The best way to design the molded keypads for your equipment is to understand each component and how it contributes to keypad function. In this entry we look at the materials Si Tech uses in our keypad contacts.
Understanding Contacts
Underneath the surface of molded keypads is the circuit board that controls the device’s operation. Beneath each key is an open circuit. Pressing a key causes the contact underneath the key to come into contact with the circuit board, closing the circuit. When choosing a contact material the two most important factors are conduction resistance and activation life.
Every component in an electrical circuit adds some resistance and for most components that resistance should be kept to a minimum. Contacts add several hundred ohms of resistance and the greatest amount of resistance the circuit can handle is based on the power in and type of circuit.
Although contacts in molded keypads have a long life, over repeated use the material wears out. The best case is to have contacts that last longer than the device so customers never have to deal with keypad failure. Engineers measure switch life in the number of activations expected.
Durable Carbon Pills
Most of the molded keypads we produce use carbon pills as contacts. Other materials can be used as well but carbon is the ideal choice for most applications.
Carbon is a durable material and contacts can be expected to last for five million activations. High-use molded keypads from TV remotes to ATM interfaces benefit from the durability of carbon pills. Carbon also provides low resistance to the circuit, on the order of 200 ohms or less.
Although carbon pills are a reliable technology there are limits to how small the contacts can be. A standard pill is about 0.5 millimeters thick, but that can be far too much if you are trying to make a wafer-thin device. If your application needs an unusual contact shape, carbon is difficult to form into anything but the simplest forms.
SiCoat V Conductive Coating
For custom silicone keypads that need an alternative to carbon pills, our SiCoat V conductive coating provides a flexible and low-cost option.
Conductive coatings can be used to make contacts of any size or shape, greatly expanding design options for custom silicone keypads. The coating layer is very thin, on the order of 10-20 microns, so is perfect for the slim electronics modern consumers demand.
Although SiCoat V contacts have worked for over a million cycles in independent laboratory tests, they will wear out a little faster than carbon pill contacts. The thinness of the layer gives it a higher resistance, approaching or passing 1000 ohms for the slimmest molded keypads.
As you can see both options have strengths and weaknesses and each is suited for different equipment. The best choice for your keypad will depend on the characteristics of your device.
