Membrane Switches

Chapter 1: What are Membrane Switches?

Membrane switches are a type of human-machine interface characterized by being constructed from several layers of plastic films or other flexible materials. Conductive materials and graphic inks are printed or laminated onto the surface of these plastic films. They function by temporarily closing or opening an electric circuit. The compact and efficient construction of membrane switches makes them suitable for a vast array of applications such as household appliances and industrial equipment interfaces.

Chapter 2: History of Membrane Switches

Membrane switches have been around for about five decades. The first membrane keypads were introduced in the early 1970s. They were made of polycarbonate plastic films printed with copper or silver-infused ink to create the electronic circuit. These were composed of two conductive layers with one spacer in between. The products were inexpensive, but they were threatened by problems such as degradation and cracking of the polycarbonate film and missing tactile feedback.

The next iteration solved the durability and quality-of-life issues by changing the plastic film into polyester and adding metal domes into the design. However, membrane switches had not been readily adopted since mechanical keyboards dominated the huge personal computer market during that time. Mechanical keyboards were preferred due to their more tactile feedback.

Come the 1990s, thinner keyboard keys and membrane switches were used to make more compact and quieter keyboards. By that time, smaller electronic devices were the future of technology. The emergence of appliances and equipment with small electronic components further elevated the need for membrane switches.

Today, the global membrane switch market is approximately $4.2 billion in 2015 and is expected to grow to $13 billion by 2025. Membrane switches are extensively used in industrial, medical, and consumer goods applications.

Chapter 3: Benefits of Membrane Switches

Membrane switches are extensively used in a variety of applications, whether domestic, commercial, or industrial. Other types and forms of user interfaces include touchscreens, keyboards, switches, and selector knobs. However, membrane switches are preferred because of their compact profile, simple construction, reliability, resistance to harmful elements, and low cost. These advantages are further elaborated on below.

Thin and Compact Profile

Each plastic layer of a membrane switch can have a thickness of about 0.005 to 0.040 inches (0.127 to 1.03 mm). They typically have three to six layers depending on the design. Even applying the conductive and graphic inks and installing other components such as the metallic domes and EMF screens, the final thickness still results in only a fraction of an inch. This makes them suitable for household appliances and equipment controllers with small form factors.

Simple Graphic Interface Construction:

The preparation process for the graphic overlay is straightforward. The graphics design or artwork can be made from software such as AutoCAD, SolidWorks, and Adobe Illustrator. After creating the artwork, it is digitally printed onto the overlay. There is no need for additional machining processes such as embossing, engraving, or stamping. These additional processes are only done to improve aesthetics and tactile quality. However, digital printing is not the only method of creating graphic overlays. Many companies in the industry also use screen printing.

Resistance Against External Elements

One popular advantage of membrane switches is their sealed construction. Sealing is achieved by pressure-sensitive adhesives or heat seals. Plastics such as polyesters and polycarbonates provide a sufficient barrier against moisture and chemicals without reducing the visibility of the artwork. There are no cavities where hazardous liquids or gasses can enter or accumulate. Membrane switches are the desired type of human-machine interface for devices with high protection ratings.

Easy Cleaning and Maintenance:

Since there are no cavities where water, dust, and contaminants can accumulate, membrane switches are easy to clean. The overlay can easily be wiped to remove any dirt. Their complete seal allows them to be subjected to equipment washdowns without any risk of damaging the control circuit. Moreover, because of few moving parts, membrane switches require almost no maintenance.

Sufficient Tactile Feedback:

Compared to touch screen interfaces, membrane switches have an advantage because they provide tactile feedback. Tactile feedback is useful in applications with a risk of equipment malfunction or shutdown. This is possible when the wrong sequence of keys is pressed. Tactile feedback helps the operator know that the key is pressed.

