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 a Dome Switch Keypad?

What is a Dome Switch Keypad?

CSI Keyboards designs domes into the majority of our membrane switches. Dome switch keypads use two circuit board traces in conjunction with a metal dome. Metal domes, which are typically made of stainless steel, are momentary switch contacts that provide tactility or “snap” when pressed. The domes become normally-open tactile switches when actuated on the circuit. 

The main advantage of dome switch keyboards is the tactile snap or feedback when actuated. When pressing the key, the user realizes they have actually actuated or successfully pressed the switch due to the feel and sound feedback received from the dome.

Another major benefit of the dome switches are the the long lifespan and reliability. Standard dome switch keypads are now rated from one million to even five million cycles. They are still the most reliable type of switch available in the membrane switch space. 

Dome Options:

  • Dome plating: can be plated in other metals such as nickel, silver or gold
  • Dome shapes: four legged, triangle, round, oblong
  • Dome sizes: standard sizes are 6mm, 7mm, 8.5mm, 10mm, 12mm, 14mm, 16mm
  • Dome forces: standard forces range from 85g to 700g
  • Other options: dimpled, hole in center (for backlighting key), double-contact closure domes. 
 
Understanding Dome Actuation Forces:

Actuation force is the amount of force required to make the dome snap. Or in other words, how much force is required to change the dome from the open position to the closed position. Dome actuation force is typically measured in grams. 
 
Internal Venting for Dome Switches:
 
A critical aspect to keep in mind when designing a dome switch keypad is internal venting. When the dome is actuated, air is trapped underneath the dome with nowhere to go. It is therefore recommended that venting is incorporated in the membrane switch design. Not properly venting the dome will result in poor tactile response, key failure and potential issues with air entrapment under the membrane surface.
 

How Do I Choose the Right Dome for my Membrane Switch?

The CSI engineers will work very closely with you in deciding which dome is best for your application. Typically the decision is based on the force of the dome (how soft or hard of a press it takes to actuate the dome) and is extremely subjective. CSI can mock up different sample keypads with different dome options so the customer can decide through a more “hands-on” approach.

Dome Arrays

When a Non-Tactile Membrane Switch Makes Sense

When a Non-Tactile Membrane Switch Makes Sense

A non-tactile membrane switch is a switch that lacks snap or tactility when pressed or actuated. Non-tactile membrane switches are constructed of copper flex circuity using polyimide Kapton as the base material. Copper flex keypad switches are manufactured by laminating a thin sheet of copper to a flexible film substrate. The copper is then chemically etched away, leaving the copper traces. An additional layer of polyimide is laminated to the circuit leaving the gold contacts exposed.

Non-tactile membrane switches are typically designed into a product due to one of the following reasons:

  1. The Need for Dense Traces to Create a Sensor or Slider Interface: typically in user interface designs where the customer is looking for a sensor or slider interface solution, where the user can run his/her finger along the surface and make contact/actuation. CSI can integrate added points, ribs, or sections providing tactile feedback. The advantage of this product is that it does not require any sensor electronics, and has no moving parts. The solution is extremely robust and cost effective. It’s a great alternative to more expensive and sophisticated capacitive touch solutions. 
  2. Size Constraints: when there simply just isn’t enough room for domes due to the small size of the design.
  3. Light Activation Force: the customer wants to simply be able to run their finger over the key with a very light press in order to make contact ie does not want any snap when pressing the key.

