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.
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.
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.
are a number of options within a membrane switch as described
graphic overlay is textured in either matt or gloss and a
range of custom finishes in between. A matt textured finish
is most common.
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.
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
alternative to a metal dome is a polyester domes which is
created by permanently forming one layer of the membrane into
a dome shape
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
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.
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.