Molded keypads are used on devices such as television remote controls and cell phones. These switches are simple in design and understanding their function makes it easier to request the right keypad for your application.

The Basic Key Structure

Molded keypads are mounted over a printed circuit. Underneath each key is a broken circuit that is completed when the key is pressed. As the circuit is completed, the electronics can detect and react to the key press appropriately.

Each key is a bead of silicone rubber held over the circuit by a web or membrane, also made of silicone. A conductive pill is mounted underneath the key to complete the circuit when pressed. The space between this pill and the circuit is small, generally on the order of a millimeter.

When the key is pressed, the web collapses and the key falls allowing the conductive pill to come into contact with the circuit board. When pressure is released, the web causes the key to spring back into its original position for another stroke.

The Webbing and Snap Ratio

Design of the web is an important aspect of molded keypads because it is the source of the click - the tactile feedback users experience when a button is pressed. The design of the web affects the activation force, the strength required to depress the key, and the contact force, the effort required to keep a key depressed. The difference between these two forces determines how strong a click the user gets when pressing the button.

Molded keypads that see occasional use, such as a TV remote, work well with a weak click. Button presses aren't rapid and the user has plenty of time to react to the key press without needing the tactile feedback. However molded keypads that are used more often, such as those used for sending text messages, require a stronger click. Users press keys very rapidly, often pressing the same key many times in succession. A tactile response registers with the user long before the character appears on the screen, allowing for faster operation.

Contact Materials

There are a number of choices for the conductive surface on the underside of each key. Each has advantages depending on the application.

The most popular choice is the carbon pill. Circular or oval discs of carbon about a half-millimeter thick are mounted to the bottom of the key. They have a long life, typically more than 10 million activations, so usually outlive the device. They have very low resistance however they come in limited shapes and some keypads require a thinner contact.

Other molded keypads use contacts printed with carbon ink. They are only a few microns thick so work in even the tightest space and can be created to accommodate any shape. However they carry substantially higher resistance than pills which can be a problem.

An experienced manufacturer of custom keypads can help you decide what features are right for your project.