One of the many interface products we manufacture at KTP is membrane switches. Membrane switches are flex circuits that turn devices off and on like a regular switch.
Traditional switches usually consist of plastic pieces and copper for conductivity. Membrane switches, on the other hand, feature circuits printed directly onto their base. They go on the front of devices, where users can access them.
Membrane switches are affordable, versatile, reliable, and effective. Membrane switches have lower conductivity than some switches and are perfect for many different low-power applications.
We make membrane switches for devices like: security keypads, cell phone touch screens, aircraft control panels, airport kiosks, personal gaming devices, tablets, home appliances, and more.
Provide us with your specifications and we will help you create a suitable design optimized for your application.
Embedded surface mount LEDs (for indicator lighting)
Backlighting with Light Guide Films and LEDs
Metal dome or Polydome for tactile feedback
Via holes (thru-holes) to simplify construction
Flex tail connectors (including female, male, and solder tabs)
Capacitive sensing technology
* Our variety of adhesive choices includes options such as such as acrylic or foam adhesives (useful for indoor or outdoor applications)
Membrane Switch Design Guide
Membrane switch technology has evolved into an effective and familiar interface solution used in a broad range of consumer products. Because of this diversity in the application, each membrane switch is uniquely designed to accommodate its intended purpose. The success of a product begins with effective communication between the design engineers, the customer and the factory. This design guide is intended as an outline for our customers to communicate their requirements to our design engineers.
Why use membrane switch?
Thin, Flexible design that can be laminated to slightly curved surfaces
Eliminates the need for a separate cable wire; the flex tail cable is integrated.
Long life and low cost integration
Customization of design and shapes
Can be water-sealed
Surface is easily cleaned
Other components can be integrated, such as LEDs and LGF (light guide film) for backlighting
Easily replaceable; upgrading your products becomes very simple and low cost
Short tooling leadtimes and inexpensive tool
Proven technology, and found in many products, such as appliances, gas stations, ATMs, and others
Versatility of a Membrane Switch
Membrane switches can be combined with many different products, and evolve into other creative parallel products. For example, a rubber keypad can replace the standard overlay but still have the standard membrane switch layers for the electrical layers and integrated flex tail. Or, LEDs can be added, to provide backlighting. There are many combinations that are possible, that this guideline only skims through the basic functions and options available. Thorough discussions regarding your product usage environment and product expectations, is recommended to provide 100% product satisfaction.
Some examples of versatility that adds product value:
Rubber keypad with membrane switch layers, where the rubber keypad can be molded as translucent color, to act as LED diffusers
Backlit membrane switches with either LEDs or LGF
Rigid membrane switches, where we can add a backer such as Stainless steel backer, Aluminum, PC, Acrylic, etc.
Water-seal membrane switches
Capacitive touch membrane switches
Tactile and non-tactile membrane switches
Basic Membrane Switch Construction Layers Description and Requirements
The most common overlay materials are either Polyester or Polycarbonate. Polyester has a longer wear-life (greater than 100,000 actuation cycles) and it has greater chemical stability. Polycarbonate shows wear at around 50,000 actuation cycles, although it is still popular because of its lower cost, high clarity and its ease of printing and diecutting. Both types of materials are available in different texture finishes and also high gloss. Certain designs requiring selective texture, can also be accommodated using texture or gloss printable ink. Hardcoating is also available for applications requiring scratch-proof.
Polyester film is available from Autotype and the following are most commonly used:
Hard coated PET
Available in gloss or antiglare finish
High abrasion, chemical and solvent resistance
Can be embossed
Available in Fine or Velvet textures
Can be embossed
High abrasion, chemical and solvent resistance
Long flex life
Can have selective gloss windows
Available in Fine or Velvet textures
Same as Autotex but for outdoor use, and resistant to yellowing an becoming brittle in outdoor UV light
Available in Fine texture
Same as Autotex but with the added benefit of an antimicrobial coating on the surface
Polycarbonate film from Sabic and the following types are the most common:
Lexan HP92S (coated film)
High performance, easily printed and diecut
High gloss polished surface (92 % light transmission)
Hard coated and shows strong durability to typical household chemicals
High abrasion resistance (4 delta haze on Taber abrasion test)
Pencil hardness HB-F
Available in gauges between 175 micron to 750 micron
Lexan OQ92S (coated film)
Offers good optical, good chemical, abrasion and UV resistance over the standard HP92W
Lexan HP92W (Coated film)
Has a very good chemical and abrasion resistance, but with good UV resistance also. Used for outdoors
Lexan OQ6DA (coated film)
Hardcoated PC/PMMA film
4H pencil hardness
Excellent chemical resistance and impact resistance
The graphics are printed on the second surface by using screen printing method or digital printing method. For windows, the top surface is printed with a gloss hardcoat and can also be tinted with colors.
