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Foam and Foil Contact Keyswitches
Like the mechanical contact keyswitch, the foam and foil contact keyswitch design also uses contact to complete a circuit and indicate when a keypress is made. However, it takes a very different approach to creating the contact.
Each keyswitch is constructed of a (usually plastic) plunger on the top, connected to a foam pad. The foam pad is coated with foil on the bottom. A spring wraps around the plunger at the top, suspending the key in its normal position. Below all the keyswitches is a circuit board, printed with many pairs of copper contacts; one pair is oriented under the foam pad of each keyswitch. When they key is pressed, the foam pad moves down and touches the pair of copper contacts, completing the circuit and telling the keyboard that a key was pressed. When the key is released, the spring pulls the plunger back up, breaking the contact.
This keyswitch design provides very little in terms of tactile feedback, and typically no audible feedback at all. The foam pads "absorb" the keypresses and this results in a very "spongy" or soft feel. In fact, keyboards made with this technology are sometimes called "soft touch" keyboards. They are very much a "love 'em or hate 'em" type of affair: those used to conventional, high-tactile-feedback keyboards often despise them as "too mushy and too quiet". Others find the soft feel wonderful on the fingers, and the silence golden. There's no accounting for taste or comfort preferences. Stop by a mattress store sometime if you don't believe me. :^)
These keyboards are also not considered by most to be amongst the best available from a practical standpoint. The main durability issue is corrosion and dirt problems that often plague the keyswitches. When either builds up on the foil or the contacts, keypresses become erratic, and problems with "bounce" or keys that won't register begin to occur. The keyswitches can be cleaned if you want to disassemble the keyboard, but as I have mentioned, this is often not a simple process.
These keyboards are relatively inexpensive to produce, but are not as common as rubber dome keyboards in the PC world, probably because rubber dome keyboards are also inexpensive, and because so many people simply don't like the soft feel of the foam design.