A customer sent us a report in July. They had tested our metal domes to 1 million actuations, and the domes failed at 800,000 cycles. The actuation force had dropped by 30%, and the contact resistance was over 100 milliohms. They said our domes were defective, and they wanted a refund.
We asked them how they tested. They said they used a pneumatic actuator, pressing the dome at 60 cycles per minute, with a force of 500 grams, at room temperature (25C). We told them the test was wrong. The actuator was pressing straight down on the center of the dome, with no lateral movement. But in real use, the user's finger presses at an angle, and there is always some lateral movement. The test was too harsh, and it was wearing out the dome faster than it would wear in real use.
We re-tested the domes in our own lab, using the correct test method: a actuator that presses at a 15-degree angle, with 200 grams of force, at 30 cycles per minute, with a dwell time of 0.5 seconds. The domes lasted 1.2 million cycles, which is well above the 1 million cycle spec.
If you are testing metal dome switches, here is how to do it correctly.
What a life cycle test measures
A life cycle test measures two things: the change in actuation force over time, and the change in contact resistance over time. A dome is considered "failed" when either:
1. The actuation force drops by more than 30%. A new dome has an actuation force of, say, 160 grams. After 1 million cycles, it should be no less than 112 grams (70% of the original). If it drops below 112 grams, the button feels too soft, and the user does not get enough tactile feedback.
2. The contact resistance goes above 100 milliohms. A new dome has a contact resistance of about 20-30 milliohms. After 1 million cycles, it should be no more than 100 milliohms. If it goes above 100 milliohms, the switch can become intermittent, especially in low-voltage circuits.
Dome life cycle test rig, with a pneumatic actuator pressing the dome at a controlled angle and force.
How to set up the test
The actuator. Use a pneumatic or linear actuator, with a rounded tip (radius 3mm). The tip must be larger than the dome, so it covers the entire dome surface. If the tip is too small, it concentrates the force on a small area, which can dent the dome and cause early failure. The actuator must press at a 15-degree angle, not straight down. This simulates the way a user's finger presses the button, which is never perfectly vertical.
The force. Set the actuator force to 1.5x the rated actuation force. For a 160-gram dome, use 240 grams of force. This is enough to fully collapse the dome, but not so much that it crushes it. If the force is too high, you will wear out the dome faster than it would wear in real use.
The speed. Run the test at 30 cycles per minute (0.5 Hz). This is slower than the 60 cpm that many customers use, but it is more realistic. Users do not press buttons 60 times per minute. They press at about 30 times per minute, with pauses between presses. The slower speed also gives the dome time to cool down between presses, because pressing generates heat, and heat can degrade the stainless steel.
The dwell time. Hold the dome in the collapsed position for 0.5 seconds, before releasing it. This simulates the way a user holds the button down briefly, before letting go. If you release the dome instantly, the dome does not have time to fully collapse, and the test does not measure the real fatigue life.
Four-legged stainless steel domes, which have a longer life cycle than round domes because the legs provide more support.
What else affects life cycle
The test method is not the only thing that affects life cycle. The PCB pad design also matters. If the pad is too small, the dome does not have enough support, and it collapses too far, which causes early fatigue. If the pad is too large, the dome does not collapse fully, and the contact is not reliable. We recommend a pad diameter that is 70% of the dome diameter, with a 0.1mm gap between the pads.
The environment also matters. If the test is run at high temperature (60C), the dome will wear out faster, because the stainless steel softens at high temperatures. If the test is run in high humidity, the contacts will oxidize faster, and the contact resistance will go up sooner. We recommend running the test at room temperature (25C) and 45% humidity, unless you are testing for a specific environment.
What we guarantee
All our dome sheet arrays are tested to 1 million cycles, using the correct test method. We send a test report with every batch, showing the actuation force and contact resistance at 0, 500,000, and 1 million cycles. If the dome fails our test, we do not ship it.
If you are testing our domes in your own lab, and you get a different result, send us your test parameters. We will compare them to our test method, and we will help you set up the test correctly. We want you to test the domes the right way, so you get accurate results.
We also produce silicone buttons, FPC circuits, and membrane switch panels. If you need help testing any of these products, talk to our engineering team. We will send you our test protocol, so you can replicate our results in your own lab.
- John Smith, Senior Application Engineer, Shanyo
