switch. Standard tolerances should be
The switch layers under the overlay will typically be fabricated smaller than the overlay. This allows for die cutting and assembly tolerances. All layers will typically be
.015" inset from the overlay at all edges and cutouts.
Laser Cutting
The various layers of a membrane switch can be cut out by using a numerically controlled laser. This technology offers two advantages. Tighter mechanical tolerances can be held, and no tooling is required. While laser cutting is a more expensive process than die cutting, in many low and medium volume applications it is quite cost effective.
Pinouts
The schematic or pinout of a switch may be specified by the customer, but if it is initially unimportant to the customer, then the pinout should be unspecified. As with any circuit layout, more freedom will allow us to produce a more efficient layout. This has the advantage of shorter development time and a simpler circuit layout, which could nominally affect switch reliability. Membrane switches can be designed with a common bus or in a matrix. Matrix layouts are desirable for keyboards with many keys to simplify the interconnect.
ESD/RFI Shielding
Several options are available for shielding membrane switches. The most common are printed carbon, printed silver, and aluminum foil. From a functional standpoint, the main difference among these materials is their conductivity. Either carbon or silver can be printed on the top of the top circuit to act as a shield. These shields have the advantage of not adding any additional layers to the switch construction. Carbon shields are less expensive than silver shields. Silver is usually printed in a grid pattern to reduce cost. A layer of aluminum foil can also be added above the top circuit. This material is the most conductive shield available. However, it does add two layers to the switch construction. The shield is usually connected to the ground through the connector, or by means of a tab with a slot for a fastener. The customer should express their shielding requirements in Ohms per square inch. The entire product packaging must be
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