Covering areas such as design of webbing, Force/travel considerations and key elements that need to be included when preparing CAD drawings, our Silicone Rubber keypad design guide gives designers an over view of all the elements that must be considered when specifying a rubber keypad design.
Our own UK design team are available to support you at any point in the process from simple concept discussions to detailed considerations of web design and keypad life. Please contact us via our web form or on +44(0)477 500206 to discuss your project in detail.
The snap ratio of a keypad is directly linked to the tactile feel experienced by the user. Designers should attempt to maintain ratios of around 40-60% only dropping below this if they are prepared to compromise tactile to ensure longer life.
Snap ratio is measured as F1-F2 divided by F1, where F1 is the actuation force and F2 is the contact force. See figure 3.
F2 = Contact Force
FR = Min Return Force
FD = Drop Force (F1-F2)
S1 = Peak Stroke
S2 = Contact Stroke
Fig 3 – Snap Ratio
The membrane shape and size of any rubber keymat can be designed to achieve almost any combination of actuation force and tactile response. Most applications simply require a positive tactile feel with a long life and as such an actuation force of 125 ~ 150g, with an accompanying snap ratio of 40 ~ 60% is a good recommendation.
Variations in tactile response can be achieved with various combinations of contact stroke, actuation force, key shape and material hardness. As a simple rule it should be remembered that the higher the force ,the longer the life but the poorer the tactile response. More specific guidelines are difficult to lay down. However, if a customer specifies key size, actuation force and stroke, Diamond can assist in the membrane design to achieve the required parameters. Always remember to specify higher actuation force for wider or taller keys.
A common problem with rubber keypad design is ensuring that the rocking action that can be a feature of a switch design is minimised. The following suggestions will assist in reducing the problem.
- Keep stroke as near 0.8 mm as possible
- Add stabilising posts on base of key or use multiple contacts per key
- Keep web length to a minimum
- Keep web angle as close to 40°
- Actuation force 80 ~ 150g for keys 10 ~ 15mm high and 150 ~ 175g for keys 15 ~ 25mm high
Return force should also be set at around 30 ~ 35g to ensure that keys do not stick.