Shielding from Electromagnetic Interference:

Unwanted electromagnetic frequencies and electrostatic discharges are potential threats to electronic devices. These can cause electronics to malfunction, especially controllers that use low-power circuits. A layer of EMF shielding can be added to membrane switches by printing a grid or mesh using conductive ink. The EMF shield can be made without any discontinuity, which defeats the purpose or lowers the efficiency of the shielding.

Low Cost:

Because of its small blueprint and readily available construction materials, membrane switches are more economical than touch screens or mechanical interfaces. They are made from lesser parts that can be easily assembled by basic processes such as applying pressure-sensitive adhesives or heat sealing. Its low cost makes it the desired interface for consumer goods or household appliances.

Chapter 4: Membrane Switch Construction

Membrane switches are composed of several components in the form of layers assembled using pressure-sensitive adhesives or heat sealing films. Its main parts are an overlay containing the graphic elements; a circuit that includes the conductive tracks, metal domes, circuit tail, and terminals; and a spacer that maintains a break between the switch contacts.

Membrane Switch Overlay

Also known as top or graphic overlay, the overlay is the outermost layer of the membrane switch. Since this layer is on the exposed side of the membrane switch, it is made from materials that have good flexibility, clarity, durability, chemical resistance, and barrier properties. There are two common materials used for making the overlay.

• • Polyester: This is a plastic material commonly known as polyethylene terephthalate (PET). Polyester is known for its clarity, flexibility, and chemical resistance. Its flexibility allows it to be more durable than other materials, especially when used on switches with tactile feedback. To achieve good resistance to puncture and tearing, the film is made through biaxial orientation.
  • Polycarbonate: Polycarbonate is the desired film for industrial applications due to its inherent flame-retarding property and abrasion resistance even without additional surface treatments such as hard-coating. Polycarbonate is also more economical and easier to process than polyester. The film can be produced and processed quickly without worrying about shrinking and warping, as experienced in working with polyester films.

  Other materials that can be used as overlays are acrylic, vinyl, and PVC.

  

  
  

  Graphics can be printed on the reverse side or front side. Reverse side or sub-surface printing is the more common method since it produces longer-lasting prints. The overlay plastic film protects the graphics from abrasion and chemical attacks. Front side or top-surface printing, on the other hand, creates various features such as selective texture and windows.

  

  
  

• Membrane Switch Domes:

  Domes are the components that provide tactile feedback. They can be made from metal or plastic. The size of the keys of the membrane switch determines what size the dome will be, with sizes ranging from 0.24 to 0.79 inches (6 to 20 mm). Additionally, the dome’s height is closely related to the size of the dome and can be 0.010 to 0.057 inches (0.25 to 1.45 mm).

  A critical aspect of using domes is the actuation force or trip force necessary to depress the dome and activate the switch, which can range from 1.41 oz to 80 oz (40 to 2250 g). Domes come in a wide assortment of shapes and sizes, including:

  • Four-legged
  • Triangle
  • Round
  • Oblong

  Metal Domes

  Metal domes are made from stainless steel or copper alloys held in place by a dome retainer layer or a spacer layer. Aside from providing tactile feedback, metal domes also function as a part of the circuit. When pressed, the metal dome shorts the open contacts of the switch. When pressed, the metal dome shorts the open contacts of the switch. Metal domes have a very few profile and can reach life ratings of up to 10 million presses, making them ideal for many applications.

  
  

  

  
  

  Plastic or Poly Domes

  Plastic domes are typically made from polyester because of their flexibility; hence the name "poly" dome. Poly domes have a layer of their own. In some designs, the poly dome layer can also become the overlay or graphics layer. The poly dome layer is a polyester film with dome or blister-like features. At the concave side of the dome is a printed conductive ink that completes the circuit when the button is pressed.

  
  

  

  
  

  Retainer Layer

  The retainer layer with the primary function of holding the metal domes in place. This is commonly made from polyester film, similar to the poly dome layer. A retainer layer can hold a dome in position without needing an adhesive layer.

