Selecting The Right Metal Dome for Real World Applications
Electrical switches are important in the world of human-machine interface (HMI) and can be used anywhere from the microwave in your kitchen to a hospital bed in the emergency room of your local hospital. Many companies rely on the activation life advertised by manufacturers, but the data is compiled in a strictly controlled testing environment. Every electrical switch is used in a different way and a different environment; numerous circumstances affect the life-cycle of metal domes and vary when used in real-world applications verses a strictly controlled testing environment.
When searching for the right dome that will fit your application, people ask themselves various questions.
- Will the switch last for the life of the product?
- How many cycles will the dome actually see?
- What is the force applied for testing standards? How does that compare to the application it will be used?
- What changes during the design phase affect the feel, or tactile ratio, of a dome?
Dome life cycle will vary depending on the application and the people using the device. Industry standards for testing of metal domes and switches were developed and published in the 1990’s, these standards were made in collaboration with industry specific professionals.
Currently, there are a variety of standard tests required for metal domes, one focuses on the life-cycle of a metal dome and does nothing but depress and release a dome until it reaches a predetermined number. Before, during and after testing the dome must be examined for changes in characteristics. For testing you must first determine the actuation force needed to depress the dome being tested, once you have the maximum force needed to depress the dome, the industry standard is to set up to depress the dome 10% higher than the actuation force needed to reach contact closure. The dome is then placed on a hard-flat surface perpendicular to a testing probe, the surface can be milled or tape can be applied to avoid movement. A typical testing probe has a diameter of ≈50% of the dome being tested, typically made of a non-elastic material (hardness number equivalent to A/45 ±5, e.g. Teflon (PFTE)) and is aligned to the center of the dome. The testing probe is then set to depress the dome for a specific number of actuations during a specific amount of time; the dome is examined at certain intervals to check for integrity and to determine if any characteristics have changed.
Testing results vary and life-cycles are different for all types of domes. Depending on the application, many items affect the life-cycle and performance of a dome; from customer specific applications, size and strength of the user/actuator, how centered the dome is, shape of the dome, dimple or non-dimpled, higher/lower trip forces, size of the dome, and whether a dome is round, double round, oblong, triangular or 4-legged.
With outside forces affecting the life of your dome, the best course of action is to submit the dome to a test within your specific application with an actuator similar to the use it will receive. If a dome is being used in an office setting, only used once in the morning to turn a machine on and once in the evening to turn the machine off, then life-cycle testing is not the most important factor. If a dome is being used in a harsh environment, like a switch for entry of an armored vehicle, where the users are large and use much more force then in an office setting, you must adjust your testing criteria. The life-cycle testing for the armored vehicle will need to have a higher actuation force than the standard 110%, some companies expose domes to testing of up to 25 lbs. to see if they will hold up to their specific application.
The actuation (trip) force, the minimum force needed to depress the dome, has a significant effect on the life-cycle of a dome. A higher trip force results in a lower life expectancy of a dome. When choosing 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 life-cycle of the dome in mind. One application may need the trip force to be low and more sensitive to touch and the other may need it to be less sensitive and use greater pressure to complete the circuitry. For example, if the users wear gloves, a higher trip force is usually desired for the user to feel the activation. The size of the dome in relation to trip force should be considered when evaluating domes for your project; two domes with the same trip force but different diameters, result in different life expectancies. A larger dome with a trip force of 220 grams 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 the life-cycle of a dome.
Every dome has what is referred 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 variety of criteria; click ratio, size, shape, or trip force. In a strictly controlled testing environment, the test probe is aligned with the “sweet spot”, in real-world applications the dome is not always actuated with 100% accuracy. The more the actuator is centered on the dome, results in the best tactile feel and life expectancy. Metal domes have a larger sweet spot compared to polydomes; metal domes will actuate even if the finger is not in the absolute center 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 aside, for example. This test is important, no matter the end-users application a dome will not be actuated in the same way or the same location each time. Some domes on the market demonstrate remarkable robustness to this test and there is almost no difference in the life expectancy results, while other domes being manufactured have a very minimal sweet spot. Looking at a variety of domes in different applications, users may notice a little dimple located in the center of the sweet spot.
Metal domes are available in a variety of styles; a dimple is an option some companies offer. The dimple is a small concaved feature located on top of the dome, with a depth usually between .0254 and .17 mm. When submitted to life-cycle tests, dimpled domes on average do not last as long as non-dimpled domes. Many believe that the purpose of a dimple is for better electrical characteristics and to reduce contact bounce, but this is not critical on printed electronics. While some small electrical advantage can be found in some applications, a dimple can significantly reduce the activation life, which is important in many applications. During the manufacturing process, the shaping of the dimple puts added pressure and gives the dome additional points of failure compared to a dome without a dimple. It is recommended to use non-dimpled domes and lower trip force domes in more difficult applications because the life-cycle is longer compared to dimpled domes.
No matter what your application is, it is necessary to test your domes in relation to how it will be used on a consistent basis. It is recommended to test the actuation life of a dome under more real-world conditions, primarily….
- In the actual switch build up
- With real-world actuation forces
- With realistic cycle expectations, specific to your application
- Possibly with purposefully offset actuation if this is a real-world concern
Be sure to keep in mind the trip force of your dome, the trip force in relation to the size of the dome, who is the typical user of the product in the application, does the dome you are selecting have a large sweet spot to compensate for irregular use, and whether the dome is dimpled or non-dimpled. Longer lasting more robust switches use larger domes, with lower forces, without dimples.