A typical silicone rubber keypad involves designing angled webbing around a switch center known as a flex wall. When the switch is pressed, the flex wall deforms and produces a tactile response. When fully pressed, the flex wall depresses the rear center of the key into the contact.
There are typically two methods of making contact with the switch itself: domes or conductive carbon pills. Conductive carbon pills are typically manufactureed from a silicone conductive material that contains carbon. Conductive pills are attached to the plunger of a keypad for closing the circuit on a printed circuit board trace. Before the silicone key is pressed, the trace located on the pad is in its open position. When the silicone rubber key is pressed, the carbon makes contact with the traces located on the pad creating a closed position allowing electricity to flow through the carbon to the traces.
Carbon impregnated silicone, also known as conductive carbon pills, make contact with a PCB on the bottom of the key. Contact pills are usually silicone based and are permanently modeled to the silicone base material. Conductive pills may be round, oval, or rectangular in shape. It is fairly common to find carbon pills available from 2.0mm to 8.0mm in diameter, though round and oval pills are normally available from 1.5mm to 10mm in diameter. Multiple carbon pills can even be designed into the part to ensure good contact is made. Other types of conductive contacts are gold plated and screen printed contacts.