How to Protect a Membrane Switch from UV Exposure

How to Protect a Membrane Switch from UV Exposure

Membrane switches intended for outdoor use are most susceptible to fading from sun exposure. Being constantly exposed to the sun’s ultraviolet (UV) rays will eventually cause the outer graphic overlay layer to fade over time. In many cases, the keys will eventually become transparent or otherwise difficult to see preventing users from being able to distinguish between the different keys.

An outdoor weatherable hardcoat material is essential when designing a membrane keypad that is used outdoors. The Uweatherable material utilizes a coating technology that resists yellowing and hazing, no matter how bright the sun is. Along with good UV resistance, this material also has chemical and abrasion resistance. This material is the perfect choice for outdoor keypad applications.

Additional Key Benefits of the UV Weatherable Material:

  • Extreme durability
  • Printable
  • Abrasion resistance
  • Chemical resistance to strong household cleaners and industrial chemicals

CSI Keyboards UV Resistant Membrane Switch

What is a Keyboard Matrix in a Membrane Switch?

What is a Keyboard Matrix in a Membrane Switch?

A keyboard matrix circuit is a type of keyboard that has a grid-like array of horizontal and vertical wires connecting the key switches. If the keyboard features 8 rows and 8 columns of wires, for instance, it can support up to 64 keys. The switches are located at the intersection of these wires. Keyboard matrix circuits contain a scanner or sensor that monitors these wires and is constantly scanning the grid determining which key has been pressed. The main advantage of using a matrix circuit design is the reduction of required wires. 

The matrix arrangement allows for current to flow backwards through part of the circuit, which can lead to phantom keys. Keyboard matrix circuits usually require diodes at the intersections of the wires to prevent phantom keys. Phantom keys, also known as “ghost keys” occur when the keyboard thinks that a key is pressed when it is actually not pressed at all. The diodes are typically placed in series with each switch (before or after).

How are Membrane Switches Constructed?

How Are Membrane Switches Constructed?


Membrane Switch Construction

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

  1. Graphic/Interface Layer – The graphic or interface layer is 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. The interface layer can also be molded silicone rubber which has become a very popular choice. 
  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.

What is the Difference Between Flexible Circuit and PCB Based Membrane Switches?

What is the Difference Between Flexible Circuit and PCB Based Membrane Switches?

When designing a membrane switch, one of the most critical aspects that must be decided early-on in the design stage is the circuit type. The whole design and construction of the part is based around the type of circuitry that is used. The three types of circuitry options are: silver flexible circuits, copper flex circuits, or printed circuit board (PCB). If you are unsure as to what circuit is best for your application, CSI can work closely with you in proposing which option is ideal for the design.

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

What is Embossing in Membrane Switches?

Embossing in Membrane Switches

To provide tactile feel to a membrane switch or graphic overlay, the material can be embossed to raise the key area. Embossing has really become standard in membrane switch designs. Not only does it give it a better look and feel, but it also allows the user of the end product to find the key and provide tactile confirmation that the button is present even before it’s pressed. 

There are essentially three options when it comes to embossing membrane switches: pillow emboss, rim or perimeter emboss, and LED or Brail emboss.  

CSI Emboss Options for Membrane Switches
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