Membrane Switch Guide
Membrane Switch Design Guide
CSI Keyboards was established in 1982. Located in Peabody, Massachusetts, CSI has been designing and manufacturing human machine interfaces for over 40 years. This membrane switch design guide is intended to serve as a framework for communicating the design requirements of our customers to CSI Keyboards. There is ultimately a lot more that goes into designing and manufacturing a membrane keypad than what was included in this Design Guide, but we hope this document offers a solid basis. structure and framework for your design. Our engineers here at CSI Keyboards are prepared to help you design a switch that will meet all of your requirements utilizing our 40+ years of expertise in the keypad industry.
GETTING STARTED: During the initial design phase, CSI works closely with you in order to generate manufacturing drawings which are ultimately provided to you for approval prior to manufacturing. If you don’t have a drawing, do not fret. We work with customers that have extensive drawing packages from the onset, to customers that only have an idea or concept.
DRAWINGS & ARTWORK: CSI supports files created in AutoCAD, SolidWorks or CorelDraw Illustrator. We support Adobe Illustrator (.ai), Corel Draw (.cdr) or other vectored graphics program for artwork.
COLOR MATCHING: CSI uses the Pantone Matching System (PMS) for artwork generation. We can also match the color a customer supplied sample if required.
POLYESTER OR RUBBER: Standard polyester overlay material was always the industry standard for membrane switches. But over the past decade, molded rubber has become extremely popular due to its customization properties and ability to both seal and backlight. More information on our rubber keypads can be found in the rubber keypad section of our website.
KEYPAD LAYOUT: The graphic layout of the faceplate usually determines the size and location of the switches. As the graphic layout is developed, consideration should be given to the size(s) and the exact placement of the keys. Typical keyboard layouts place 0.50” square keys on 0.75” centers.
Regardless of the size or shape of the keys, the space between the keys and around the perimeter of the keypad should be .100” of an inch or greater. This spacing allows room for internal layer die rules and circuit traces, and leaves enough exposed adhesive between layers to insure proper switch operation.
OVERLAY MATERIALS: The most commonly used material for membrane switches is polyester. CSI Keyboards utilizes a wide range of high quality polyester in all of our membrane keypad designs. Polyester not only has a superior life cycle compared to other materials, but also offers exceptional resistance to a wide range of chemicals, solvents, surface wear, and abrasion. Polycarbonate can also be used if desired for certain applications. Both polyester and polycarbonate are available with a variety of textures and hardcoats. The latest polyester technologies include Environmentally Resistant; Soft Touch; and Antimicrobial polyesters.
ELASTOMER MATERIALS: Our elastomer or rubber keypads are manufactured from silicone rubber properties that are typically processed from either injection or compression molding. Elastomer keypads offer a wide range of design possibilities due to the many sizes, shapes, and colors that it can be molded into. The rubber is typically over-sprayed with a matte PU (polyurethane) coating to protect your keypad from chemicals, moisture, scratches, and any other wear and tear that the keypad might encounter.
FLEX CIRCUITRY: There was a time when screen printed silver-filled epoxy ink was the most commonly used product for flex circuitry. Copper flex circuitry, also known as Kapton circuits, is now used in the majority of CSI Keyboards’ keypad designs however. CSI prefers copper flex over printed silver due to its excellent dielectric strength, thermal stability, chemical resistance and flexibility. Copper flex has become the superior choice over printed silver especially for outdoor applications. Copper flex circuitry construction designs offer a significant advantage over printed silver and a printed silver circuit can be replaced with a copper and polyimide construction with minimal additional cost.
DOMES: CSI typically utilizes stainless steel domes in all of our keypads. Stainless steel domes have better “feel” and increased reliability over any other type of actuation technology. The customer can specify actuation force, which is normally 12 to 18 ounces, or can rely on CSI’s expertise to choose the best actuation for that particular application. The domes used in our designs are all rated for at least one million cycles. Higher cycle-rated domes can be designed into the keypad if necessary reaching up to five million cycles. Dimple domes can also be designed into the keypad which can give a keypad, especially elastomeric switches, a better tactile feeling switch. Polydomes can also be utilized if necessary.
EMBOSSING: In many applications, it is necessary to emboss the keys of a switch. Plateau Embossing is used to describe keys that are raised and flat on the top. Rim Embossing is used to describe embossing or raising only the border of a key. Embossing is typically .010″ high, but Raised Plateau Embossing can also be requested, which can increase the height of the plateau to up to .050” in height.
ADHESIVES: The majority of our membrane switches have pressure sensitive adhesive (PSA) on the rear side. The most commonly used adhesive is 3M 467MP, which is an excellent adhesive for bonding. We normally recommend 3M 468MP for rougher surfaces. We also use special silicone/acrylic adhesives for adhering silicone rubber keypads to certain layers and surfaces.
