Membrane-switch basics
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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 circuit layer is die cut to the right shape after printing. Alternatively, copper-etched layers or a conventional PCB can also serve as the base circuit. Membrane-switch material can incorporate surface-mounted LEDs if need be. LEDs are glued to the screen-printed silver conductive traces with a heat-cured conductive epoxy. The height of the LED increases the overall switch thickness by about 0.02 in. Switch contacts in membrane switches typically carry a rating of 28 Vdc/30 mA max. They comprise a loop resistance of about 100 Ω. Nontactile switches can be designed with actuation forces starting at 3 oz. The spacer thickness and diameters of the spacer hole determine the actuation force. For example, a switch with a thin spacer and large diameter spacer hole will have a light actuation force. Some switches incorporate shielding to ward off interference from ESD and EMI. The usual method of shielding is to screen-print conductive ink in a grid on the top of the circuit layer, thus eliminating the need for a separate shielding layer. But when the keypad uses only one circuit layer, a shield comprised of laminated aluminum foil and polyester is less expensive than a printed shield. In either case the shield is thin so it doesn't affect the tactile feel of the switch. |