How to Design Membrane Switches for Harsh Environments

Membrane switches are prominent in modern electronics devices, providing ample design flexibility to create user interfaces that perfectly balance functional requirements and user expectations and demand for intuitive controls. The standard materials and design options work well enough for most applications, but there are special considerations to weigh when designing membrane switches for devices that are used in or exposed frequently to harsh environments.

Always Consider the End-Use Environment

First and foremost, engineers designing a membrane switch for any purpose should carefully consider the environment in which the device will ultimately be used. For devices that may be used in a range of environmental conditions and situations, you should design based on the worst-case scenario to ensure robust durability in all use cases.

It’s important to realize that many design options intended to add usability or to enhance the user experience may not be practical for certain end-use environments. Design options that primarily exist to enhance usability include features such as tactile feedback, embossed graphic overlays, and embedded LED lighting. Embossing and tactile feedback features can suffer from stress under continual outdoor use, where materials are exposed to moisture, UV rays, and even extreme temperatures. This environmental exposure can stress the materials to the point where they crack or fracture, leading to premature switch failure. There are several key environmental conditions that can pose particular challenges in membrane switch design:

• Continued exposure to moisture – water droplets, spray, submersion, or even extreme humidity

• The need to use harsh cleaning solutions and caustic chemicals

• Exposure to extreme heat or cold

• Dust and debris, often resulting from outdoor use

• Atmospheric pressure, which can be a problem for devices used at high altitudes.

It’s critical for engineers to weigh the benefits of appealing ergonomic features against the potential for a shorter membrane switch lifespan based on the end-use environment. There are alternative materials and design options that can offer some degree of the desired effects for end users while providing more robust, durable solutions that can withstand regular exposure to the elements.

Careful Material Selection Is a Must

As membrane switch assemblies comprise many layers, each serving a specific function, the integrity (or lack thereof) of each material used in each layer can lead to a complete failure of the switch as a whole. Materials, therefore, must be carefully selected and individually evaluated in terms of durability within the context of the end-use environment. The wrong adhesive can wear down with exposure to the elements and allow dust, debris, and moisture to access critical circuit layers.

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While every material used in a membrane switch should be carefully selected, durable overlay materials and adhesives play the biggest role in preventing contaminants and environmental elements from reaching the sensitive inner layers of the assembly. In some cases, an impervious bond can be created with the use of an adhesive sealing process, which lends further integrity to the adhesive layer and allows it to stand up to the harsh environmental conditions that can wreak havoc on standard adhesive options.

In addition, there are a variety of materials and components that offer greater durability for devices likely to be exposed to harsh end-use environments. Silicone rubber keypads, for instance, are suitable for some applications, providing a rugged seal to protect sensitive underlying components from exposure to moisture, dust, harsh chemicals, and cleaning solutions. There are also membrane switch assemblies that offer exceptional moisture resistance, meeting NEMA 4 and IP 67 specifications.

Mounting Considerations Play a Role in Overall Durability

As equally important as the materials used in constructing the membrane switch are the mounting options used to mount the assembly to the device. This may mean a bezel, recessed area, cutout opening for the membrane switch assembly, or a case, depending on the design of the device or equipment for which the membrane switch assembly is intended.

One such consideration is the mechanical dimensional tolerance. It’s important to remember that membrane switch panels are not fabricated using metal fabrication equipment. Therefore, associating the membrane switch with the typical metal fabrication industry tolerance of +/- .005 inch is a grave mistake.

Membrane switch panels are typically cut with steel rule dies and low-to-medium-speed gantry-style CO2 lasers, resulting in a dimensional tolerance of +/- .010 inch. But that’s not all: The bezel openings, recessed mounting areas, and other mounting options also introduce tolerances, so the total stack-up tolerance is really what engineers should consider. A membrane switch panel that fits too tightly into equipment or has gaps between the membrane switch panel and the housing can lead to a multitude of problems, almost all of which will contribute to premature device failure.

Finally, when choosing the proper adhesive, you should also consider the surface to which the membrane switch will bond on the device. Engineers should consider the surface energy as well as the finish of the mounting surface in order to choose the proper adhesive that will stand up to the conditions expected in the end-use environment. Low surface energy materials (e.g., polypropylene and powder-coated paints) require modified acrylic adhesives, while high surface energy materials (e.g., aluminum, polycarbonate, and ABS) require acrylic adhesives. Likewise, some mounting surfaces have smooth finishes, whereas others are rough and textured, which will require a thicker adhesive caliper to maximize the surface area.

There are many design options in modern membrane switch design, including a variety of adhesive, lighting, and sealing options that lend themselves to greater durability in a multitude of environments. Carefully considering the materials and production options within the context of the end-use environment, as well as giving equal consideration to every layer and component, will result in a more robust, durable membrane switch assembly that can stand up to anticipated environmental conditions and withstand the test of time.

Trey Hooper is vice president of sales and marketing at Pannam Imaging, a maker of custom membrane switch assemblies. Trey has more than 32 years in the membrane switch and product identification industries. He contributes regularly to the Pannam Imaging Blog, offering advice and tips on membrane switch design and manufacturing, as well as reporting on industry news and trends.