There are two basic keypad designs in today’s Silicone rubber keypads. You have the choice of designing your keypad as tactile or non-tactile. Within these options come a multitude of variations of both types which could be included in a silicone rubber keypad. The keypad type we will focus on today is the tactile keypad.
There are multiple ways you can have a tactile silicone keypad. The three main ways are: Key Diaphragms, Metal Domes, and Tact-Switches. A key diaphragm is the thin hinged area of a keypad that allows the keypad to move and flex. Diaphragms are also known as webbing or webs. Both metal domes and Tact-switches accurately control the force requirement of the tactile snap of the key, and are both mounted to the PCB.
By far the oldest method in silicone keypad technology is controlling the tactile feedback within the silicone rubber itself. This is done with the use of diaphragms. Due to the inherit nature of silicone rubber, this method has the greatest tolerance on the force of the key press. Determining the force of a specific silicone key is complicated and often requires multiple prototypes before the desired force is achieved.
To determine the force of a tactile silicone rubber key a force curve chart is used. The force curve of a tactile silicone key is made up of a couple different variables. First at the top of the curve you have the actuation force. The actuation force is the force required to collapse the membrane of a silicone key. Next you have the Contact force. The contact force is the amount of force required to maintain the closure of the silicone key. With these two forces, you are able to calculate a snap ratio. The formula for determining the snap ratio as followed: (Actuation Force – Contact Force) / Actuation Force. This ratio is what gives the keypad a tactile response or more simply put the snap feeling. The last force to determine is the return force. This is the force created by the keypad’s webbing as it returns to its neutral position. These forces are all mostly determined by the design of the webbing. Different variations in the thickness, angle and travel will give different forces across the board.
When designing a silicone rubber keypad, it is important to consider how you want to apply force to the shorting pad, and how much force is needed. The simplest and most accurate method is by using metal domes. The use of metal domes within your keypad assembly eliminates the need for the use of a diaphragm. Metal domes have accurate force profiles with small tolerances that allow for the same tactile feel for usually a million plus actuations. Typically metal domes are the number one choice for tactile silicone rubber keypads, but sometimes, due to size restrictions, they may not be able to be used.
Tact-switches are very similar to metal domes, except they contain the shorting pad inside the switch. The biggest difference is the height requirement for tact-switches. Tact switches require the most amount of space due to the increased height from containing the circuit pad. Typically tact-switches use metal domes inside the switch.
Here at SiTECH, we have experience using all types of tactile silicone rubber keypads. If you are interested in having your custom silicone keypad quoted or simply wish to learn more about your tactile options, give us a call at: 757-887-8488, or submit a quote.