Environmentally Sealed Switch

Environmentally Sealed Switch

CSI Keyboards was asked by our customer to design an environmentally sealed switch with backlight assembly that would withstand the destructive temperatures of the arctic as well as the dry heat of the desert without these extremes degrading its functionality. The reliability of these switches was paramount as they were to be used as lighting controls for temporary military field housing. 

CSI recommended an assembly consisting of a silicon rubber keypad, a Kapton etched circuit, and silicone adhesive based on the bullet points below:

· Silicone rubber is extremely durable and performs well in all temperatures.
· Etched copper circuitry on Kapton film is a more durable material than silver ink on
polyester, able to withstand moisture and extreme temperatures.
· Silicone adhesive is extremely durable and reliable regardless of extreme temperatures
or wet environments.

After approval of the proposed design, CSI produced first articles for test and qualification. These were tested to expanded temperature standards and have been qualified from +150 C down to -50 C. After test and qualification our customer went into production with our design. Over the first 2 years of production they have experienced zero field failures. If your application needs to be reliable and stable in extreme temperatures, CSI Keyboards is the supplier to partner with.

Molded rubber keypad: copper flex circuit based with domes and backlit keys

Parking Meter Problem Solved

Parking Meter Problem Solved!

A  customer recently contacted CSI Keyboards to help them design and manufacture a new custom tactile keypad for outdoor use. The application was a parking meter that was currently using a non-tactile capacitive switch using molded polycarbonate. The customer’s end goal was to create a keypad that had raised profile and tactile feedback with each button press, while maintaining a waterproof seal against the weather’s elements.

 

CSI presented them with a new design that used molded silicone rubber to give height to the keys, improve button feedback using tactile domes on a Kapton copper flex circuit, while also using the silicone rubber to provide a water tight seal for their parking meter. The final result was a more robust HMI that was completely waterproof, provided the user with tactile feedback, which was also outdoor weatherable by using a UV resistant coating. All of this was achieved while maintaining a sharp visual aesthetic for their parking meters. This keypad can now be seen on city streets across the country. 

What is a Membrane Switch?

What is a Membrane Switch?

Membrane Switch

A membrane switch is an interface between user and machine, enabling an operator to communicate with equipment, instrumentation, or machinery. A membrane switch is a printed electronic circuit that uses pressure to open and close a circuit. The membrane switch circuitry is most often screen printed using conductive inks which are typically made of silver or carbon. Membrane switches are part of a range of devices considered to be user interfaces or human machine interfaces (HMIs) along with touch screens and mechanical switches.

Membrane Switch Construction

A typical membrane switch assembly typically consists of six to seven main layers:

  1. Graphic Overlay – Graphic overlays are typically constructed of polyester, the material of choice due to its superior chemical resistance and flex life compared to polycarbonate. CSI can either digitally print, screen-print, or employ a combination of both methods to insure you get the right colors, textures, and finishes your Silver Flex membrane switch design requires.
  2. Overlay Adhesive – The overlay adhesive layer bonds the graphic overlay to the top circuit layer. This overlay adhesive is typically an acrylic adhesive, selected for its durability and ability to maintain adherence in atypical environments, such as moist environments.
  3. Top Circuit Layer – Typically a .005″ – .007″ heat-stabilized, polyester printed layer with silver-filled, electrically conductive inks and dielectric inks. This layer can also encapsulate metal domes or incorporate polydomes, which are used to achieve tactile feedback, an important design consideration impacting usability.
  4. Circuit Spacer – This layer separates the top circuit from the bottom circuit, so the switch remains normally open until the keypad is pressed. The circuit spacer is a polyester spacer with adhesive on both sides.
  5. Lower Circuit Layer – The lower circuit layer is typically a .005″ – .007″ heat-stabilized, polyester-printed layer with silver-filled electrically conductive inks and dielectric inks. This layer terminates as a flexible tail that serves as the interconnect to controller PCB’s or other electronics.
  6. Rear Adhesive Layer – This adhesive layer bonds the entire membrane switch package to the product enclosure, housing, or to a rigid support panel. CSI can specify the appropriate adhesive type and thickness to bond your membrane keypad to your equipment.
  7. Rigid Support Layer – This optional layer can add structural integrity to the membrane switch assembly. Materials can be aluminum, FR-4, steel, etc. Mounting hardware such as studs and standoffs can also be utilized in this layer.

Silicone Elastomeric Rubber Keypads

Silicone rubber keypads use compression-molded silicone rubber with conductive carbon pills or with non-conductive rubber actuators.  They have exceptional resistance to extreme temperatures and aging, making them an ideal choice if reliability is a prominent concern due to likely environmental influences.

A rubber membrane switch uses compression-molded silicone rubber with conductive carbon pills or with non-conductive rubber actuators. Rubber keypads are relatively inexpensive on a per-piece basis, but require fairly expensive tooling, usually making them a design choice for higher-volume projects.

Silicone rubber keypad switches have numerous features that set this type apart from other traditional membrane switch designs. Some of the main differentiating features of this type make the silicone rubber keypad switch an ideal choice for applications requiring more durability or better resistance to exposure to moisture, chemicals, or other compounds.

Some of the primary distinctive features of silicone rubber keypad switches include:

  • Work as a conductive shorting device for Silver Flex membrane switches, PCB membrane switches, and Copper Flex membrane switches
  • Can utilize carbon pills, non-conductive rubber actuators, or stainless steel tactile domes
  • Actuation forces and switch travel can be customized
  • Any shape or size can be designed
  • Multiple colors can be achieved by flow molding the color during the compression-molding process
  • Rubber keypad top graphics can be customized by screen-printing
  • Rubber keypad switches can be PU spray-coated for enhanced durability
  • Rubber membrane switches have excellent weatherability for outdoor use
  • Can be designed to seal the keypad assembly from moisture and contaminants
  • Silicone rubber is resistant to chemicals and moisture
  • Laser etching the rubber keypads can allow for backlighting individual keypads
  • Backlighting options

Silver Flexible Membrane Switches

Silver Flex membrane switch panels utilize screen-printed silver and carbon conductive inks printed on flexible polyester layers separated by an adhesive spacer. This is the most common design used in flexible, custom membrane keypads, offering a slim, space-saving design.

