Although the structure of molded keypads is basically simple, understanding each part of the system allows manufacturers to make better decisions on what kind of keypads are appropriate to their products and will best serve their customers. Let's consider options for key contacts.

Contact Basics

On the printed circuit boards below molded keypads, an open circuit runs under each key. When the key is pressed, a contact under the surface of the key presses against the board and completes the circuit. Two important features of contact materials are activation life and conduction resistance.

Contacts wear out, making molded keypads useless. Manufacturers prefer materials that last longer than the equipment using the keypad. Customers won't be happy when a million dollar machine becomes inoperable because a few cents of contact material has worn away. Contact life is measured in activations and the choice of material depends on how many activations a key will experience.

When a contact completes an electrical circuit, it adds a small amount of resistance to the circuit. Engineers try to keep resistance low but the switch adds a few hundred ohms of resistance. The acceptable maximum resistance depends on the device and the circuit involved.

Carbon Pill Contacts

An older technology still used in molded keypads is the carbon pill. The pills are occasionally made of other materials such as gold, but for these contacts carbon provides the best balance of functionality and cost.

Historically carbon pills have been used because they are reliable and durable, offering several million activations before failing. Carbon adds little resistance to the circuit, often less than 200 ohms, so provide minimal interference to electrical function.

Modern manufacturers often avoid carbon pills because of their size and shape. Carbon pills are difficult to manufacture in shapes other than circles or ovals. The contacts are a few millimeters across and generally about a half millimeter in thickness, which by today's standards is very bulky. With the "smaller is better" attitude common with customers, some manufacturers want a smaller contact.

Conductive Ink Contacts

An alternative to carbon pills is to use a conductive ink which is painted or silk screened onto the back of molded keypads.

Conductive inks give engineers tremendous versatility in design of custom silicone keypads. They can be screen printed onto a key of any size or shape, so don't limit the design of the keys in the way carbon contacts can. Ink layers are a fraction of the thickness of carbon so allow manufacturers to produce the very slim molded keypads consumers want.

Although conductive inks are thinner than carbon, they are still very durable and have a usage life of a million or more contacts. Carbon pills are rapidly being replaced by this technology which offers more flexibility in device size and shape with nearly the same contact life.