When appliance manufacturers adopted membrane switches as the standard interface for microwave ovens, they did more than just settle on a design solution for a kitchen appliance. They also established a model for how the technology could be employed in a wide variety of applications with similar interface requirements, fueling the migration of membrane switches into a diverse array of product segments.

The most obvious virtue of a membrane switch — an environmentally sealed, easily cleaned surface — soon proved beneficial in commercial appliances, medical equipment, business machines, fitness equipment, and more.

Membrane switches offer a number of advantages over mechanical switches, including longer life and lower cost. Furthermore, a thin, space-saving profile measuring only a fraction of an inch, makes it simple to design membrane switches into an application. Another significant plus is that the core technology behind membrane switches permits them to be easily and cost-effectively customized for a product, from both a functional and aesthetic standpoint.

A membrane switch generally consists of two polyester films that are screen-printed with a conductive ink on the sides that face each other to form the circuit layers of the switch. A spacer, which is die-cut at contact points, is then sandwiched between the circuits. The die-cut holes permit a touch of the finger to close the contact, which in turn completes the circuit. Upon release, the circuit is broken. A graphic overlay is added to the membrane switch to allow the user to identify specific locations for switch contacts. The panel back is laminated with an adhesive for mounting to a backer or enclosure, and is terminated with a means for interfacing with the customer’s electronics.

Enlarge this picture Cross section view of ClickTouch technology that delivers strong tactile feedback without requiring a metal dome. The level of response can be varied from hard to soft. (click to enlarge)

It is true that early generations of membrane switches lacked the reliability of current products due to occasional deficiencies in, and incompatibility of, various materials. But since then, suppliers have made significant improvement in the quality of materials and properly matching them, resolving most of those reliability issues.

OEMs today have a wide range of options available to them when specifying a membrane switch, so it is important for designers to define the requirements of the application early in the product development process. In addition to considering the physical configuration and appearance of the component, design engineers must establish desired life expectancy in terms of number of actuations, desired resistance to abrasion and chemicals, expected temperature range of operating environment, desired tactile response, and what type of illumination is desired, if any. There are even shielding options available to prevent electromagnetic interference or damage from electrostatic discharge, both key considerations in medical equipment and other sensitive electronic devices. And, as always, these choices must be evaluated within the framework of cost targets.

Overlays

ClickTouch membrane panel utilizing electroluminescent backlighting to enhance key identification.

The selection and design of the overlay is one of the most important decisions, as the overlay is the part of the assembly that the user sees and touches. The overlay displays the informational and decorative graphics, which are typically printed on the backside to protect them from being worn or scratched off.

Common overlay material choices include polyester, polycarbonate and acrylics, which are available in different thicknesses, gloss levels, textures, and degrees of pencil hardness. Polycarbonate is more flexible, but polyester delivers longer life, so the expected number of actuations is an important criterion in overlay selection. In some tests, polyester overlays have been actuated more than 1 million times without showing signs of wear.

Other overlay materials such as polyvinyl fluoride and silicone rubber can provide enhanced resistance to harsh environments and weather, making these suitable for outdoor or industrial applications.

Thickness of the overlay material also affects both durability and tactile response. Overlays typically range from 0.005 in. to 0.015 in. The lower end of the range provides better tactile response, while the higher end delivers longer life.

Different overlay coatings are available that can improve the gloss or resistance to abrasion and chemicals. Newer film materials aimed at the medical market can even include antimicrobial properties.

Overlays can also be embossed to achieve various effects, such as raised pads, domes, or raised edges to serve as locator features. Maximum embossing depth is limited by the overlay thickness, and the shape of the embossment must be curvilinear to avoid creating stress points.

Tactile response

Embossed membrane switch with silicone overlay and display window, made by Jayco.

A number of things can influence the tactile response felt by the user when operating a membrane switch. In a standard switch, the degree of required deflection, or travel distance, combined with the required amount of force to actuate the switch, will be the two things sensed by the user. Actuation travel is determined by the thickness of the circuit spacer between the two films. Actuation force is influenced by the combination of spacer thickness, the overlay thickness, and the diameter of the hole in the spacer. For example, the lowest actuation force would be achieved with a thin overlay, thin spacer, and large diameter hole.

When tactile response is low, audio feedback (beepers) and/or visual (lights) feedback components are often incorporated into the application.

In many applications, a higher degree of tactile feedback is considered desirable, either to give the product an upscale “feel,” or to provide more verification when a product is used in a noisy environment where audio feedback might not be heard.

Such applications often employ either polyester or metal domes to provide a “snap-action” tactile response. Actuation forces for these domes can be customized by changing either the diameter or height of the dome. Metal domes, which are more expensive, typically provide a crisper response, along with superior reliability and temperature resistance.

