Switch Tips: Membrane-switch basics

Membrane-switch pads often serve as a way of giving an operator interface distinctive graphics or an attractive look. The overlay or top layer of the switch is screen-printed with graphics (usually on the back of the overlay for protection from the environment). Typical overlay materials include polyester and polycarbonate and may have a thickness ranging from 0.005 to 0.015 in. Polyester is the usual material of choice for overlays that will be embossed or which will see a large number of actuations (25,000 or more). Overlay thickness tends to depend mostly on the amount of desired tactile feedback. The thicker the overlay, the less the tactile feedback. Membrane-switch overlays sometimes incorporate embossing to get a better look and add switch functions. There are two basic ways to emboss. The usual way is with metal-to-metal, male and female dies. There are height-of-emboss-area limitations with this method that sometimes make a second method, hydroforming, a possibility. Here fluid pressure applied to ductile blanks forms embossed shapes. This technique is more expensive but fosters design flexibility. Embossing can produce three different switch profiles: a raised pad, a rim around the key area (often used as a locator feature), and a dome where the keypad takes a hemispherical shape. There are some limitations on embossed features. The emboss height on a polyester overlay generally ranges from 1 to 2.5 times the thickness of the base material. It is possible to emboss thicker substrates but at the cost of durability. This is due to a thinner wall thickness at the point of stress. The minimum emboss width is eight times the thickness of the base material. Square corners are not possible because they will crack the overlay material. The typical circuit material for membrane keypads is silver conductive ink screen printed on a polyester substrate. The