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October, 2016
Switch Reliability: How to Choose the Right Metal Dome
Continued from previous page
domes to testing of up to 25 lb (11.3 kg) to see if they will hold up to specif- ic applications.
Dome Characteristics The actuation (trip) force, the
minimum force needed to depress the dome, has a significant effect on its lifecycle. A higher trip force results in
actuated in the same way or the same location every time. Some domes on the market demonstrate remarkable robustness to this test, with almost no difference in life expectancy results, while other domes being manufactured have a very minimal sweet spot. Looking at a variety of domes
for different applications, users may notice a little dimple located in the
Dome array — used for applications that require many switches, such as a keyboard interface.
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a lower life expectancy. When choos- ing a dome that needs to last a long time, it is best to choose a low trip force; find a balance that gives enough tactile feedback to the user while keeping the lifecycle of the dome in mind. One application may need the trip force to be low and more sensitive to touch while another may need it to be less sensitive and use greater pres- sure to complete the circuitry. For example, if the users
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wear gloves, a higher trip force is usually needed in order for them to feel the activation. The size of the dome in relation to trip force should be considered when evaluating domes for a particular project; two domes with the same trip force but dif- ferent diameters will have dif- ferent life expectancies. A larger dome with a trip force of 220g has a greater life expectancy than a smaller dome that has same trip force, both domes also give a different tactile feedback to the user. Whether the pressure used to actuate a dome is high or low, the location where the user attempts to actuate the dome is important in determining its lifecycle. Every dome has what is referred
center of the sweet spot, which is an option some companies offer. The dimple is a small concave feature located on top of the dome, and can be as deep as 0.2 mm (0.008 in.). When submitted to lifecycle tests,
Dimpled metal dome marking its “sweet spot,” generally considered to have a shorter lifecycle.
to as the “sweet spot.” The sweet spot is the tolerance on the location of the actuator to the dome center. The sweet spot varies depending on a vari- ety of criteria — click ratio, size, shape, or trip force. In a strictly-con- trolled testing environment, the test probe is aligned with the sweet spot, in real-world applications the dome is not always actuated 100 percent accu- rately. The more the actuator is cen- tered on the dome, the better the feel and its life expectancy. Metal domes have a larger
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WWW.ACLSTATICIDE.COM INFO@ACLSTATICIDE.COM (800) 782-8420
sweet spot compared to polydomes. Metal domes will also actuate even if the finger is not in the absolute cen- ter of the dome. Certain companies conduct a separate sweet spot test where the actuator is not directly in the center of the dome, but 0.5 mm (0.02 in.) aside, for example. This test is important because no matter the application, a dome will not be
dimpled domes generally do not last as long as non-dimpled domes. Many believe that the purpose of a dimple is to provide better electrical charac- teristics and to reduce contact bounce, but this is mostly irrelevant on a membrane switch. While small electrical advantages can be found in some applications, a dimple can sig- nificantly reduce the activation life, which is usually more important. During the manufacturing pro -
cess, the shaping of the dimple puts added pressure and gives the dome additional points of failure as opposed to a dome without a dimple. It is recommended to use non-dim- pled domes and lower trip force domes in more difficult applications, because their lifecycles are longer. No matter the application, it is
necessary to test a dome in relation to how it will be used. Be sure to keep in mind the trip force of the dome, the trip force in relation to the size of the dome, who is the end user of the product in its application, the size of the dome’s sweet spot, and whether the dome is dimpled or non-dimpled. Contact: Nicomatic, 165
Veterans Way, Unit 200, Warminster, PA 18974 % 215-444-9580 fax: 215-444-9581 E-mail:
sales@nicomatic.com Web:
www.nicomatic.com r
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