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Electrostatic discharge protection (ESD) for “punch through” is not often a concern in membrane switches due to the high dielectric strength of the overlay. Typical ESD “punch through” values for overlay materials are 1) .007” polycarbonate: 14000 volts ; 2) .007” polyester: 17000 volts
However ESD can create issues for membrane switches and graphic overlays in two ways:
- electric charge may travel to ground through overlay cut outs or around the perimeter of a membrane switch or graphic overlay. Once on the backside of the overlay, charge can then travel to ground through LCD displays or other pcb components that come in contact with the overlay.
- rapid electric discharge across the front of a graphic overlay or membrane switch can induce current behind it with the same effect as above.
Printed ESD shields are used to collect stray charge and bleed it to ground. Shields can be printed 1) on the backside of the overlay or 2) on the top of a circuit layer. The effectiveness of the shielding is dependent on the conductivity of the printed material (e.g., carbon, graphite or silver), and the geometry of the printed shield. Two broad geometry options are typical:
- Grids or solid blocks of conductive material printed across the entire surface
- Rings around cutouts and perimeters
Grounding of the shielding traces can be accomplished:
- “Horizontally” through a trace on the switch tail, a separate ground tail, or through an eyelet that is attached to the chassis.
- “Vertical” grounding to a PC board ground plane can be achieved by including a z-axis conductive adhesive (e.g., 3M9702) in the membrane switch or overlay stack up.
Shielding from electromagnetic (EMI) and radio frequency interference (RFI): Membrane switches can include shielding from induced currents created by EMI, and RFI. Three materials are used to create these shields:
Aluminum Shields: Increasingly engineered vs printed silver on PET because of rising silver costs. Aluminum shields are grounded via tabs to a ground point.
Printed Shields: See ESD shielding described above.
ITO coated polyester is used in to provide a conductive surface in clear window areas. ITO polyester is transparent and typically used in conjunction with other shielding materials (e.g., aluminum foil, printed shields) in the membrane switch stack up.
More on membrane switches
Membrane ESD, RFI, and EMI Shielding