Membrane Switches I

Membrane switches are a frequently used interface component in industrial, commercial and consumer electronics. Their simplicity of design, durability and reliability, in combination with an infinite choice in configurations, make them a favorite of product design engineers.

In general a membrane switch is a multilayer, wafer assembly, consisting of - Operator-Machine Interface (Graphics command layer), Spacer (dielectric polyester), Trace layer, Contacts and Conductive Tail/ Connector.

Design options centre around switch life and cost, each of which is traded off against the other.

The graphics layer is the most visible portion of the switch. The range of decorative options is limited only by imagination and budget. Some options may include colour, texturing with UV inks, tinted display windows, and embossing.

The spacer provides a non-conductive separation of the command overlay and the underlying conductive trace. The adhesive on the spacer also provides a seal for the trace(s). As with the command overlay, the spacer can be fabricated from either poly carbonate or polyester materials. Spacers vary in thickness from 0.005 to 0.011" depending on switch type and required sensitivity. Thinner spacers are used on designs requiring small buttons or high sensitivity. Generally, as button size increases, so does the thickness of the spacer layer.

The trace layer comprises the circuitry. This is composed of a silver based ink applied to a non-conducting polymer film. In complex designs, conducting traces may be crossed, but are kept separate by means of UV cured dielectric ink bridging pads.

Two types of connectors are commonly used in membrane switch manufacture, the crimp on and press fit types. Both connectors are applied after the switch is assembled, but the choice must be made during the design stage as each affects the method and processes used in fabrication of the switch.

The crimp on connectors are invasive, utilizing a series of teeth driven through the protecting polyester film to make contact with the conductive leads.

Press fits are non-invasive, but require that the traces be exposed on one side. A carbon film is often applied over the exposed trace, adding to trace resistance which should be taken into consideration in the design. The carbon will also increase the cost of the switch as an extra step is added. Where a conformal coating is to be applied to the connector and board assembly, the carbon cap may be omitted.