Advantages of Copper Flex Membrane Switches

  • Improved creasability – the ability to fold or crease without causing open circuits which you are susceptible to using silver ink
  • Ability to put solder components directly on flex, better adhesion than bonding to silver ink
  • Much more resistant to thermal shock
  • No potential problems with silver migration
  • Greater conductivity
  • Lowering the closed loop resistance and switch bounce will be reduced by gold plating the contacts. We will have a gold to gold contact with no bounce versus silver to gold dome
  • Applying stiffener to tail end using heat lamination versus cold lamination which provides better adhesion and better for pinning
  • Ability to have a 0.5mm pitch vs. the 1.0mm minimum with printed silver
  • Tighter trace routing capabilities

Backlighting Copper Flex Membrane Switches

LEDs: 

LEDs are the most popular and economical method for keyboard backlighting. LEDs are a great option for backlighting non-tactile membrane switches, as they can be easily integrated into the flexible circuit and act as indicator lights providing visual feedback for users, since they don’t feel the snap of a switch when they pressed. LEDs are most commonly used to backlight keys, icons and symbols. LEDs are also typically used as indicator lights. A combination of LEDs, Light Guide Film and proprietary CSI backlighting methods can be designed to backlight the entire surface of a user interface.

Light Guide Film (LGF): 

Light Guide Film is designed to evenly distribute light from top or side firing LEDs, providing bright, uniformed illumination. It also reduces the amount of LEDs needed, saving power consumption. More on light guide film technology below. The design and utilization of light guide film (LGF) technology has become one of the most common methods of interface backlighting. CSI Keyboards uses proprietary techniques to design the light guide film so it is optimized for light redirection and reflection giving the customer the brightest possible backlighting solution. Light guide film dots are also designed and implemented which allow for the optimization of light distribution to obtain maximum brightness and uniformity. Common problems that many of our competitors face are light leakage and hot spots. CSI’s backlighting designs prevent any light leakage and hot spots from occurring, and also result in much brighter light guide film and interface.

Electroluminescence (EL): 

EL is applied on a very thin layer between the graphic overlay and the circuit. EL uses a printable ink deposit to illuminate the switch and provide a uniformed illumination.

Fiber Optics: 

Fiber Optics provide a flexible back lighting layer that can be incorporated between the graphic overlay and the circuit layer allowing the entire surface area of the membrane switch to be evenly backlit.

Benefits of Using Copper Flex Circuitry

The Benefits of Using Copper Flex Circuity vs. Printed Silver

CSI Keyboards uses copper flex circuitry in the majority of our custom keypad designs due to its excellent dielectric strength, thermal stability, chemical resistance and flexibility. Copper flex, also known as Kapton circuits, have become the superior choice over printed silver especially for outdoor applications. 

A printed silver circuit can be replaced with a copper and polyimide construction with minimal additional cost. Copper flex circuitry construction designs offer a significant advantage over printed silver.

Copper Flex membrane switch panels are produced using polyimide (Kapton) as the base material. Copper flex keypad switches are manufactured by laminating a thin sheet of copper to a flexible film substrate. The copper is then chemically etched away, leaving the copper traces. An additional layer of polyimide is laminated to the circuit leaving the gold contacts exposed.

Advantages of Copper Flex Circuitry vs. Printed Silver

  • Improved creasability – the ability to fold or crease without causing open circuits which you are susceptible to using silver ink
  • Ability to put solder components directly on flex, better adhesion than bonding to silver ink
  • Much more resistant to thermal shock
  • No potential problems with silver migration
  • Greater conductivity
  • Lowering the closed loop resistance and switch bounce will be reduced by gold plating the contacts. We will have a gold to gold contact with no bounce versus silver to gold dome
  • Applying stiffener to tail end using heat lamination versus cold lamination which provides better adhesion and better for pinning
  • Ability to have a 0.5mm pitch vs. the 1.0mm minimum with printed silver
  • Tighter trace routing capabilities

Printed Silver vs. Copper Flex

The Case Against Printed Silver - "Silver Migration"

The biggest case against using printed silver circuitry and thus using copper flex circuity is a phenomenon known as silver migration which occurs in microelectronics, components, PCB assemblies and membrane switches. Silver migration is the ionic movement of silver between two adjacent traces that inevitably results in a temporary electrical short.

