A Definition of Carbon Tactile Keys

Silicon rubber keypads are very popular in consumer and industrial electronic products. These keypads are low cost and mostly reliable. A typical silicone rubber keypad involves creating angled webbing around a switch center. When the switch is pressed, the webbing deforms and produces a tactile response. As soon as the switch is no longer under pressure, the webbing returns to its neutral position. The carbon tactile key comes into play when making electronic switches, as a carbon pill is placed at the switch center’s base. This carbon tactile key contacts onto a PCB when the web is pressed.


Tactile Responses and Snap Ratios of Keys

Tactile responses are the clicks you feel and/or hear when you press a key. A key’s tactile response and travel can be altered by changing the webbing design or the shore hardness of the silicone base. Depending on key size and shape, the tactile forces may be as high as 500g. Ultimately, the users’ tactile feel is determined by the snap ratio of a keypad. The standard snap ratio is 40% to 60%, but keys will lose their tactile feel if dropped below 40%.

Electrical engineers and designers often choose carbon tactile keys for their silicone keypads. There are several benefits to using carbon tactile keys, including the way in which they deliver different actuation feels from traditional silicon web keys. Tact switches and metal domes offer a different actuation feel from conventional silicone web keys. The stroke is typically lower (0.8 – 0.4mm) and the snap is higher (70% – 30%). Additionally, the life of metal domes and tact switches is usually longer (500,000+ actuations) than those of silicone key webs. Generally, you should specify higher actuation forces for keypads with large keys than for those containing small keys in order to develop good tactile feel.

Conductive Contacts in Tactile Keys

  • Carbon Impregnated Silicone – Carbon impregnated silicon, also known as conductive carbon pills, make contact with a PCB on the bottom of the key. More often than not, contact pills are 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. Sometimes, multiple pills are used to ensure good contact is made. Carbon pills have an average life that exceeds 5 million actuations, and they are known for their long life and low contact resistance.
  • Conductive Contacts – Other types of conductive contacts are gold plated and screen printed contacts. Designers and engineers choose different contacts to produce various contact resistances.
    • Gold plated contacts – Gold plated contacts produce click or soft tactile experiences. While gold plated contacts are expensive, they deliver reliable contacts for a great deal of time because gold does not rust and is known for its longevity.
    • Screen printed contacts may be silk screened or made with conductive ink. These types of contacts are available in any size or shape, as they are printed on switch-contact areas. One disadvantage of screen printed contacts is their shorter life and higher contact resistance when compared to carbon pills.
  • Gold Plated PCB – Carbon tactile keys and their related conductive rubber keypads are most reliable when their environments include well-designed printed circuit boards (PCBs). Gold plated PCBs are plated over nickel. These PCBs have the lowest possible contact resistance, at less than 100 ohms, for any keypad application. Gold plated PCBs are known for their reliability, but they are much more expensive than nickel plated PCBs.

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