Backlit Membrane Switches: Design Options and Considerations

Backlighting options are often used in membrane switch design. This is a visual effect, used to highlight or draw attention to certain key areas of the graphic overlay or to aid visibility in low lighting conditions. There are several options for incorporating backlighting into a membrane switch assembly, each with advantages and disadvantages for certain applications.
Benefits and Uses of Backlighting in Membrane Switch Design

There are several reasons for incorporating backlighting in a membrane switch assembly. If the final product or device will often be used in dim lighting conditions, backlighting can make the graphic overlay elements more visible, enhancing usability. Unless users have committed the location of various controls to memory, using a device in poor lighting can be a frustrating experience.
In some applications, backlighting aids users in identifying the correct control options or serves as an indicator that certain functions are active. This adds usability to devices for any user, but particularly for special populations where users have difficulty reading smaller fonts and interpreting other types of visual cues. When certain controls are illuminated based on current control settings, users can more easily identify the appropriate keys.
Backlighting Design Considerations
Backlighting can increase power consumption and heat production, two factors that influence the selection of backlighting technology. Some backlighting technologies are available today which require little power, ideal for both battery-powered devices and general applications in which minimizing power demands are desirable. Additionally, the availability of backlighting technology that produces little heat is generally valuable for most product designs and alleviates some thermal considerations for the functionality of various components, such as PCBs (printed circuit boards).
Backlighting also plays a critical role in the aesthetics of a membrane switch. Backlit displays have a modern feel for many users, and can enhance the visual appeal and general perception of a product. But backlighting options should be considered and chosen carefully based on the usability requirements and general layout of the graphic overlay. When the design is not planned carefully, backlighting can have the opposite effect and actually detract from the aesthetic appeal or even make the display less intuitive, particularly in complex designs.
LED (Light Emitting Diodes) Backlighting
LEDs are surface-mounted, using silver conductive epoxy, and are best-suited for selectively backlighting text and graphics, rather than backlighting the full graphic overlay or display. LEDs are, typically, a cost-effective alternative to other backlighting options.
LEDs are efficient, and they radiate little heat. LED backlighting can be configured using a single color or multiple colors, depending on the desired result. This backlighting technology is used under polyester graphics or rubber keypads.
Embedded LEDs are most often used to illuminate small areas of a membrane switch or small indicator windows. They can be embedded on the flexible circuit of a membrane switch , making them a practical option when illuminating a single icon or small portion of a graphic overlay is desired. But because the LEDs are close in proximity to the graphic overlay, using embedded LEDs as backlighting for larger areas or to illuminate an entire graphic overlay is not as cost-effective as other backlighting options, in most cases.
Fiber Optic Backlighting

Fiber optic backlighting uses optical fibers, which are glass or plastic fibers that carry light along their lengths. Fiber optic panels are often used in membrane switch designs where uniform lighting is desired in larger areas.

Sheets of fiber optic cloth are ideal for use in membrane switch design. Fiber optic cloth can be cut into specific shapes and sizes, creating a thin, flexible lighting layer that is placed between the graphic overlay and the dome switches.
Using this process, the optical fibers extending from one end of the panel are bundled into a circular ferrule, which is then coupled to one or more LED light sources. Because LEDs offer a long life span and the configuration used in fiber optic lighting allows for easy replacement of the LED light source, fiber optic backlighting offers an impressive life span that, when considering the ease of LED replacement, is virtually endless.
Fiber optic backlighting is a desirable option when suitable for the application. In addition to the extended life span of this backlighting technology, fiber optic backlighting consumes little power and a compact size that’s easily integrated within a membrane switch assembly. Additionally, fiber optic backlighting is low-heat, creates no electromagnetic interference, and offers exceptional durability, as well as design flexibility and reasonable cost.
Electroluminescence (EL) Backlighting
EL backlighting is a unique technology, using a printable ink embedded with light-emitting phosphors. It’s a very thin layer which, like fiber optic panels, can be cut into any desired size and shape and easily layered within a membrane switch assembly. Electroluminescence backlighting emits very little heat.
EL backlighting is used in a variety of electronic devices, including the widely recognized Timex Indiglo Watch. You’ll also see this backlighting technology used in applications such as automotive dashboards, mobile devices, GPS devices, and pagers.
EL backlighting typically costs more than backlighting technologies like LEDs and fiber optic backlighting, but the consistent, even illumination possible across large areas is highly desirable in some applications. Because EL backlighting can be applied selectively, it also allows for design flexibility and customization.
The typical position of the EL layer, directly beneath the graphic overlay but above the circuit layer, also circumvents some design challenges in applying backlighting to keypad areas in certain applications. For example, conductive ink and metal domes used to add tactile feedback do not allow light to pass through, making EL backlighting a viable option in applications using these components. There are other design options for getting around this issue, such as the use of clear conductive ink and poly domes, but the passing of light is one of many critical considerations in membrane switch design.
With several backlighting options available for membrane switch technologies, the right combination of elements will add usability and enhance aesthetic appeal in electronic devices. But as each backlighting technology has advantages and disadvantages, your membrane switch design team is a valuable resource to help analyze your requirements and the design considerations that come into play when configuring the proper backlighting options to produce the desired result.
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