One of the main decisions that you will make when designing your keypad is how you want your keys to feel when they are pressed. Silicone keypads are extremely versatile and Silicone Dynamics can produce keypads ranging from extremely sensitive to extremely tactile.
Reasoning and Analysis
Actuation force is important for keypads, giving the user some sort of feedback when they press a particular key. Keys with extremely low actuation force are very sensitive to input and prone to accidental pressing, but keys with extremely high actuation force might be too difficult to push easily in succession. The high actuation force reduces the life of the keypad because the silicone gradually wears out with each press.
Depending on your application, you might want a keypad closer to one of these extremes. Keypads where multiple keys are pressed in quick succession typically require a lower actuation force in order to provide a better user experience. On the other hand, keys where the operators may be wearing gloves may require a higher actuation force to confirm that the keys are being pressed by the operator.
Recommended Actuation Force and Snap Ratio
Silicone Dynamics firmly recommends that customers choose a snap ration of around 50 percent with an accompanying actuation force of 130 grams for most consumer applications. This produces a key that is relatively easy to press while still providing tactile feedback, and it extends the life of the keypad to a moderate level. This is also a good target range because it allows the tolerance level for the manufacture to vary without damaging the usability of the keypad.
The snap ratio of the keyboard (also called the click ratio) dictates the tactile feel that the keyboard will produce for the operator. In general, the snap ratio is also a trade-off between tactile feel and long life of the keyboard. Most silicone keypads outlive the devices that they are attached to. Regardless, this is still a consideration for many products.
In general, you want a keypad with higher actuation force for keys that are higher. For keys between 15 and 25mm, it is best to aim for an actuation force of around 150 grams. This supplies gives the keys the ideal tactile feedback level. Keys that are lower can use an actuation force of 100 grams or less.
Most rubber keypads follow a predictable curve for return force as compared to contact force. Keys that are harder to press will also return with more force (the force that is stored by the compression of the material)
Return force needs to be (at a minimum) the same as the contact force to prevent keys from sticking and causing multiple presses.
One of the most frustrating design flaws in any type of input device is the propensity for keys to become stuck underneath the bezel temporarily. This occurs when keys are able to depress beyond the “horizon” of the bezel. The effect of which is the corners of the keys becoming stuck, and resulting in multiple key presses or a long key press.
Anti-wobble pins are off-set on either side of an oblique key top, ensuring that the key top will not become lower than the surface. If you are planning a key pad with small conductive pills accompanied by relatively large buttons, make sure you incorporate anti-wobble pins into your design with the dimensions and specifications listed in our design guide.