  Spacer Layer

  This layer is used to create a break in contact between the two conductors of the switch. This allows the switch to have its open position. In some designs of tactile-type membrane switches, it can also act as a retainer to keep the metallic dome in place. The spacer layer has channels between the empty cavities or the sides of the keypad for venting air. This prevents air from being compressed in the cavity when the key is pressed.

  
  

  

  
  
• Circuit Layer This layer is where the conductive paths of the switch are applied. These conductive paths can be produced through two main methods: screen printing and photochemical etching.
  • Screen Printing: This method uses a stencil containing the pattern of the circuit. Silver conductive ink is flooded on the stencil placed above a substrate. The substrate used is typically a polyester film. This method is used for thinner and more flexible membrane keypads.
    
    

    

    
    
  • Photochemical Etching: In contrast, this method uses a copper-laminated substrate selectively patterned through photolithography and chemical etching. The result can be a printed circuit board (PCB) or a flexible printed circuit (FPC) that is thicker and more durable than screen-printed membrane keypads.
    
    

    

    
    

    Depending on the type of membrane switch, the circuit can be designed and built in two ways.

  • Two-layer Circuit: In this design, the circuit layer is separated into the upper circuit and lower circuits. Each circuit layer contains a conductive path that leads into or goes out of the switch. The spacer layer separates the two layers. When a switch is pressed, the upper circuit deflects and touches the lower circuit completing the circuit.
    
    

    

    
    
  • Single-layer or Single-sided Circuit: As the name suggests, a single-layer switch has only one circuit layer. A discontinuity creates a break in the circuit in the conductive path that is printed onto the substrate. The circuit is completed using a metallic dome or conductive ink printed on the reverse side of a plastic dome. When a key is pressed, the dome flattens against the circuit layer creating a single conductive path.
    
    

    

    
    

• Circuit Tail

  The circuit tail is the part of the circuit that connects the membrane switch to the machine's control unit. It is a flat, flexible ribbon composed of several conductive tracks printed on a polyester strip. At the end of the circuit tail are standard connectors that match to the termination block of the control unit. Common connector options are plain header, latching header, or solder tabs. The circuit tail can also be a ZIF (zero insertion force) style, which differs on the force applied between the circuit tail and the control unit terminals. ZIF is used for more delicate circuits where the control unit terminals are weak and easy to damage.

  
  

  

  
  

• Mounting Adhesive:

  Mounting adhesive is placed at the back of the membrane switch to create a secure bond between it and the mounting surface. They are chosen according to their bonding strength, thickness, and operating temperature. Mounting adhesives are an elastomeric compound composed of high strength or modified acrylic.

  
  

  

  
  

Acrylic Adhesives

Acrylic adhesives are the industry standard due to their exceptional adhesion to metal and plastics. They allow repositioning for greater placement accuracy when bonding with plastics. The benefits of acrylic adhesives include:

• Humidity Resistance: Humidity does not affect acrylic adhesives. They show no reduced bonding after exposure to 90 °F (32 °C) heat with 90% relative humidity.
• UV Resistance: Membrane switches are designed to withstand all climatic conditions, including ultraviolet rays.
• Water Resistance: Being submerged in water does not affect the bond strength of acrylic adhesives.
• Temperature Cycling Resistance: Bond strength is maintained at 158 °F (70 °C) down to temperatures as low as -20 °F (-29 °C).
• Chemical Resistance: Acrylic adhesives are unaffected by exposure to oil, mild acids, and alkalis
• Bond Build-up: The bond strength of acrylic adhesives increases over time and after experiencing fluctuations in temperature
• Heat Resistance: When exposed for short periods to temperatures up to 400 °F (204 °C), acrylic adhesives remain bonded. If exposure is for longer periods, such as weeks or days, the bond strength can withstand temperatures of up to 300 °F (149 °C).

The thickness of the mounting adhesive is a crucial factor in choosing the right adhesive to fit the needs of the specific membrane switch. When bonding a membrane switch to a smooth surface, an acceptable thickness is 0.079 in (2 mm). For textured surfaces, the thickness of the mounting adhesive should be 0.2 in (5 mm) to maximize the surface to which the adhesive may bond.