Suppliers of membrane switches may also offer some unique or specialized tactile feedback options. For example, ClickTouch America, Saint-Laurent, Canada, has developed a double-touch key that can support two functions. When pressing the key, the operator receives the normal click from actuating the first function. When the operator presses harder, a second, non-tactile contact closes, activating a second function. In a complex application, double-touch keys can reduce the number of input locations and help the designer create a smaller interface surface area.

Illumination

As shown here, Jayco can apply texturing to selected areas to create a striking visual effect on a membrane switch panel.

Different illumination options are also available for membrane switches. Surface-mount LEDs can be bonded to the circuit layer to provide point-of-contact visual feedback for actuation. In membrane switch assemblies backed by a PCB, a wider variety of lighting configurations can be employed.

For general backlighting illumination, where the product may be operated in low-light situations, a thin, electroluminescent lamp can be incorporated into the assembly to highlight large touch areas. Common colors for EL lamps include, blue, green, white and amber, but additional colors can also be specified.

Other design approaches involve the use of light guides or fiber optics to transport the light from its source to the point of emission.

Another option that can be employed in conjunction with illumination is the use of a deadfront panel. On a deadfront panel, the information it contains remains invisible until illuminated from behind. This technique can be used to help avoid visual clutter on the interface, and allow the user to focus on the selected function. It would be particularly beneficial in critical applications, such as medical devices, where the user has to operate swiftly without distraction, or become quickly aware of warning messages.

Application diversity

For this hand-held terminal product, White Electronic Desgins integrated elastomer and membrane keypads, an LCD, a microcontroller, printer, and printed circuit board into a single package.

Stock, off-the-shelf membrane switch products may be used for generic interfaces requiring simple inputs such as on/off, alphanumeric, or basic up/down or left/right scrolling in conjunction with a display. But the true strength of membrane switch technology lies in its ability to be customized for so many diverse applications and operating conditions.

As with specifying any other customized component, OEMs should involve the membrane switch supplier early in the product development process. This will not only allow the supplier to help produce an optimal membrane switch design, but also to aid the OEM to design the product to which the membrane switch assembly must be attached. This is especially true in applications where sealing is essential, since a chain is only as strong as its weakest link.

For example, Memtron Input Components, Frankenmuth, Mich., assisted a customer designing a beverage-dispensing gun that is used in bars and restaurants. The challenge was to provide a moisture-resistant switch on a small mounting area with a high key population. The keypad size was 1.75 in. x 3.5 in., and it contained 18 keys.

The solution was to create an internally vented switch with a vented tail design. The graphic overlay was embossed with an egg-crate pattern that permitted non-visual key identification without compromising the adhesive properties required for the high-moisture environment.

“Our customer came to us looking for a solution to the old mechanical key design that was failing in the field due to soda getting into the mechanical keys, causing them to stick and fail. The customer came to us early, and listened to our design recommendations regarding the plastic enclosure that the membrane mounted to,” says David Hildner national sales manager, Memtron Input Components. “They took our advice, and used a tooled gun enclosure that mated well with the membrane and performed admirably in the field.”

Listening to supplier suggestions is even more critical when the switch and product are destined for tough environments.

“Harsh environments always present greater challenges because the membrane switch alone does not completely protect the end product from the elements,” Hildner notes. “We have to hope that our customer took our recommendations into consideration when developing their device to ensure the entire unit will function well in the intended environment.”

Future

For this medical application, White Electronic Designs mounted elastomer keypads over sealed membrane keypad circuitry to create a system resistant to contaminants.

Over the years, the membrane switch has evolved from an upstart innovation to a reliable workhorse serving many industries. Now, as a mature technology, the prevalence of the membrane switch is threatened by many other alternative interface technologies, such as resistive touch screens and capacitance-based touch controls. But membrane switches won’t be disappearing any time soon. For one, the technology still beats most of its competitors on cost. Secondly, suppliers are constantly coming up with new ways to improve the technology, often fueled by customers’ increasing demands.

“There are numerous challenges with custom designs today where customers want all the features they have ever seen, packaged into half the size of last year’s model, for half the price,” says Dwayne Anstead, national sales manager, Jayco Interface Technology, Corona, Calif. “Some of the most specific challenges include matching cost expectation with features and options, then putting them into smaller panel sizes that push the limits of the designers and materials to the very edge of the technology.”

Another way to beat competitive technologies is to imitate them. For example, the Interface Electronics Div. of White Electronic Designs, Phoenix, has developed an innovative membrane switch product called SimTouch. The SimTouch membrane switch combines a flexible overlay with a very thin switch construction, significantly reducing the travel distance and actuation force needed to close the switch, thus simulating the feel of a touch control.

Furthermore, the SimTouch employs a rich-looking overlay with a glassy appearance. In essence, the SimTouch simulates the look and feel of a glass-capacitance touch control, while still offering the benefits of lower-cost membrane switch technology. The overlay on the SimTouch can also be metallized to simulate a stainless-steel touch control. Innovations like the SimTouch and others, along with the never-ending pressure to reduce costs, will likely keep membrane switch a popular choice for many design engineers for a long time to come.