Snap Ratio for Siliocne Keypad

Designing for Snap Ratio & Tactile Feedback

As mentioned above, being able to design silicone keypads for specific snap ratio and tactile feedback is one of the many favorable qualities of silicone. Much like membrane switches, it is important for a user/operator to recognize when he or she has fully pressed a key. This action is done when the operator depresses the key which collapses the webbing and connects the contact area on the bottom of the key to a circuit or mechanical type key below, then once pressure is removed the webbing returns to its neutral position. The differential between the pressure combined with the movement creates tactile feedback and can be designed optimally by design engineers using the proper snap ratio.

To better understand snap ratio, it is important to know the ratio is determined by actuation force (the force required to collapse the membrane/web of a rubber switch, also referenced as F1 in the formula below) and contact resistance (F2).

F1 - F2

-----------

F1

The recommended snap ratio to maintain is 40% - 60%; if dropped below 40%, the keys will lose tactile feel but have an increased life. Loss of tactile feel means the user will not receive a 'click' feedback during actuation. Since most applications require a positive tactile feel and a long life, it is best to design for an actuation force of 125 - 150 grams and an accompanying snap ratio of 40% - 60%.

Since silicone is an adaptable material, the membrane shape and size can be designed to achieve a variety of actuation forces and tactile responses. Additional customization can be achieved by changing the contact stroke, actuation force, key shape and material hardness. For instance, to create taller or wider keys, the keypad must to be designed with a higher actuation force.