Mix & match for theperfect switch panel

April 4, 2002
An assembly technique called island construction integrates different magnetic switch elements into a single flatpanel assembly.

By David VanZoest
Tony VanZeeland
Duraswitch Industries Inc.
Mesa, Ariz.

Edited by Miles Budimir

In a rotary switch, two magnetically attracted balls function as the armature. The balls are attracted to the conductor side of the film layer as well as to each other by a magnetic field of opposing polarity. Moving the actuator rolls the individual balls into and out of contact with fixed switch contacts on the film layer. The rolling action of the balls maintains good electrical contact and produces low wear and erosion of the individual switch contacts.

Directional switches use a multidirectional armature magnetically held in place. The armature is a single stamped piece of metal with a center contact pad and several outer contact pads. When not actuated, it is held in place by two magnets on either side of the armature. A substrate below the armature contains electrical contacts which can be laid out in any number of ways depending on the application.

A magnetically coupled pushbutton switch uses a bonded ferrite magnet and a plated steel moving contact armature.

Traditional appliance control panels are made of a variety of switching elements such as pushbutton, rotary, sliding-type encoders, and directional switches. These get connected using wire harnesses, solder, or conductive adhesives. The connections, however, often break down from normal wear and tear. A new switch assembly technique promises to eliminate these interconnect woes.

With the technique called island construction, engineers can build different switch types into single flat-panel circuits less than 3-mm thick. Individual switching elements are packed together in a thin adhesive foam matrix. The foam provides structural integrity as well as environmental sealing. VHB acrylic foam from 3M is used for harsh environments, and Volara fine-cell, cross-linked polyethylene foam is used in general-purpose cases.

Island construction differs from other fabrication techniques because switching elements are inserted into individual cutouts in the foam. The foam fills in the spaces between the individual switching elements, which eliminates the need for failure-prone interconnections such as solder, conductive adhesives, or wiring harnesses.

The switch circuitry is either screen printed or etched onto the substrate layer to form the entire circuit. The screen-printing processes are those typically used in the membrane-switch industry. In a nutshell, the process works this way: circuits are printed on industry-standard 0.005-in. polyester with conductive inks that can be silver-based, carbon-based, or a blend of the two. Copper circuits can be etched onto either rigid printed-circuit boards or on flexible substrates like polyester or polyamide.

A switch overlay, or graphics layer, is then placed over the entire assembly where it is held in place and sealed from the environment by the foam adhesive layer. Tests show this sealing technique is effective in water, harsh chemicals, and sodas for extensive periods without damaging the assembly seal.

Even conventional membrane switches can be used with island construction. But these have drawbacks in mounting and packaging, for example. Also, it's difficult to easily integrate pushbutton and rotary switches onto the same panel.

Thickness is another issue. A typical island-switch panel is only 3-mm thick yet includes the substrate, foam matrix, pushbutton elements, and the graphics overlay. Even the thinnest electromechanical rotary switches are double the thickness.

Magnetic switches are best suited for island constructions. Pushbutton switches are magnetically coupled using a bonded ferrite magnet and a plated-steel armature. The magnet holds the armature away from the fixed contact pads until actuated. When released, the switch functions as a magnetic return spring. The magnetic spring action provides an almost limitless life cycle, with lab tests demonstrating a life of over a half-billion cycles.

Plus, a hemispherical-on-flat contact system is preferred over the butt-type, flat-on-flat contacts commonly used in traditional membrane, metal dome, and elastomeric dome switches which tend to trap debris. The hemispherical contact shape tends to push debris out of the way. Also, there is some redundancy because there is more than one contact.

Rotary encoders use magnets as an integral part of the design. Switch armatures are attracted to the film layer by a field generated by magnets in the actuator. As the actuator moves, the armature follows the magnets and maintains contact with the substrate on the opposite side of the film layer. Electrical contacts are printed or etched on the armature side of the film layer. This seals the switch armature and the electrical contacts from the environment. It also gives good electrical contact and produces low wear and erosion of the switch contacts.

Pushbutton arrays were previously built on a membrane surface. An electromechanical rotary switch was added by drilling a hole in the backing plate and mounting the switch with a nut. A wiring harness or circuit board was then attached. With island construction, however, the rotary switch is dropped directly into the foam matrix, eliminating the need for mechanical fasteners.

Directional switches use a multidirectional armature that is magnetically held in place. These switches are used in appliances as temperature selectors, for time control and other specific functions. These might include switches with arrows indicating up/down or right/left selection.

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