KTP provides complete artwork service. The customer should specify the font styles, colors and sizes. Artwork for logos should be supplied to use. We use Adobe Illustrator to create artwork. Proofs of artwork will be sent to the customer for approval prior to production.
The overlay keys can be dome embossed to add a tactile feedback. Other types of embossing are also available that does not provide tactile feedback, such as full key emboss that are mainly used when there is a metal dome and Perimeter emboss for which only the rim is embossed. Full key emboss is used to describe keys that are raised and flat on the top. Perimeter or rim emboss is used to describe raising only the border of the key. Embossing is typically 0.010” high and 2 dimensional. A higher emboss of about 2.5x material thickness can also be achieved using hydroform. A combination hybrid of the embossing is also available, such as dome emboss with perimeter emboss. Both polyester and polycarbonate can be embossed but polyester is recommended due to it being more flexible and gives a longer life. Polycarbonate tends to emboss with much sharper details but does not have as long as life as the polyester.
UV mold print 3D
UV hardcoat: The most durable hardcoats are those that are cured by exposure to UV light. These hardcoats are selectively added to many materials to produce parts with textured background or gloss or antiglare windows.
The colors can be matched using Pantone Color System or a sample color chip. Although we can match as close to the Pantone Color book, however, since the Pantone Color system is based on paper, there will be slight variations on the color match due to the type of substrate material and gloss level of the final product.
If a sample color chip is supplied to be color matched, the substrate and gloss level should also be the same as the final product. The sample color chip should be at least 2” x 2” and opaque.
If there is any doubt, we will produce a variety of color chip samples using the actual ink and actual substrates for you to approve. These sample color chips will be DE less than 1.5, and show variations of Da and Db values, that will fall in the low, middle and high values close to the master color chip. Once you approve a color chip, that will then become the new standard master color chip and will be kept in our records.
It is also recommended to inform us of the lighting condition of where the product will be used. Colors appear differently under different lighting conditions. We will use a light booth to simulate different lighting conditions helps to obtain objective color assessment, improves communication and reduces product rejections. This can be taken into consideration when using the spectrophotometer when taking color measurements.
Steel rule dies are usually used to fabricate the various layers of the membrane switch. Standard tolerance is +/- 0.015”. Tolerances of +/- 0.010” can be held on critical dimensions such as perimeters and cutouts. Hole to hole center tolerances of +/-0.005” can be held. Tolerances on very large parts will be greater. Tighter tolerances can be held by laser cutting or with hard tooling.
The switch layers under the overlay will typically be smaller than the overlay. This allows for diecutting and assembly tolerances. All layers will typically be 0.015” inset from the overlay at al edges and cutouts.
The various layers of a membrane switch can be cut out by using an industrial laser. This technology offers two advantages. Tighter mechanical tolerances can be held and no tooling is required. While laser is more expensive, in man low and medium volume applications, it will be more effective.
Circuit material and Layout
The circuit material we use is a heat-stabilized PET that is 0.005” thick. In membrane switch technology, the circuit is screen printed using silver-filled conductive ink. You will need to provide us with a pin out/schematic and the tail exit location and we can set the layout for you. In many situations, cross overs bridge or even via holes may be necessary to be designed. Rest assured that cross overs or vias are effective and reliable, but increases printing operations and cost. It is recommended that in very complex circuitry, that customers should allow KTP to determine the best possible pin outs instead.
Can be a standard matrix, or for maximum efficiency, KTP can determine the pin outs that minimizes cross overs
Tail exit point
Flexible membrane switches are connected by means of a flexible tail that is cut from the circuit material. The tail cannot exit under or within 0.125” of the active keypad area. The area where the flexible tail is cut, will have a tail filler to fill in thickness. In design situations where the tail has too many traces and making its width too wide, we will recommend to design a funnel-shaped tail, to minimize the “bump shadow” seen on the front of the overlay when the tail is extended and also to give more real estate for the traces to be routed.
The flexible tail that exits a membrane switch usually has single row traces on 0.100” centers. This tail can be connected to a circuit board with many different single row connectors designed for flex circuits. Tail connectors can be a female connector, male connector, solder tabs or ZIF (no connector). The most common standard pitch is 0.100” for connector options, whereas for ZIF, it can go as close as 1mm pitch. ZIF tails will have a carbon-overprint on the exposed contacts on the end of the tail. When using ZIF connectors, customers should specify the connector or the requirements for the connector as some will require locking holes.
It is also important to designate the orientation of the contact surface and also the location of where Pin 1 is.