LED INDICATORS: It is common to integrate small surface mount LEDs into our membrane switches. We can build up the internal layers to accommodate the LED or can also emboss the LED window. Additionally, the indicator window can be printed translucent white to hide the LED underneath.
BACKLIGHTING: We offer many backlighting solutions for dark and low light applications. The most common backlighting methods used are Fiber Optics, Electroluminescent panels, LEDs, and Light Guide Film. LEDs are the most popular and economical method for keyboard backlighting. LEDs are most commonly used to backlight keys, icons and symbols. LEDs are also typically used as indicator lights. Electroluminescence (EL) is applied on a very thin layer between the graphic overlay and the circuit. EL uses a printable ink deposit to illuminate the switch and provide a uniformed illumination. Fiber Optics provides a flexible back lighting layer that can be incorporated between the graphic overlay and the circuit layer allowing the entire surface area of the membrane switch to be evenly backlit. Light Guide Film is designed to evenly distribute light from top or side firing LEDs.
ENVIRONMENTAL SEALING: We offer many environmental sealing solutions. CSI Keyboards designs and assembles sealed switches and keypads built to last in extreme and harsh environments and meet requirements for water, temperature, humidity, dirt, dust, altitude, impact resistance, vibration, and chemical resistance. CSI’s engineering team can assist in deciding which sealing methods are right for your design.
ESD/RFI/EMI SHIELDING: CSI uses a few different methods for shielding membrane switches depending on the application. The most common methods are printed carbon, printed silver grids, and aluminum foil. Using a ground trace around the perimeter of the circuit layer is another effective technique for ESD shielding. The shield can be connected to the ground through the connector, by using a tab with a slot that can be mechanically connected to ground, or by wrapping around the membrane and grounding to a chassis.
TAIL BREAKOUT: The flexible tail in a membrane switch is created by die cutting the tail shape from the bottom, top, or both circuit layers. From a switch design standpoint, this means that the tail can exit from any clear area within the switch that is larger than the tail, or from any edge. Center to center for a ZIF tail, silver printed circuit is 1mm. An etched circuit can be 1/2mm. The tail exit should be a minimum of .25” from the edge of the part and, optimally, be .25” from keys, LED’s and other components on the circuit. It is important for the customer to provide tail dimensions to CSI.
VENTING: When a keypad is depressed, air pressure within the switch cavity increases. In order for the switch to close properly, air within a switch cavity must be displaced, equalizing the internal pressure. This is usually only a problem in switches that are environmentally sealed. There are two standard venting methods that can solve this issue: internal and external venting. Internally vented keypads have narrow channels between key location cutouts are cut into the spacer layer, permitting the air from one location to move elsewhere when that key is pressed. Externally vented keypads also have narrow channels that have been cut into the spacer layer connect each key location. These channels then exit through the sides, rear, or internal cut-outs of the membrane switch. This design allows pressure within the switch cavities to be equalized with the surrounding atmosphere, thus allowing switch closure at any atmospheric pressure. Because external venting increases the risk of contamination, it is only recommended when the membrane switch is exposed to rapid or extreme atmospheric pressure fluctuation and will generally not come into contact with a harsh environment.
WINDOWS: CSI can include windows in the design, which essentially the base clear overlay. CSI will design the window utilizing hardcoats or textures depending on the application (LCD, VFDs, LEDs, etc.). CSI has also perfected the manufacture and application of clear polycarbonate to reinforce membrane switch display windows (what we call Opti-Bond Windows). By optically bonding durable, clear polycarbonate, CSI is able to strengthen the viewing areas over LCDs, VFDs, etc. while retaining the clear clarity necessary in reading these displays. The Opti-Bond advantages include: abuse protection, readability, and a sealed window created using one continuous overlay surface.
BACKERS: Many of our membrane switches require backers for rigidity or other design related purposes. The most commonly used material is aluminum. Aluminum backers can be supplied with a variety of hardware installed. Other options include G10, stainless steel, polycarbonate or acrylic backers. If the keypad is PCB (printed circuit board) based, we can typically utilize the PCB as the backer and install hardware directly on the board.
PINOUTS: The schematic or pinout of a keypad is usually provided by the customer, but the circuit layout can also be developed by CSI’s engineering team utilizing our expertise of laying out the pin-out and circuit as efficiently as possible.
MECHANICAL TOLERANCES: Steel rule dies are typically used to construct the various layers of a membrane switch. Standard tolerances are +/- .010″. If tighter tolerances are required, laser cutting can be utilized.
KEY NOTES:
Space between keys and around perimeter of keypad should be .100” or better.
- .100” center to center min for tail with connector.
- 1mm center to center min for ZIF type interconnect.
- Tail exit should be .25” min. from edge, keys and other components (tail cannot exit under key).
- Polyester is the recommended overlay material (we can specify the correct material for your environment).
- Normal final assembly tolerances = size +/- .010”, die cut to image = +/- .010”
Please note that these are optimum suggestions. We will work closely with you to design the best possible keypad to meet your requirements.
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