What’s Changed?

Membrane switches were first adopted by industries that valued their inherent ability to resist moisture and liquid egress. Furthermore, the low profile and nature of these input devices allows for easy cleaning, making them an excellent choice for products such as appliances and medical instrumentation. Though membrane switches are still in high demand for their ability to deliver these unique qualities, a new trend is starting to gain momentum. This trend is to replace and/or combine the thin polyester overlay of a membrane switch with a rubber overlay. Although rubber keypads have been used in handheld devices such as remote controls and calculators for many, many years, this design twist has provided engineers with many more design options. Moreover, advances in adhesive technology now allows rubber keypads to be permanently affixed within the layers of the switch assembly using compatible pressure sensitive adhesives. The ability to eliminate mechanical fastening methods provides design opportunities never before experienced with rubber keypads.

New Twists on Rubber Keypads Can Help Transform Your Interface Devices

Rubber keypads can provide a greater perceived value due to the higher depth for which the keys are molded. While overlays for membrane switches can be embossed to heights of .015”-.020”, rubber overlays can be practically molded to heights over .5”. The height of the mold coupled with high-end fin-ishing features such as epoxy coating can provide a valuable look.

Above (Left)—Keypads and other entities can easily be highlighted by molding these areas into the rubber overlay. Above (Right)—Advanced features such as coating the top of the keys with epoxy can be inte-grated onto the rubber keypad giving a high-end look for a fraction of the tooling cost associated with injection molded keycaps.

Advantages of Incorporating Rubber Keypads in Your Interface Designs

Top (Left)—According to a study conducted at MIT, the average human finger ranges in diameter of 16-20mm (.63”-.79”). When a 12mm dome is used, which is standard in most membrane switches, the result is typically a crisp, positive tactile response. Top (Center)—As the domes get smaller (necessitated by space constraints), the relatively large actuating surface area of a human finger can result in a button which is hard to actuate and even “mushy” feeling. Top (Right)– By using a rubber keypad as the graphic overlay, molded bosses can be incorporated which aid in transferring the force from the human finger to the smaller metal dome. This results in a very lively tactile feel, even with domes as small as 6mm in diameter. Bot-tom—This keypad designed for a handheld mobile device requires a full QWERTY keypad. Using a rubber keypad interface was the only way to achieve good tactile feel while fitting 52 keys within a very small foot-print.

16-20 mm

While typical membrane switches use tactile domes that are 12mm (.472”) in diameter, space constraints often require the keys and respective metal domes to be smaller. Whereas the tactile feel of membrane switches often diminishes as the size of the keypads get smaller, rubber keypads offer unique design at-tributes allowing for excellent tactile feel even using small metal domes. Thus, rubber overlays allow for a tighter cluster of keys than traditional membrane switches.

Overlay Rubber Overlay Membrane Switch

Top–An illustration showing the typical stackup using a rubber keypad in conjunction with a graphic overlay. The graphic overlay is adhered to the top of the rubber keypad using adhesives specifically designed to ad-here to the low surface energy characteristics of the silicone rubber material. Bottom (Left)– By using an overlay in conjunction with a rubber keypad, it is easy to incorporate logos by printing on the graphic overlay. Bottom (Right)– This keypad design uses the graphic overlay for instructional copy as well as graphical highlights. It would be impractical to incorporate this verbiage directly onto a rubber keypad.

One of the largest drawbacks of rubber keypads is the decorating challenges in areas other than the key-pads. With overlays used on membrane switches, the entire surface area of the part is open to add logos, illustrations, instructions, etc. So, by selectively die cutting the graphic overlay and using it with a rubber keypad, this limitation can be mitigated. In these cases, the overlay is screen printed with the desired graphics and the buttons protrude though cutouts in the graphic overlay.

The un-pigmented base silicone material used in rubber keypads acts as an excellent diffuser of light. Thus, the use of rubber keypads in your switch design can open up some unique and inexpen-sive backlighting options. Often a single surface mount LED can effectively backlight a .5” x .5” area of the keypad when a rubber overlay is utilized.

A Customer’s Perspective: CEM Corporation CEM Corporation, head-quartered in Matthews, NC, is a world leader in utilizing microwave technology to innovate unique labo-ratory and scientific equipment. CEM has been using membrane switches as their preferred user in-terface for their equipment for nearly 30 years. In order to have their interfaces keep pace with the ad-vances in their equipment technol-ogy, CEM’s team has been looking at ways to integrate more sophisti-cated input devices.

In one of their most important recent product launches, CEM de-cided to combine several interface technologies by incorporating a rubber keypad assembly underneath their touch screen. Christy Davidson, a senior engineer at CEM, explained some of the feedback that they have received from their customers on this new inter-face. “It is clear to us that many of our users enjoy the flexibility of the touch screen interface. However, we have found that we still have users that ap-preciate the look and feel of discrete keys”, Davidson offered. “The use of both of these technologies has been extremely popular among our custom-ers. Moreover, the rubber overlay used on the membrane switch pro-vided a totally new and unique look for us”, she added. “The ultimate compli-ment for us is that our customers have enthusiastically embraced the technol-ogy of this equipment as well as the new interfaces used to operate the equipment! SSI has been an excel-lent resource for us in helping us in-corporate great interfaces for our products”, Davidson concluded.

For more information about our rubber keypads, backlighting and other interface devices visit our website at: www.ssielectronics.com or call us at 800-866-8510.

8080 Graphic Drive Belmont, MI 49306

800/866-8510 fax: 616/866-8882 www.ssielectronics.com