Chapter 5: Specifications and Additional Features

Before selecting a membrane switch to use or supplier to order from, it is best to understand of its specifications and features. And like any other electronic or electrical device, it is important to fully determine the characteristics of the system where the interface will be installed. In addition, the electrical specifications of the membrane switch must be applicable to the system to prevent any electrical shorting or premature failure of the membrane switch or control unit. Moreover, other features such as coatings, backlighting, and precision cutting are worth noting.

Performance and Electrical Circuit Specifications:

These data provide the characteristics and performance of the electrical circuit. Some of these specifications are enumerated below.

• Rated Voltage and Rated Current: The design voltage and amperage of the circuit.
• Maximum Load: The maximum power that the circuit can withstand.
• Loop Resistance: Resistance of the circuit when the switch is closed.
• Open Resistance: Resistance when the circuit is open.
• Design configuration: This can be either be matrix type, common bus, or a combination of both. A matrix keypad has unique pairs of row and column wires. In contrast, a common bus has a single bus wire where one terminal of each switch is connected.
• Termination: The type of connector standard matches with the control unit termination block.
• Contact Bounce: The period of intermittent continuities and discontinuities upon switch actuation due to the actuation force and inherent elasticity of the contacts, typically in the order of milliseconds.
• Capacitance: The amount of charge the insulation can store.
• Dielectric Strength: The maximum electric potential that the insulating material, material with a specific thickness, typically polyester in the case of membrane switches, with a specific thickness can withstand.
• Breakdown Voltage: This is the maximum electric potential where the material loses its insulating properties. However, its value depends on the thickness of the material.
• Actuation Force: The amount of force required to activate the switch.
• Actuation Life: The typical range of the number of cycles before the switch fails.
• Protection Rating: The degree of protection or sealing effectiveness applied to the construction of the switch.
• Operating Temperature: The design ambient temperature for operating the switch without affecting its design functions and incurring damage over time.

Safety Certifications

Certifications assure that the product conforms with the general safety standards mandated by national and international organizations. Widely accepted certifications are Underwriter Laboratories (UL Listed or Recognized) and CE.

Electrostatic Discharge or Electromagnetic Frequency Shield

Since membrane switches operate with low voltages and low currents, stray electrostatic discharges (ESD) or electromagnetic frequencies (EMF) can short the circuit or disrupt the electric current. An ESD/EMF shield is included in the membrane switch by adding a layer of conductive material underneath or above the circuit. Other shield designs feature a complete wrapping of the circuit layer. An ESD/EMF shield is made of a thin layer of polyester with conductive ink printed in a grid or mesh-like pattern. Another form is copper or aluminum foil with or without polyester lamination. The shielding can be grounded by connecting it to the metal enclosure, metal backer, or grounding connection from the circuit tail.

Tactile or Non-tactile Feedback

Tactile feedback is provided by metal or plastic domes. As mentioned earlier, this feature is necessary for helping the operator know that a keypress is registered. Different dome designs have varying actuation forces. Non-tactile membrane switches, on the other hand, are used in applications where a thin profile is more valuable than feedback.

Backing Panel or Support Layer

A backing layer is used to provide rigidity to the membrane switch. This layer can be omitted depending on the application since keypad support can be the panel on the device's panel itself. The backing layer has a pressure-sensitive adhesive applied on its underside for mounting.

Selective Texturing

Selective texturing is the application of a transparent, scratch-resistant, matte finish on specific areas of the overlay to accentuate graphic elements for improved aesthetics. This finish also helps minimize scratches that are easily developed on glossy finishes. This makes a textured finish desireable for heavy-duty use, such as interfaces for industrial equipment.

Hard Coating

This is a common surface treatment for surfaces with low durabilities, such as plastics, paper, wood, and glass. Hard coating is done by applying a layer of ultraviolet-curable polymer resin through different surface deposition methods. The composition of the polymer and the deposition method varies from each manufacturer. The polymer used has better durability and chemical resistance than the polyester or polycarbonate film underneath.

