Editing Foam and foil

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'''Foam and foil''' refers to a design of [[contact mechanism#Capacitive|capacitive]] [[keyswitch]] that uses foil discs affixed to foam pads as capacitor plates.

==Design==
Capacitive switches require three capacitor plates. Two are placed side by side on the PCB. The third is a movable part, whose proximity to the PCB alters the capacitance of the switch and permits detection of keypresses. In foam and foil switches, this third plate takes the form of a disc of metal foil attached to the slider. To allow for [[travel#Overtravel|overtravel]], a thick foam pad is placed between the slider and the foil disc, giving the switch its moniker.

Foam and foil switches were referred to by both [[Key Tronic]] and [[Cherry]] as "solid state". This is an inaccurate approach used to describe the fact that such switches do not use metal switch contacts, relying instead on electronic sensing; they do of course have moving parts.

Foam and foil switches have no capacity for an internal [[return spring]], so this must be fitted to the outside of the switch. A [[buckling rubber sleeve]] can be used instead, to provide a tactile response.

==Longevity==
The foam pads degrade with time, and ultimately require replacement.<ref name="OneTrueKeyboard-Lisa" />

==Implementations==
Several companies have manufactured foam and foil switches, with those from Key Tronic and [[BTC]] being the most prominent.

See: [[:Category:Foam and foil switches|Foam and foil switches]]

==Gallery==
<gallery widths=250 heights=187>
File:KT foam and foil switch internals.jpg             | [[Key Tronic foam and foil]] sliders and guide shafts (tactile verion)
File:BTC-5060-Switch-Components.JPG                    | Linear [[BTC foam and foil]] switches
File:Alphameric foam and foil -- disassembled, top.jpg | [[Alphameric foam and foil]] parts
File:Cherry KFN3-8351 key switch parts.jpg             | [[Cherry solid state capacitive]] parts
</gallery>

==References==
<references>
<ref name="OneTrueKeyboard-Lisa">OneTrueKeyboard — [http://z13.invisionfree.com/OneTrueKeyboard/index.php?showtopic=35 Keyboard on the Apple Lisa]</ref>
</references>

[[Category:Switch designs]]