Conductive rubber
Conductive rubber is a popular term for electrically conductive elastomers . Their specific electrical resistance is typically less than 1000 .
In electronics , this also means anisotropic elastic electrical elements for multiple contacting , which consist alternately of thin, electrically conductive channels and insulating intermediate layers. They are also known as zebra gum .
The electrical conductivity of conductive rubber is considerably lower than that of metals and higher than that of elastomers, which are classified as semiconductors . The low conductivity does not play a role with the low currents of typical applications (e.g. contacting liquid crystal displays , dissipating electrostatic charges, signal buttons).
Conductive rubber is used either instead of resilient electrical multiple contacts, as a switching contact or to prevent electrostatic charging .
composition
Vulcanized natural rubber is used for the production of e.g. B. soot is added to car tires, but does not achieve the conductivity of conductive rubber. Synthetic elastomers , mostly silicone rubber with fillers , are used as conductive rubber . H. conductive particles such as B. coal dust or metal powder. Conventional black conductive rubbers use synthetic carbon black, which is particularly active in electrical terms, as a filler.
Multiple connections
The anisotropic electrical contact elements were patented in 1974 by Timothy R. Ponn . The marketing took place in the following years under the brand name ZEBRA .
An example of elastic permanent connections, the connecting webs ( English rubber plugs or display rubbers ) between liquid crystal displays (LCD) with the associated circuit board . Based on the originally protected brand name ZEBRA, they are also known as "zebras" or zebra rubber , as they consist of a large number of parallel pieces of conductive rubber (black), interrupted by non-conductive rubber (light). The connection piece often contains many more conductive rubber elements than there are connection surfaces, so that the gaps between the connection surfaces are not short-circuited even if the position of the zebra deviates during assembly. The exact positioning of the conductive rubber in production is therefore not critical and allows cost-effective production processes.
The use of such a conductive rubber connection is necessary in particular with liquid crystal displays, since their contact surfaces are very thin and the electrical conductor structures on the glass plate cannot be soldered or metallically contacted. This connection method can also be used to connect two circuit boards. The advantage of these conductive rubber compounds is also their elasticity. B. in small housings such as in cell phones , quartz watches, digital panel meters or bicycle tachometers ensure reliable contact between the many connections between the displays and the circuit board even in the event of vibrations. The conductive rubbers are exposed to a certain pressure, which is created by the housing and the mechanical fixation, and thus connect the circuit board and LC display electrically and relatively shock-proof.
However, problems can arise in high humidity. The capillary effect causes water to get between the conductive rubber and the display or circuit board and can cause short circuits. The conductive rubber connections must always be dry and be contacted without dust during production.
The long-term stability of conductive rubber connections is also limited, since the plastic deforms and hardens plastically over time. As a result, reliable contact is no longer guaranteed as soon as the contact surfaces are slightly removed again, e.g. B. due to temperature fluctuations, mechanical deformation or change in position. In this case, individual columns or lines fail on displays with a dot matrix or individual symbols or image segments fail in LCDs with symbols.
Keys
In keys , conductive rubber can be used as one of the contact partners if they are operated frequently and currents are low. The switching contacts consist of a comb-like interlocking structure of two conductor surfaces (graphite or gold-plated, for example on a circuit board ). The conductive rubber piece connects these two surfaces when the button is pressed.
Conductive rubber keys can be found in keyboards that can be manufactured at low cost, such as computer keyboards, mobile phones , remote controls or even in individual buttons for circuit board assembly.
The conductive rubber piece is usually embedded in an elastic piece of non-conductive rubber, which closes tightly to the contact structure or the circuit board or the key housing. Conductive rubber keys are therefore often washable. They are very safe and reliable with little bounce .
Trivia
Conductive rubber strips (zebra) are often jokingly called "suffering rubber" by electronics engineers and radio and television technicians. The reason is the hardening found in conductive rubbers and the resulting change in shape that prevents reliable contact after years of use. The conductive rubbers are therefore regarded as the main cause of the failure of the display and thus for the defect of a device that was previously expensive to purchase and is therefore dubbed "rubber of suffering".
Web links
Individual evidence
- ↑ a b https://www.eriks.de/medien/540/atei/363/Elektrisch-leitfaehige-Elastomer-Compounds.pdf Company publication from Eriks on conductive elastomers, accessed on November 11, 2018
- ↑ Patent US4008300 : Multi-conductor element and method of making same. Filed October 15, 1974 , published February 15, 1977 , Applicant: A&P Products Inc., Inventor: Timothy R. Ponn.