is a Membrane Keyboard (i.e. a
Keyboard with Membrane Keyswitches)?
The majority of computer keyboards
currently in production use a rubber dome membrane
keyswitch (which is actually usually made of silicon, but
is referred to as rubber). There are three physical
elements in a membrane keyswitch mechanism which are
simple and inexpensive to manufacture, making them popular
for most mass market keyboards.
The first is the keycap which is object
which the user touches with their fingertip. The keycap
typically has a plunger (or stem) which extends below the
keycap and serves two purposes. The primary function
is to compress the rubber dome when the keycap is pressed
downward. The secondary purpose is to maintain the
position of the key by restricting twisting or tilting of
the vertical movement vector over the key travel distance.
The second component is the rubber dome
which is where the feel of the keyboard and force curve of
the keyswitch is derived. The third component is a
three-layer membrane sheet which covers the entire base of
the keyboard, which is responsible for capturing the
physical action and sending a signal to the
computer.
When the user presses the keycap the
full key travel distance, a dimple at the top of the dome
pushes the top membrane 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 switches. 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 cushioning.
Durability: Most standard
membrane 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, compression 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 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.
