In its simplest form a membrane switch comprises of two flexible printed circuits, physically separated by a spacer. When the button is pressed, holes in the spacer allow the two circuits to contact completing the circuit. The printed top layer is known as the graphic overlay. It is normally made of clear polyester, and reverse printed to protect the image from wear, solvents, and abrasion. As membrane switches are momentary switches, it may be necessary to introduce a method of electronic latching to replicate the actions of mechanical switches.
There are many variations on this simple design, and by laminating further layers into the switch, it is possible to add additional features such as snap domes, ESD/RFI shielding films, lighting panels, and embedded surface mount components.
The membrane switch is mounted on a flat enclosure, and typically a single ribbon cable connects the switch to the PCB through a single slot in the mounting panel. In contrast, an overlay requires cutouts in the enclosure beneath each switch adding to machining costs.
MEMBRANE SWITCH OPTIONS
The are a number of options within a membrane switch as described below
The graphic overlay is textured in either matt or gloss and a range of custom finishes in between. A matt textured finish is most common.
Windows can be "printed" in the matt polyester to allow clarity for LCD and LED displays and the "windows" can be tinted in a variety of colours.
Metal domes are tactile devices that are inserted within the membrane switch to give a physical tactile response when pressed (Clicked). They can be designed as part of the circuit or above the circuit. A range of sizes are available
An alternative to a metal dome is a polyester domes which is created by permanently forming one layer of the membrane into a dome shape
Often switches may require backlighting. There are a number of ways to do this however the most effective are Electroluminescent Backlighting (EL) and Lumitex woven fibre. These are covered more fully in another section
Radio Frequency Interference (RFI) is becoming an important consideration is legal restrictions are tightening in many countries. Electronic equipment must be protected from unanticpated behaviour caused by RFI and Markit Graphics has developed specialist solutions in this area.
SURFACE MOUNT DEVICES
It is possible to incorporate surface mounted devices such as LED's and resistors with the membrane switch. This may save allow costs savings in PCB design while allow flexible placement of the PCB