Silver is a very active metal and is thus highly susceptible to silver migration or dendrite growth. Yet it is also a very cost effective metal for the electronic industry because of it’s conductively and usability. With the reduction or elimination of lead in electronics, silver is a very attractive choice because of its solderability and conductivity.

Silver Migration with Membrane Switches

Silver migration in membrane switches was a much bigger problem in the 1970’s and 1980’s; mainly because of the technical inability of the membrane switch manufacturers. In some cases these manufacturers were graphic screen printers who could screen print silver paste, but had little understanding of electronics or reliability issues associated with the electronics industry.

Today, with competent membrane switch manufacturers, silver migration is less of a problem. However, there are situations such as severe environments or design constraint issues where silver migration is still a risk. As in all aspects of electronics, the industry drive to reduce space and reduce costs with increased functionality continually pushes the envelope for designers and manufacturers of membrane switches.

Causes of Silver Migration

Two factors are typically required to create silver migration in a circuit using silver as the conductor:

1) A voltage potential between two traces. 

2) The presence of moisture. 

Ways to Reduce or Prevent Silver Migration

Some or all of the following solutions can be used to reduce or prevent the occurrence of silver migration.

  • Modifying the silver composition with palladium or copper. 
  • Covering the silver traces with an inert coating such as a protective carbon layer and/or an overcoat dielectric.
  • Increasing the conductor spacing between traces that have a voltage potential.
  • Reducing the voltage.
  • Preventing moisture penetration greatly reduces the risk of dendritic growth. Gasketing and sealing technology can stop the ambient penetration of moisture; elevated temperatures will make it necessary to utilize other methods to reduce migration.
  • There are several areas of a membrane switch that are more susceptible to water damage because they are collection points for moisture or allow moisture to easily penetrate such as terminations, tail break-outs, areas with low adhesion

Understanding IP Ratings for Keypads

Understanding IP Ratings for Keypads

What is an IP rating?

IP or (ingress protection) is the resistance offered by the fixture to the penetration of solids and liquids is indicated by the IP rating. This is a 2 digit number, the first number identifies the degree of protection against the ingress of solids and the second number against liquids e.g. IP65 indicates total protection against dust and protection against low jets of water.

 

FIRST DIGIT PROTECTION AGAINST SOLIDS

0 No Protection

1 Protected against solid objects greater than 50mm (e.g. accidental contact with hand)

2 Protected against solid objects greater than 12mm (e.g. accidental contact with finger)

3 Protected against solid objects greater than 2.5mm (e.g. tools and wires)

4 Protected against solid objects greater than 1mm (e.g. fine tools and wires)

5 Protected against dust

6 Total protection against dust

 

SECOND DIGIT PROTECTION AGAINST LIQUIDS

0 No Protection

1 Protection against vertical water drops (e.g. condensation)

2 Protection against direct sprays of water, up to 15° from vertical

3 Protection against direct sprays of water up to 60° from vertical

4 Protections against water spray from all directions

5 Protection against low pressure jets of water from all directions

6 Protection against jets of water of similar to heavy seas

7 Protection against immersion between 15 – 100cm

8 Protection against submersion

CSI Sealing Capabilities​

CSI Keyboards has become the go-to membrane keypad manufacturer for environmentally sealed membrane switches and rubber keypad designs. Many of CSI Keyboards’ customers require environmentally sealed switches and keypads built to last in extreme and harsh environments. CSI Keyboards has over 40 years of experience designing keypads that are completely environmentally sealed and switches that need to meet specific requirements for Water, Temperature, Humidity, Dirt, Dust, Altitude, Impact resistance, Vibration, and Chemical resistance.

Some of our technologies for environmentally sealing keypads include the Seal Frame design, rubber keypad sealing designs, copper flex circuitry, potting solutions, environmentally resistant polyester technology, and other proprietary sealing techniques. 

Please visit the Sealing section of our website for additional information or reach out directly to discuss.

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.