Two types of technologies can be offered in these keyboards.
Piezo Sensing Switches:
The pressure applied by the SS or Aluminium on the piezo sensor generates a voltage pulse which is sensed by an electronic circuit and used as a signal. SBL is a pioneer in developing this technology in India and has applied to patent the same.
Using a snap dome gold contact:
Metal caps are placed on top of a silicon sheet which can activate the snap dome switch on a gold plated PCB. This entire assembly is embedded inside a SS plate to seal it and thus keep out dust, moisture, humidity, etc.