Membrane technology offers design opportunities

Developments in membrane switch technology can help designers improve the performance, reliability and functionality of products and systems, says Andy Birch, Production Manager for Danielson.

Developments in membrane switch technology can help designers improve the performance, reliability and functionality of products and systems, says Andy Birch, Production Manager for Danielson.

Membrane switches and keyboards are used in a wide range of industrial and consumer products, from process instrumentation and machine controls to washing machines, refrigerators and microwaves.

Although membrane technology has evolved considerably since it was first used in low cost children's games and toys, there are still applications where traditional types of switch have until now been preferred or where requirements such as backlighting, close switch spacing or key size have limited the choice of membrane switch.

Now, however, recent developments in membrane technology designed to increase operating life, functionality and overall performance are beginning to extend still further the application of membrane switches and keyboards, into areas such as the automotive, aerospace and telecommunications sectors.

Backlighting membrane keys has in particular always presented a challenge to product designers, as it can be difficult to avoid creating shadows from circuit tracks on upper circuit layers while achieving even illumination at high light outputs.

Conventionally, backlighting has been achieved by the use of fibre optics, light guides, point light sources or electroluminescent lamps.

Although these systems can provide a reasonable level of illumination, they each have their limitations; for example, light guides often produce hot spots or are difficult to assemble in large volumes, while EL lamps have a relatively short operating life at high outputs and require separate inverters to be incorporated, thereby increasing overall costs and complexity.

To overcome these issues a new method of low voltage backlighting has been developed that ensures consistent and even light distribution, without affecting the tactile response or other properties of the keyboard.

Perhaps as importantly, it has an exceptionally long operating life and, as it can be manufactured using traditional assembly methods, can easily be produced in small and large volumes at low unit cost.

The new system consists of a custom designed plastic layer that is inserted either directly beneath the normal outer protective graphics layer of a standard membrane keyboard or below the switch and tactile layers.

Illumination is provided by LEDs that are positioned within specially machined pockets in the base of the custom layer and which act as the light source for individual switches, groups of switches or graphics areas.

Although this construction is similar to that of competing light guides the new system incorporates a series of innovative plastic switch actuators that, through a special shape and coating, allow the light path and degree of diffusion to be carefully controlled.

This is achieved without affecting the normal operating life of the switch, interfering with switch functions or significant reduction in lamp intensity.

This new lighting solution creates extremely even lighting patterns, while enabling light to be directed precisely into those areas that require illumination.

In addition, the structure enables fewer LEDs to be used than would normally be the case with light guides and offers the option of using different coloured LEDs within the same keyboard.

In real terms, the unit cost of LEDs has fallen considerably in recent years and this has led to an increase in their use within all types of keyboards.

In membrane keyboards LEDs are often mounted either on the lower circuit layer or some form of base layer to project upwards into the graphics overlay.

Adding extra components such as LEDs, extending the functionality of the keyboard or increasing the packing of membrane keys does, however, have an impact on the design and layout of switch circuits.

Circuit design can become especially complex if tactile keys with metal switch domes are being used, as sufficient space must be left around each key to enable the switch mechanism to function.

This type of construction increases the overall switch dimensions and in keyboards with large numbers of keys can significantly reduce the space available for routing circuit tracks; this in turn leads to tracks being tightly packed causing potential problems with interference and increasing production costs.

One solution is to redesign the layout of the circuit layers so that the upper circuit is positioned above the tactile metal dome and the lower layer beneath it.

Pressing the switch therefore forces the upper circuit into contact with the dome, which then depresses, makes contact with the lower layer and completes the circuit.

This vertical construction effectively allows the entire switch mechanism to be confined to within the diameter of the dome, reducing the total area required by up to 50%.

In addition, through the use of a shaped dome retention collar, that fits between the circuits, it helps to eliminate movement at the base of the dome that can otherwise lead to dome misalignment or movement, causing unnecessary wear on the lower circuit tracks.

The advantage of this type of switch design is that it retains all the features of tactile metal domes, without affecting keyboard performance, functionality or reliability; it also frees up more space for the switch circuits.

One final point worth noting is that developments in digital print technology now make it possible both to produce high quality working switch prototypes far faster than ever before - typically within days rather than weeks - and to enhance the quality and variety of keyboard graphics by incorporating four colour half tone images.

In many sectors of industry membrane technology has already had a major impact on product design.

These new developments in backlighting, circuit construction and digital print now look set to extend that impact still further.

Back to top