Top 3 Considerations for Designing Robust Membrane Switches

Silicone & Rubber Keypads
Membrane switch technology is an ideal user-interface technology for a wide range of products and applications. The sleek design and space-saving thin profile of membrane switches helps to enhance and maximize the user experience with your product. Additionally the reduced cost of membrane switch technology vs. electro-mechanical switches makes membrane switches a compelling design choice for many OEM design engineers.
In some industrial design circles, however, there is sometimes a perception that membrane switch technology is not a sufficiently robust design for certain user-interface applications. This perception is usually the result of membrane switch failures. While there are surely certain environments that are simply not suited to the technology, there are 3 primary areas of design and end-use that must be considered to maximize the ability of a membrane switch assembly to effectively function without failure.

1. End-Use Environmental Considerations

Perhaps the most important consideration is the need to carefully consider the environment in which the membrane switch must operate, also called the end-use environment. There are many ergonomic features that can be incorporated into membrane switch designs to enhance the user experience, such as tactile feedback, embossing, and embedded LED’s, just to name a few. Depending on the end-use environment, some of these ergonomic features may not be practical to incorporate into a membrane switch design.
For instance, if the end-use environment calls for continual outdoor use with exposure to temperature extremes, moisture, and the weathering effect of UV rays from sunlight (aging & yellowing), while a membrane switch can certainly operate in this environment—adding features such as embossing and tactile feedback (in combination with the outdoor effects) could stress the membrane switch materials to the point where they fracture and crack, causing premature switch failure. Design engineers must be willing to consider the trade-off of adding appealing (but life degrading) ergonomic features vs. the potential for switch failure. A non-tactile, non-embossed membrane switch construction with visual and audible feedback for the user would typically offer a more long-lasting, robust solution.
In outdoor situations where the design must incorporate tactile feedback of some kind, there are alternative or “hybrid” constructions that can be used for your user-interface. IMG_2467
Elastomeric (rubber) keypad assemblies
, and also DuraSwitch PushGate technologies are worthy design considerations in these circumstances.
If the end-use application involves potential operator abuse, vandalism, or actuating keypads with potentially sharp objects then Dynapic Piezoelectric Keypad technology is a suitable alternative to traditional membrane switch technology.  With Dynapic keypad constructions, the outermost graphic layer can be constructed of rigid, durable materials such as Metalphoto anodized aluminum, or thick acrylic.  These thicker more rigid materials can withstand operator abuse more easily than traditional membrane switch constructions, which utilize a thin, polyester layer for the graphics, and can be more likely pierced by sharp objects.
Another end-use environmental consideration is the exposure of either UV rays (from sunlight) or exposure to chemicals or specific cleaning agent and wipe-down chemicals. There are a number of application-specific graphic overlay material choices that are designed to maximize resistance to these effects.

2. Mounting & Bonding Considerations

A second primary consideration to ensure the reliability of your membrane switch keypad design is understanding the factors involved with mounting or bonding the membrane switch assembly to your equipment, whether it involves a bezel, case, cutout opening, recessed area, etc.

  • A critical mounting/bonding consideration is the understanding and expectations regarding the mechanical dimensional tolerances typically achieved with membrane switches. Design engineers often associate membrane switch dimensional tolerances with the metal fabrication industries, which is usually +/- .005″. However membrane switch panels are not fabricated with metal fabrication equipment. Instead they are cut using steel rule dies and low-to-medium speed CO2 gantry-style lasers. The result is typically a dimensional tolerance for membrane switches of +/- .010″. In addition to the +/- .010″ tolerance for the membrane switch, the total “stack-up tolerances” must be considered, also taking into account the tolerances for the bezel opening or recessed mounting area in your plastic or metal housing. Determining the total stack-up tolerance will ensure a proper fit into your equipment, eliminating either a too-tight situation, or one where there are large “gaps” between the edge of the membrane switch and the housing.
  • The geometry of the mounting surface is an important consideration, especially with tactile membrane switch assemblies. It is essential that the mounting surface be rigid, and most importantly, flat. Mounting a tactile membrane switch to a concave surface will most certainly cause the tactile domes to invert past the “zero plane” into a collapsed position, causing a permanent switch failure.
  • Another bonding issue that deserves attention is the nature of the material to which the membrane switch will bond. While certain membrane switch constructions utilize rigid support panels with mounting studs, the purpose of this discussion is to address membrane constructions using a pressure sensitive adhesive as the mounting mechanism. It is extremely important to understand the surface energy  and surface finish of the material to which the membrane switch is being applied. Does the mounting surface have a smooth or rough-textured finish? If textured, then a thicker adhesive caliper (.0035″ – .005″ thick) will be necessary to maximize the surface area to which the bonding adhesive must adhere. Is the mounting surface a High Surface Energy (HSE) or Low Surface Energy (LSE) material? HSE materials such as aluminum, polycarbonate, and ABS require acrylic adhesives, while LSE materials such as polypropylene and powder coated paints require special modified acrylic adhesives.

3. Handling Issues & Laminating Techniques

Proper handling methods and laminating techniques are essential to ensuring the long-term life of the membrane switch. One problem often encountered is when a membrane tactile switch is held in one’s hands and the tactile keypads are pressed, with no rigid surface behind the switch. This will often deform the tactile domes so that they either permanently collapse, or make them susceptible to future collapse. It is imperative to never actuate any membrane switch while the switch is in an unsupported position. Flexing the tactile domes prior to final mounting on a rigid surface or subpanel can cause over-travel of the dome or bending of one or more legs on the dome.
Another laminating mistake is to  roll the membrane switch on to the surface of the final backing substrate. The switch should be aligned and then lowered down as a flat plane. Rolling a membrane switch onto the surface of the subpanel (as one might with a label or graphic overlay) can damage the domes (similar to stress on keys in unsupported manner) or dislodge SMT components embedded in the switch array.
The proper laminating technique is to laminate the switch assembly to a rigid surface with a suitable roller, typically a 35-45 Durometer, Shore “A” hardness roller and/or a proper template. Never burnish or press the keypads with hard/sharp objects.
Nontactile membrane switches should never be actuated with sharp objects such as a pen, screwdriver, stylus or any other actuator. Membrane switches are specifically designed to be activated by finger actuation. Actuation with any hard, sharp and/or small diameter object can cause damage (dents) to the transition ring of the tactile dome. Once damaged–even slightly—the dome is unstable and prone to premature failure.
When installing membrane switches with printed silver conductive flexible tails, it is important to understand that these flex tails (as opposed to polyimide copper flex tails) are not designed to be creased. While a bend radius is acceptable, creasing of the tail will cause the conductive ink to fracture and flake off the substrate, resulting in a switch failure.
Care should be exercised with the use of any ZIF and LIF Connectors. These connectors have delicate and fragile sliding locking mechanisms, unique to each part number and manufacturer. When installing the membrane switch, the installer should be aware not exert too much force or torque on the locking slider, or it will break.
Membrane switches are designed to be laminated to a substrate one time. Parts that have been removed and/or re-positioned for final assembly will often void any manufacturers’ warranties.
When design engineers carefully consider these three primary issues, they can properly design and specify a durable, robust membrane switch construction designed to thrive in its intended end-use application.

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