Many different types of electric switches are used in pointing devices, but most frequently they're one of three major kinds, commonly known today as "Omron-style", "Panasonic-style", and tact button:
Dominant type of switch used in mice is definitely "Omron-style". Actually, that company is just one of many manufacturers, while history of microswitch invention is worth separate article: viewtopic.php?f=12&t=120
The design is simple but effective: spring-loaded contact move between two limiters. Actuating pin is separate part. Switch could be opened without desoldering from PCB and serviced: contacts cleaning and spring re-tensioning may prolong its life if the replacement is not available at the moment. Many variants are available, with different leads, gold plating, additional actuators ets. - but most widely used is a simplest form:
Another popular type is ~6x6mm square, known mostly by Matsushita (Panasonic) EVQP0E07E model. Actuating force and feel are pretty much the same. Size is more compact, and it does have just two leads: no need to provide additional space and hole on PCB for third (normally-closed) one, useless in mouse. Lead pitch and location of actuating pin allow this switch to be used instead of 3-pin "Omron-style" ones described above. Disc-shaped membrane is pushed by soft elastic insert, with plastic piston attached to it:
One more type - tact switch, used mainly for secondary buttons. Hard and loud click is suitable for fax machine or printer controls, and relatively acceptable for rarely used mouse functions (like DPI change, Bluetooth pairing, etc). Design is similar to the previous one, but lacks elastic insert:
2-pin and 4-pin versions are manufactured, as well as variants with upper metal plates with additional pin(s), including side-mounting form:
Formerly, in many early trackballs the keyboard switches were used (e.g. made by Cherry), but they've been eventually abandoned as they're less convenient for clicking because of too long travel.
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