A key does not need to have electrical contact to be mechanical. A membrane keyboard has electrical contact between membranes - but that does not make it mechanical.
I would define "mechanical key" is one that has:
* A "
mechanism", in classic terms. There is at least two parts that together form a
simple machine: a joint, a lever, wedge, etc. and...
* This mechanism plays a
part in actuation of the key.
+ A IBM Buckling Spring switch (membrane or capacitative) is mechanical because the foot pivots.
+ Cherry MX is mechanical because the plunger has an inclined plane that presses against a leaf spring.
+ Alps SKCL/SKCM is mechanical because it has a plunger that presses against a leaf spring - that is an inclined plane.
+ Fujitsu Peerless is mechanical because the coiled spring interacts with the rubber dome that it sits on!
- A scissor switch is not mechanical: there is a mechanisms but the mechanism is not used to actuate the key.
- Topre is not mechanical because there is no classic mechanism: only a sensor, which provides a value in a range.
- Linear Flaretech is not mechanical because there is only a plunger: only a sensor which provides a value in a range.
? I am not sure if I should call Clicky Flaretech mechanical or not. It uses the same mechanism as Cherry MX, but it is not the force on the leaf spring that actuates - it is when the sensed position of the inner plunger crosses a threshold.