In some cases, there can be 2 or more tails that exits a membrane. Usually, this is the case when a separate switch layer and LED layer are required, therefore having 2 separate connectors, or it can have double tails that can be crimped together with one connector, when needed.
Connector pin design and crimping guidelines: (information below taken from Nicomatic website and provided here as a reference)
Our standard female terminals and connector is Nicomatic. The terminals are pierced through the tail conductor at 6 points, creating a conductive path. Crimp Pad Design for female pin connectors are as follows: Silver trace width of 0.060” and length of 0.275” is standard, with a smooth transition to a thinner traces, if needed.
In designs where a carbon –overprint is needed, the silver and carbon trace design is as follows:
Single Sided Circuit:
A 0.275” of exposed ink should be designed so that the terminal can be crimped through it. The flat side of the terminal should be touching the exposed ink.
Double sided Circuit:
A dummy trace should be provided opposite the actual circuit. Ie. Opposite a trace on the top circuit there should be a dummy trace printed on the bottom circuit. The dummy trace should have a width of 1.57 mm (0.062”) nominal and a length of 7 mm (0.275 “). This provides a thicker layer of ink to insure a reliable electrical connection. Note: There should be no adhesive or dielectric between the dummy trace and the active trace
Three Layer Circuit (Double Circuit with a shield for example):
The two internal layers should be designed the same as a double sided circuit. A dummy trace should be provided opposite the actual circuit. For example, opposite a trace on the top circuit there should be a dummy trace printed on the bottom circuit. The dummy trace should have a width of 1.57 mm (0.062”) nominal and a length of 7 mm (0.275”). This provides a thicker layer of ink to insure a reliable electrical connection. The shield layer should then be exposed and crimped into, as shown below:
LED Circuit/Spacer/Upper Circuit:
Both layers are prepared with tails that have exposed traces as in ‘A’ above. The tails are then ‘scalloped’, so the upper circuit is cut away to expose the bottom circuit and the bottom circuit is cut away so the combined tail is the same thickness across its entire width. The end of the tail then has the identical design to the single layer circuit shown in ‘A’ above. Care must be taken to insure that the two layers are registered well enough to maintain the 2.54 mm (0.100”) spacing where the tails meet.
The purpose of inspecting the crimp height is to insure that gross errors have not been made in the set-up of the machine. For tails with a thickness of 0.125 mm to 0.35 mm (0.005” to 0.014”) in the crimp area. This includes any stiffener in the crimp area. The height of the crimp should be between 0.7 mm (0.028”) and (0.9 mm) 0.035”. While this height does vary by machine and by set-up to some extent, it is scalable. The following table shows a more specific correlation for the height of the crimp to the thickness of the tail.
Tail ThicknessApproximate Crimp Height
0.075 mm 0.55 mm to 0.68 mm
0.100 mm 0.57 mm to 0.70 mm
0.125 mm (0.005”) 0.59 mm to 0.72 mm
0.250 mm (0.010”) 0.78 mm to 0.83 mm
0.350 mm (0.014”) 0.88 mm to 0.93 mm
Note: The crimp height should NEVER be used to reject a part, only to identify it for further investigation. If the crimp is tight and not cracking the ink it is acceptable, regardless of the height, but a good crimp will typically fall within the ranges shown above. The crimp height should also never be used as the sole criteria for accepting a crimp. The part should always be checked for cracked ink traces and for gaps between the contact and the tail, as well as, misalignment and the other items mentioned above. A crimp that is too high will have a noticeable gap between the barrel of the contact and the tail, as well as, misalignment and the other items mentioned above. A crimp that is too high will have a noticeable gap between the barrel of the contact and the tail. A crimp that is too low will show signs of cracking the ink trace.
To insure good electrical conductivity of a crimp, it is important to make sure that each contact is aligned properly over the ink trace. This will enable all the crimping portions to penetrate the ink properly, providing a sound electrical connection. When checking the alignment of a contact, it is suggested that the edge of the contact be no more than 0.30 mm (0.012”) from the edge of the trace. Even though electrical conductivity will be present if only a few crimps penetrate the trace, this margin is suggested so that all of the crimps will come in contact with a portion of the ink.
Creasing of Flex Circuits
The tail portion can withstand slight bending (at around 0.010” radius) but creasing is not recommended. As for the entire membrane switch, it is highly recommended not to bend or crease it, especially when metal domes or LEDs are present. Doing so will damage the switch, have potential failures and will impact reliability.
When assembling the membrane switch to the fascia or panel, extreme care should be taken to make sure tail slots are not sharp or have burrs that can potentially scratch the traces on the tail. In addition, once the membrane switch is applied with an adhesive, removing it and reapplying, will impact reliability when not done properly or when metal domes and LEDs are present.
