Other Membrane Switch Design Considerations

Tail Length & Exit Positioning: Membrane switches are connected to the customer’s printed circuit board by means of a flat cable. The flat cable is cut from the circuit layer(s) of the switch. The tail exit position and tail length must be specified before Melrose can begin design of the switch. In volume production tail length, and exit position can substantially affect productions costs. Melrose can advise you when a change in tail exit position or tail length results in a material affect on cost.

Multilayer Circuits: Multilayer membrane switches circuits can be designed as screen printed conductive inks on heat stabilized polyester (typically .005”), or with circuit boards. With screen printing technology, multiple circuit layers are created (if necessary to achieve needed trace routing) with dielectric insulating ink printed in areas where circuits cross over one another. With flexible or rigid printed circuit boards feed throughs are used to: create multiple circuit layers, as well as to produce two sided circuits.

ESD Protection. ESD protection 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:

• .007” polycarbonate:                14000 volts
• .007” polyester:                       17000 volts

Printed ESD shields are often used to collect stray charge and bleed it to ground. The shields can be printed on the backside of the overlay or on the top side 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

EMI/RFI Shielding: Membrane switches can include shielding from induced currents created by electro magnetic radiation. Three materials are used to create these shields:

• Indium Tin Oxide (ITO) coated polyester
• Aluminum Foil
• Printed Shields

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.

Aluminum foil is typically clad to a polyester carrier. It can achieve superior attenuation of radio frequency interference to printed shields. AutoShield aluminum clad polyester achieves a 20db greater attenuation of frequencies greater than 1GHz than does .040” wide traces printed on a .240” spacing with a typical conductive silver ink. (see Autoshield pdf)

Membrane Switch Use Considerations. If properly designed and manufactured membrane switches are very reliable system components. To achieve maximum reliability, membrane switch users should:

• Avoid creasing the switch tail as this can result in broken circuits or high loop resistance. Tails should not be bent around a radius of less than .020”. If desired Melrose can supply the membrane switch with a preformed radius.

• Keep the switches in a flat condition prior to and during installation. Bending the membrane switch backwards in a dome area can result in dome failure or unseating of domes from their nominal position. If membrane switches are bent in areas where LED’s are placed, the LED’s may become detached from the circuit.

• Membrane switches should be applied to clean, dry, oil free surfaces if pressure sensitive adhesives are used. Maximum adhesive bond strength is not achieved until approximately 72 hour after application. Of course specifying the proper adhesive for the applied surface, and environmental conditions is essential. Melrose can assist you in selecting the proper adhesive. This link will take you to more information on adhesives.