Hardware Reference
In-Depth Information
made. Preferences in keyboard feel are somewhat subjective; I do not favor the foam element switch
design.
Another problem with this type of design is that it is more subject to corrosion on the foil conductor
and the circuit board traces below. When this happens, the key strikes can become intermittent, which
can be frustrating. Fortunately, these keyboards are among the easiest to clean. By disassembling the
keyboard completely, you usually can remove the circuit board portion—without removing each foam
pad separately—and expose the bottoms of all the pads. Then, you easily can wipe the corrosion and
dirt off the bottoms of the foam pads and the circuit board, thus restoring the keyboard to a “like-new”
condition. Unfortunately, over time, the corrosion problem will occur again. I recommend using some
Stabilant 22a from D.W. Electrochemicals (
www.stabilant.com
) to improve the switch contact action
and prevent future corrosion. Because of such problems, the foam element design is not used much
anymore and has been superseded in popularity by the rubber dome design.
KeyTronicEMS, the most well-known user of this technology, now uses a center-bearing membrane
switch technology in its keyboards, so you are likely to encounter foam-switch keyboards only on old
systems.
Rubber Dome Switches
Rubber dome switches are mechanical switches similar to the foam element type, but they are
improved in many ways. Instead of a spring, these switches use a rubber dome that has a carbon
button contact on the underside. As you press a key, the key plunger presses on the rubber dome,
causing it to resist and then collapse all at once, much like the top of an oil can. As the rubber dome
collapses, the user feels the tactile feedback, and the carbon button makes contact between the circuit
board traces below. When the key is released, the rubber dome re-forms and pushes the key back up.
The rubber eliminates the need for a spring and provides a reasonable amount of tactile feedback
without special clips or other parts. Rubber dome switches use a carbon button because it resists
corrosion and has a self-cleaning action on the metal contacts below. The rubber domes themselves
are formed into a sheet that completely protects the contacts below from dirt, dust, and even minor
spills. This type of switch design is the simplest, and it uses the fewest parts. This made the rubber
dome keyswitch reliable for several years. However, its relatively poor tactile feedback has led most
keyboard manufacturers to switch to the membrane switch design covered in the next section.
Membrane Switches
The membrane keyswitch is a variation on the rubber dome type, using a flat, flexible circuit board to
receive input and transmit it to the keyboard microcontroller. Industrial versions of membrane boards
use a single sheet for keys that sits on the rubber dome sheet for protection against harsh
environments. This arrangement severely limits key travel. For this reason, flat-surface membrane
keyboards are not considered usable for normal touch typing. However, they are ideal for use in
extremely harsh environments. Because the sheets can be bonded together and sealed from the
elements, membrane keyboards can be used when no other type could survive. Many industrial
applications use membrane keyboards for terminals that do not require extensive data entry but are
used instead to operate equipment, such as cash registers and point-of-sale terminals in restaurants.
Membrane keyswitches are not just relegated to fast food or industrial uses, though. The membrane
keyswitch used with conventional keyboard keytops has become the most popular keyswitch used in
low-cost to mid-range keyboards, and even some high-end units. Although low-end membrane
keyswitches have a limited life of only 5-10 million keystrokes, some of the better models are rated
