Membrane Switches II

As mentioned in the last issue of The Omni Review, membrane switches are a frequently used interface component in industrial, commercial and consumer electronics. This is the second part of the article.

There are two primary contact configurations used in membrane switch design;

i ) screenprinted shorting pad and

ii) a metal convex dome.

Each of these can be used in combination with graphic layer options to produce different switch response. Configuration choices will be dependent on the life cycle of the switch, target cost, and desired activation force.

i) Shorting pad

The simplest and least costly contact method, utilizing a simple conductive disc pressed down onto inter-woven leads, Shorting pads can be screen-printed directly to the command overlay, or onto a secondary layer (generally where stringent design specifications are required, as in some medical or aviation applications). Shorting pad switches are often designed in combination with an audio feedback, as no physical or audio response to activation is provided.

ii) Metal Dome

The metal dome, utilizes a flexible metal dome which is placed directly over the individual switch contact. Pressure on the dome results in a temporary deformation of the dome from the convex to concave. This deformation produces the tactile and audio click we have come to identify with membrane switches.

Metal domes come in a variety of sizes, configurations (disc, pyramid etc.) and activation pressures. Metal domes can add to the cost of the membrane switch, due to both the increase in material and labour costs during fabrication of the switch.

iii) Polydome

Polydomes is the embossing of a polymer film and is often used in combination with the shorting pads to provide enhanced operator feedback. Embossed domes create a tactile feedback when depressed at lower costs than metal domes. The polydomed layer is laminated beneath the graphics layer. Embossing of the graphics layer is often combined with polydoming.

Contact choices must be made in the early stages of design as each affects the design and mechanical stresses differently. Shorting pads create a shearing force on the surrounding adhesive as the downward pressure on the command overlay creates a stretching or pulling action on the surrounding material. Domes or embossed buttons create an upwards, separating force on the surrounding adhesive.

Materials and configuration flexibility plus the ability to operate in hazardous environments is driving the popularity these switches. Small volume, custom designs and prototypes can be produced relatively inexpensively and with the use of automated processes, the membrane switches can be economical in high volumes.

It is important to remember that design choices have associated costs. Padtech Industries can provide you with options to produce most cost effective designs.