A membrane switch (also referred to as a membrane keypad) is an HMI (Human Machine Interface) that can be found just about everywhere; the buttons on your microwave, on the pump at a gas station, on the side of hospital beds. They are the buttons you push to increase the steam level in the steam room at the spa, or powering on machines in a factory. People use membrane switches on a daily basis without even realizing it! So what exactly is a membrane switch?
A membrane switch assembly is comprised of several flexible layers that make up a complete switch package. This assembly can have as few as one active area (key or button), or several dozen active areas. How many keys or buttons is only limited by the physical size of the finished application.
A membrane switch is built in such a way to provide a momentary contact when force is applied to a key area. The most common materials that are used in the construction of a membrane are polyester (PET) and pressure sensitive adhesive (PSA). There are two polyester layers required, these layers are selectively printed with conductive inks and separated by an adhesive layer that has been die cut as to provide an opening between the polyester layers. In its most basic form, when a human finger pushes on an active area, one layer of polyester flexes or moves through the opening in the adhesive layer and shorts to the other, stable, polyester layer causing the momentary contact. The distance that the polyester travels can be as small as .007”, once the force is removed, the switch opens again.
Telling the user where to push and what to expect is the job of an additional layer, the graphic or overlay. This is the top portion of a membrane switch assembly and is the only portion that the end user will see once a switch is mounted to an appliance, medical instrument or industrial control panel. It provides a layer of protection for the switch below and also conveys information to the end user. Graphic layers are screen printed and can have as many colors as needed; they can be simple or elaborate. Key areas can be labeled with text or icons, and rings or borders can be printed around the key areas. Company logos or other symbols can be added in non-active areas. Clear windows can also be incorporated into the graphic if a LCD or other type of display is mounted behind the switch/graphic. Just by changing background colors, a different look can be achieved if a product line has different color combinations much like the appliance industry.
An important feature of a membrane switch is the interface with the electronics that it is controlling. Membranes will have some sort of flexible tail that just plugs into the electronics that are behind. Tails can come with connector housings attached (.100” center, female) or could be built as a zero insertion force tail that plugs into connectors with as small as a 1mm pitch on the electronics board.
Another twist on the basic membrane construction is to swap the stable polyester layer (the non-moving layer) with a printed circuit board or kapton circuit. This provides other options such as attaching displays or mounting hardware directly to the printed circuit board part of the membrane switch assembly.
Other features or components can be added to enhance the end user experience, these include:
- Tactile metal domes for a more “snappy” type of feel when pressing the switch.
- Embossed keys to give the active area a raised feel.
- Customized graphics that can show functional aspects of the switch as well as branding information.
- Indicator LEDs to show when a switch has been closed (activated).
- Backlit areas to illuminate icons.
- Gaskets to prevent water ingress.
- Polyester or Dielectric tail covers to protect traces on the switch tail.
- XyWeld construction for the ultimate in water ingress protection.
- Backing panels or bezels.
Because they are lightweight and flexible, membrane switch assemblies might be thought to be products that are only fit for a home or office environment but that couldn’t be further from the truth. When designed properly, with the correct materials, membranes can stand up to some of the toughest challenges. Outdoor applications that see extreme temperatures, humidity, and wash down tests are not a problem!
Membrane switches have been a versatile and durable option for human to machine interfaces since the late 1970’s and are still used across the globe today.