In situations where the tail needs to curve around or bend around or when it needs to be creased in order to work inside the housing, please consult KTP engineers so we can take this into consideration when designing the flex tail.
Membrane switches can also be supplied with a rigid subpanel, such as an aluminum panel. These subpanels can be supplied with a variety of hardware installed.
In order to make the subpanel not visible after assembly, the subpanel should be 0.020” smaller than the membrane switch in both height and width. All cutouts and holes should be 0.030” larger. Cutouts behind windows should be 0.060” larger than the windows.
Feedback can either be visual, audible or tactile. An example of a visual feedback is using an LED. An example of audible feedback is using a metal dome. To provide tactile feedback, domes can be added to the membrane switch. There are two types of domes that we use in membrane switches, metal domes and polyester domes. Metal domes have a better snap feel, and no tooling cost. It is recommended when the membrane switch will be used in extreme temperatures. Polyester domes are usually formed into the top circuit of the membrane switch. It requires tooling , but unlike metal domes, does not need to be assembled individually. One drawback of polyester domes is that they tend to relax and lose their tactile feel at elevated temperatures and are not recommended for applications above 55C (131F)
We can add LEDs to the membrane switch. Because of the LED height, usually, the LED indicator area will be embossed on the overlay. If embossing is not possible, we can add layers to the switch construction to build up the height.
LEDs are mounted using silver conductive epoxies with an addition of a Loctite glue for added security, and an encapsulant. It is highly recommended not to bend areas with the LED. We take extreme caution to make sure the parts remain flat during manufacturing and transit.
The LEDs can be bicolor or single color. New LEDs that are very low profile are also available although they cost more. Our most common single color package LED size is 0603 (1.6mm x 0.8mm), top emitting. In designs requiring Light Guide Films, we will use a side firing LED such as from Everlight 3.8mm x 1.2mm x 0.62mm
The most commonly used rear adhesive on membrane switches is the 3M 467MP. This is an excellent adhesive for bonding to smooth metals and high surface energy plastics. For rougher surfaces, we recommend 3M 468MP. For automotive applications, we recommend closed cell foam adhesives. There are many other types of adhesives available for your demanding applications. For low surface energy plastics, high temperature or low temperature requirements, outdoor environment and other specialty applications, there is an adhesive available. Please contact KTP to discuss your requirements.
Always clean the surface with alcohol to remove residues such as oil and dirt prior to assembling the membrane switch. Never bend the membrane in the dome area or LED area. After the membrane switch is correctly assembled, apply pressure firmly and press down. These adhesives need pressure for a strong bond. The adhesive will continue to cure for about 72 hours. It is recommended that any testing to the adhesion strength be done after the full cure of 72 hour have been met.
Several options are available for shielding. The most common are printed carbon, printed silver and aluminum foil. From a functional point of view, the main difference among these materials is their conductivity. Carbon and silver can be printed on the top circuit layer to act as a shield. These shields have the advantage of not adding an additional layer. Carbon shields are less expensive than silver shields. Silver is usually oriented as a grid patter to reduce cost. A layer of aluminum foil can also be added above the top circuit. This is the most conductive shield however it does add two layers to the switch construction. The shield is usually connected to the ground through the connector or by means of a tab with a slot for a fastener or by using a 3M 9703 (a conductive z-axis adhesive).
In some cases, the window also needs to be shielded but because it is clear LCD window, neither the silver, carbon or foil is transparent enough. We are able to use an ITO film, or even print using a clear, PEDOT conductive ink, on the window areas, without affecting clarity and transparency.
There are a few ways to provide backlighting to a membrane switch. A Light Guide Film uses a piece of polycarbonate with a white reflector on one side and etched or printed UV dots on the other side. These will be lit by side firing LEDs. The benefit of this technology is that it utilizes the minimum amount of LED to backlight a bigger area and minimizes hot spots. It can also be used over metal domes and it can be embossed (if the polycarbonate layer is thin) to become a polyester dome. The disadvantage is the cost of tooling and the brightness of the backlight is affected by the thickness of the polycarbonate layer, and for areas that do not backlighting, a cutout is needed.
Using fiber optics is another way of backlighting. It is very costly since it is custom made, but it yields very even backlighting without hot spots. It can be used over metal domes but not recommended over polyester domes.
LEDs are also used for backlight but almost always together with a diffuser. A diffuser can be a Bayer diffuser film or a customer silicone rubber diffuser piece. Hot spots can be an issue and a lot of testing and trial will need to be done, to make sure uniform lighting is achieved.