Silver Flexible circuity is also more cost-effective when compared to electromechanical switch assemblies and the process of screen-printing conductive silver inks onto a flexible, film substrate poses less potential threat to the environment compared to chemically etched copper.

Silver Flex membrane switches offer you a variety of design options: 

  • Tactile and non-tactile with either metal or polyester tactile domes
  • Embedded LED’s
  • Fiber Optic backlighting
  • EL (Electroluminescent) backlighting
  • Rigid backers such as aluminum and FR4
  • EMI/RFI shielding
  • Standard connectors on .100″ centers, or prepared for ZIF connections
 

Additionally, a Silver Flex membrane switch uses a graphic overlay, which also has a number of design options:

  • Digital printing, screen-printing, or a combination of both
  • Pillow or rim embossing
  • Transparent and/or tinted display windows
  • Selective textures
  • UV hard-coat surface finishes
Copper Flex Membrane Switches

The Copper Flex Membrane Switch constructions are ideal for smaller designs, where space is at a premium, or where dense circuit patterns or trace routing limitations exist. Copper Flex membrane keypads utilize silver or copper layers which are laminated to a dielectric layer and etched away.

This switching technology combines the ability to accommodate the complex circuit patterns of a FR4 rigid printed circuit board with the flexibility of a membrane switch. Copper Flex keypads also have the advantage of being able to “hard” solder both active and passive components into the assembly, making it a good choice in high-vibration environments.

Copper Flex membrane switch panels can be produced using polyester or polyimide (Kapton) as the base material depending on your interface requirements. A very thin sheet of copper is laminated to the flexible film substrate then chemically etched away, leaving copper traces. 

Copper Flex membrane switches offer you a variety of design options:

  • Single and double sided designs
  • Lower electrical resistance and higher conductivity vs. traditional Silver Flex membrane switches
  • Tight trace routing capabilities
  • Thin profile and flexibility of Silver Flex membrane switch
  • Plating options can be tin-lead, nickel, or gold
  • Tactile and non-tactile with either metal or polyester tactile domes
  • LED’s and other components can be soldered

PCB Based Membrane Switches

The PCB Membrane Switch construction utilizes a printed circuit board (PCB) which can serve a dual purpose in your membrane switch design. PCB Switches are typically more costly than Silver Flex membrane keypads, but can accommodate dense circuit patterns and more complex circuit patterns compared to Silver Flex membrane keypads.

A PCB membrane switch also allows the electronic components to be “hard-soldered” into the PCB, whereas membrane switch components are placed using a polymer thick film conductive paste. With a PCB membrane switch, the PCB can serve as a rigid backer, and is also a very durable and reliable method to incorporate LED’s, resistors, LCD’s and other components.

PCB membrane keyboards offer you a variety of design options:

  • Tactile and non-tactile with either metal or polyester tactile domes
  • Pillow or rim-embossed graphic overlays
  • Embedded LED’s that are soldered directly into the PCB
  • Fiber Optic backlighting
  • EL – (Electroluminescent backlighting)
  • Rigid backers such as aluminum and FR4
  • EMI/RFI shielding
  • Unlimited choice of connectors, which can be soldered directly into the PCB 
CSI Keyboards PCBA Rubber Keypad

Requirements for Handling Membrane Switches

Requirements for Handling Membrane Switches

CSI Keyboards takes pride in manufacturing quality products and wishes nothing more than to maintain the quality of your products through years to come. But even the highest quality products can be damaged stemming from all different reasons such as shipping, handling and application factors.

CSI has been working with membrane switches for over 35 years and consider ourselves the experts in product care and handling. Below is some information on what care must be taken to prevent damage to your products as well as what maintenance suggestions.

Prevention:


Windows are typically the most easily damaged portion of a membrane switch assembly. CSI suggests that you use a clear hard coat to prevent scratching, which minimalizes the chances of scuffing that could be caused by everyday handling. CSI also coats the uncoated (reverse) side of the window with a peel-able masking film. We suggest this stays on the product until the very last moment prior to assembly in order to protect the window during assembly handling. Upon request, CSI will also apply a poly-masking film that goes over the top of the overlay window and can also be peeled off upon assembly. 

Adhesive choice may not matter when it comes to the damage that your product can take, but it will matter in accordance with the overall life expectancy of your product. Thicknesses in adhesives vary and certain adhesives are better suited for certain surfaces. Specifically, you will need to know what kind of surface you are applying your product to and whether or not it has high surface energy (HSE) or low surface energy (LSE). Other surfaces to consider would include powder coated paints, silicon or rubber, and metal finishes. CSI’s staff will be able to help you choose the right adhesive for your application and determine what kind of surface you are looking to apply your product to so that your product won’t end up peeling from its surface.

Shipping can play a huge role in damaging your products. CSI’s experienced shipping staff has procedures in place to ensure your products will be delivered to you in perfect condition. Bubble wrap, foam and shipping paper is also available to fill in empty gaps to secure the product. If you are still concerned about how your product is packaged to prevent any damage, ask for specific packing instructions to be sent to our shipping team.


Maintenance, Care and Handling:

 

  1. Do not bend the product.

  2. Do not attempt to take the product apart.

  3. Do not press or push the membrane switch keys while it is unsupported. The keypad must be on a flat surface.