Panel of Expertise

Douglas Sellers, technical manager at Danielson, discusses the design issues faced when specifying flexible membranes for front panel design

Designers and manufacturers are continually seeking reliable and cost effective front panels for Man Machine Interfaces (MMIs), to reduce both production and assembly costs, and combine enhanced performance with greater reliability. Although membrane systems have been used in front panels for some years, the technology has continued to evolve and develop in order to meet modern demands.

Consequently, membranes today, are able to offer greater flexibility, longer operational life, improved performance, and a wider range of design and overlay protection. As a result of increasing levels of automation in all areas of industry and leisure, MMIs, in particular the front panel presented to the user, are subjected to an increasing range of environments, stresses and strains. This, in turn, creates a number of key factors to be considered when specifying and designing a membrane system.

The modern membrane panel is a simple but effective concept. Two flexible circuits are held a short distance apart, and when pressure on defined key areas pushes the circuits together, a connection is made.

Unlike other switch arrays, which comprise bulky assemblies of discrete standard components, the membrane panel normally contains a number of laminated layers integrated within a single, custom designed structure, frequently containing a membrane switch. This switch is available in a variety of constructions to suit individual applications. Sometimes products such as Electroluminescent (EL) lighting may compliment the switch for applications with low ambient lighting levels, with surface mount LEDs, PC boards, encoders and touch screens, to provide a total integrated solution.

The environment in which the panel is used and the substances it may be exposed to, are important factors when specifying design. Typically, these can be solid objects, moisture, mechanical impact, levels and fluctuations in temperature, humidity and chemical exposure.

Solid objects and moisture are generally specified using the International Protection (IP) system whereby the levels of protection provided by an enclosure are classified.

Generally, the materials and adhesives holding the layers together in membrane panel construction offer good moisture and dust protection, although an additional, heavy duty, unbroken seal around all layers of the structure will ensure compliance with the most stringent IP rating. One critical factor is the point at which the circuit tail exits the panel; this is often a weak spot for moisture and dust ingress. In these cases a gasket can overcome the problem.

MECHANICAL IMPACT

In applications where impact damage or scarring, caused by tools or excessive wear, may occur, membrane panels can be combined with a wide range of high performance overlays, usually polycarbonate, polyester or Bayfol, to provide additional protection. In addition, overlays can incorporate hard coatings, antiglare or textured finishes and graphic images; fine-textured polyester is particularly suitable for the selective application of gloss or anti-glare coatings to window areas.

Occasionally, where multiple language options are available, overlays need to be available in a number of different formats. The use of removable inserts is a cost effective method for eliminating the need to print separate overlays for each task. It is possible for the inserts to be positioned directly over switches with no discernible increase in actuation force. An additional option available with overlays, which ensures more pronounced finger location and greater visual impact, is embossed and debossed features. It should be noted that some loss of tactile effect may occur where keys are small or densely packed.

All membrane switches are constructed with a small pocket of air in the cavity between the two contacts. When a panel is subjected to frequent severe temperature changes, the expansion and contraction of these air reservoirs may cause delamination of the electrical switch contacts. One possible solution is to vent the keys to the atmosphere, although this may not be suitable where moisture ingress is critical. In these situations, a larger, sealed pocket of air may be constructed within the panel itself to form an internal air reservoir and minimise pressure level fluctuations. High temperature operation may demand special materials, offering viability up to 70 degrees C.

CHEMICAL EXPOSURE

On the whole, the principle overlay material specified for protection against chemicals, is polyester. Currently, the types of materials in use offer almost total protection against short-term exposure to chemicals normally encountered by membrane panels in use, including solvents, acids and alkalis. Vented panels may require additional protection against air-borne gaseous attack by the use of selective materials and careful circuit design.

In certain applications, the feel of a switch is important. The operator needs to know that the actuation has occurred, and the equipment will subsequently respond, especially in cases where there is no obvious visual or audible response.

Membrane front panels can be designed to incorporate a metal or plastic domed switch within the structure to provide a tactile response, especially useful for an operator encumbered by gloves. Polyester switch domes are made using proven processes to develop switches that give extended key travel and stability across a range of operating temperatures, from -30 degrees C to +70 degrees C, and which are designed to have an operating life in excess of three million actuations.

Alternatively, metal domes can be used for high temperature applications. To improve performance still further, these can be selectively plated with gold to improve circuit characteristics and can feature an anti-inversion pip to eliminate any possibility of the domes becoming permanently inverted when depressed.

In both cases, the domes can either be passive, where they sit over a normal flat switch to give an enhanced tactile feel, or active, where the domes form part of the circuit when actuated. Active domes are generally more reliable, while passive versions may be specified where electrical regulations require laminated protection within the membrane. It should be noted that a typical unembossed 125 micro m thick overlay has a dielectric strength of 15.6kV.

HOW SHOULD THE PANEL LOOK?

Overlay design is an important facet of any membrane panel. Careful consideration must be given to the choice of materials, textures, mechanical and graphical effects that ensure the best solution for the application.

NEW TECHNOLOGY

New technology often plays an important part in the type of front panel specification available, with membrane panel companies investing in new equipment and processes in order to push front panel design forward and offer greater economic solutions.

For example, innovative technology capable of quickly and accurately placing surface mount components and conductive adhesive onto membrane panels, dramatically cuts costs, as production time and reject rates are reduced.