Sigma 6 Electronics brings knowledge and expertise to user interface design and pride ourselves on service, quality and competitive pricing. This guide is a basic introduction to membrane switch design and should be valuable information for customers looking to add a membrane switch to an application.

CIRCUIT LAYER

The circuit layer includes the membrane switches electronics: tactile domes, LED’s, shielding, tail length and connector termination. Circuit material is constructed from thin polyester, durable polyimide flex or PCB material of any thickness.

When designing a circuit layer it’s important to consider the length of the flexible tail, where to locate its exit point and how to terminate the membrane switch circuit layer to a controlling board. As circuitry continually becomes smaller, electronic interference and shielding options should not be overlooked.

Traditional membrane switch circuit layers are screen printed to polyester using a silver-filled epoxy ink that is heat cured. This process is carefully monitored to insure maximum conductivity, flexibility and adhesion. Extreme caution should be used to never sharply crease the flex as the traces damage or be prone to future failures.

For some customers the advantages of using a flexible polyimide printed circuit outweigh the additional tooling and unit costs. Flexible printed circuit applications are used when customers need: folding, creasing, higher operation temperatures, higher circuit densities/layers, soldering, plating, narrower pitches, or more durable tail traces.

Constructing your switch with a rigid circuit board removes all of the flexibility; however, the design gains the ability to congregate additional intelligence and components to the board such as LEDs, resistors and connectors.

CONNECTORS

Customers also may consider how to connect the membrane switch with the brains controlling your product. Sigma 6 Electronics offers a variety of ways to accommodate connector durability and the minimization of unwanted space when deciding on how to terminate your membrane switch. As a general design rule the tail may not exit under or within 0.125” of any key.

A (ZIF) connection has no physical connector on your membrane switch and relies on a receptacle connector. It’s important to choose a ZIF connector designed for your interface application. The part number of the chosen connector should be provided at the time of order. Sigma 6 Electronics can prepare a screen printed circuit for a ZIF connection in the following pitches: (0.100”, 0.050”, and 0.020”). Smaller pitches are available in polyimide flexible printed circuits only.

Male/Female/Solder connectors are considered to be a more reliable termination against heavy shock or vibration. Since these connectors physically snap together, they are also easier to assembly. These connectors have pins crimped to the circuit and require more space in a design. Housings are available with many different features from a number of suppliers.

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SHIELDING

ELECTRONICS

Several options are available for protecting and shielding membrane switches from electromagnetic interference (EMI), electric static discharge (ESD), or radio frequency interference (RFI). The shield is usually connected to the ground through the connector, or by means of a grounding tab with a slot for a fastener.

Shielding may add another layer between the graphic overlay and circuit layers. The customer should express shielding requirements in Ohms per square inch. More often than not, shielding is not required; however, Sigma 6 Electronics can help determine the best shielding approach for your membrane switch based on your protection requirements.

Today’s membrane switches are becoming increasingly more sophisticated and Sigma 6 Electronics prides itself on being at the design forefront. Determined by a membrane switches schematic, the circuit layer carries all the electrical traces. In recent years, tactile domes, ultra sensitive buttons, low profile embedded LED’s and backlighting have all have made significant technological .advances

The pinout of a switch may be specified by the customer or determined by Sigma 6 Electronics. When given more freedom in the circuit design we can make sure a pinout will have an efficient layout for manufacturing. Membrane switches are designed with a common bus, a matrix or a combination of both layouts.

DOMES

It’s important to give the user of your membrane switch at least one or a combination of different feedbacks when touching a button: visual, audible or tactile feedback. Domes are available in stainless steel or polyester, in numerous shapes, sizes and with varying actuation forces.

There is no substantial reliability difference between the metal dome and polydome. Stainless steel domes require lower initial tooling costs over polydomes and are more affordable in lower quantities. Polydomes can start to save cost as the volume of individual buttons reach the thousands.

Metal domes are the most popular design and provide a user with a snap feedback which is both tactile in feel and audible in sound. Metal domes are most always used in PCB membrane switches. Sigma 6 carries a variety of metal domes in stock ranging from 6mm to 12mm; each size is available with different actuation forces.

Needing additional tooling to form the buttons, the difference being you only feel the click but do not hear the snap of a polydome. The tactile actuation force is adjustable.

EMBEDDED LEDS

BACKLIGHTING

Embedding LED’s into a switch layer is a common practice, especially for visual indicator light applications. Sigma 6 Electronics uses an ULTRA-thin LED profile for membrane switches available in all colors and bi-colors.

The most common backlighting method used are Fiber Optics and Electroluminescent panels. Sigma 6 Electronics works with a proprietary LED design which offers the same uniform backlighting over a large area. Both Fiber Optics and Electroluminescent have large prototyping costs due to tooling. Some applications require an exterior light source or additional layers. Our LED technology embeds a few LEDs to light up a large area and is significantly less expensive in the prototype stages.

MOUNTING ADHESIVES

INTEGRATION

The majority of our membrane switches are shipped with a pressure sensitive adhesive on the back side. The adhesive is designed specifically for bonding to smooth metal and high surface energy plastic surfaces. Pressure is required when applying membrane switches to ensure a strong bond which will continue curing for 72 hours after being applied.

As a general rule, we recommend removing a small corner of the backing, align and apply part of the membrane switch. Then remove the rest of the liner and apply firm pressure. Never bend a membrane switch in an LED, dome area or try to activate a dome without a firm backer.

Membrane switch panels are most often mounted to a rigid substrate panel, most commonly aluminum or acrylic. Sigma 6 Electronics can design, manufacture, and assemble any panel at customer’s request. Sigma 6 does this interface integration process for rugged applications in: industrial, medical, and military applications.

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