Surface Embossing

Embossing is done to improve the aesthetics and tactile features of the overlay by raising some of its surfaces. There are different types of embossing, namely pad, dome, and rim. Pad is a pillow- or plateau-like embossment of the whole key or keypad. Dome-emboss is spherical as described by a poly dome key design. Rim or rail-emboss is done by raising the edges or perimeter of the keypad. Aside from these three common embossings, raised patterns can also be custom shapes such as Braille patterns, texts, and logos. The height of the embossing can be made multi-level to enhance the look and feel of the graphics.

• Pad embossing is a pillow- or plateau-like embossment of the whole key or keypad.
• Dome embossing is spherical, as described by a poly dome key design.
• Rim or rail embossing is done by raising the edges or perimeter of the keypad.

Aside from these three common embossings, raised patterns can be custom shapes such as Braille patterns, texts, and logos. The height of the embossing can be made multi-level to enhance the look and feel of the graphics.

Display Windows

Windows are transparent or translucent areas intended for including light crystal displays (LCD) or light-emitting diode (LED) displays. Windows are designed according to the requirement of the display to maintain readability. LCDs require clear windows with minimal filtering. For LED segment displays, optical filtering is required to maintain readability in bright light. Readability is increased by enhancing the contrast of the LED segments with their background. This is done by printing a gray or amber filter on the display window with varying degrees of transmission. The color of the filter depends on the color of the LED.

Indicators and Backlighting

Indicators are used for pointing out activated keys, while backlighting is used to improve the interface's readability and aesthetics. The four main types of backlighting are:

• Light-emitting diodes (LED) are a common choice for indicator lights and displays because of their reliability, long life, and low cost.
• Optical fibers offer uniform brightness, long operating life, low heat emission, and low power consumption.
• Electroluminescence Backlighting (EL) lamps offer the same functionality and features as optical fibers but at a lesser quality.
• Light Guide Films (LGF) are thin sheets of optical films that can be placed over surfaces to be illuminated.

Backlighting is an added feature for membrane switches that illuminates or lights up the front surface of the switch. Backlighting can be used in several ways, such as lighting one area, the entire area, or several selected areas. Backlighting is not a necessity for membrane switches, but it does add benefits such as:

• Improved visibility in dark or dimly lit areas
• Easier-to-read inputs and keys
• Highlighted areas like indicator lights on control panels
• Enhanced appearance and up-to-date modern appeal
• General accessibility for disabled users

Dead Front

Dead front is when a symbol blends into the background of the front view of the membrane switch and becomes visible when it is illuminated. It is a method of display used on instrument panels as an additional option for the inclusion of legends against a dark backdrop.

Cutting Technology:

Membrane switches are designed to fit into a plastic molding or metal panel assembled with the control unit. Specific dimensional tolerances must be complied with to ensure proper mounting and sealing of the electrical components. This is influenced by the type of cutting method used. The most popular methods are steel rule die cutting and laser cutting. Steel rule die tooling is used due to its low capital cost and high cutting speed. On the other hand, laser cutting is preferred in applications where precision and burr-free cutting is necessary.

Conclusion:

• Membrane switches are a type of human-machine interface characterized by being constructed from several layers of plastic films or other flexible materials. These films are printed or laminated with conductive inks. They function by temporarily closing or opening a circuit.
• Membrane switches are extensively used in a variety of applications, whether may it be domestic, commercial, or industrial. They are preferred because of their compact profile, simple construction, reliability, resistance to harmful elements, and low cost.
• The main parts of a membrane switch are an overlay containing the graphic elements; a circuit that includes the conductive tracks, metal domes, circuit tail, and terminals; and a spacer that maintains a break between the switch contacts.
• Aside from its electrical performance and characteristics, common features of membrane switches are UV hard coats, selective textures, clear or optically filtered display windows, indicator lighting, backlighting, precision cutting, and embossing.