How do Silicone Rubber Keypads Work?

A rubber keypad membrane switch uses compression-molded silicone rubber with conductive carbon pills or with non-conductive rubber actuators. The compression molding process creates an angled web around a keypad center. When a keypad is pressed, the webbing collapses or deforms to produce a tactile response. When the pressure on the keypad is released, the webbing returns the keypad to its original position with positive feedback. The closure of the switch circuit occurs when the conductive pill or printed conductive ink makes contact to a PCB when the web has been deformed. 

What are the Benefits of Using Silicone Rubber Keypads?

• Cost-effective: 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.
• Outdoor Weatherability: Rubber keypads have exceptional resistance to extreme temperatures and aging and to chemicals and moisture.
• Design Flexibility: Rubber keypads offer a multitude of cosmetic and aesthetic options as well as tactile feedback customization.
• Superior tactile feedback: the geometry of the keypad webbing can create a 3-dimensional keypad with a firm tactile response and longer switch travel. Actuation forces and switch travel can be customized to your requirements.
• Can utilize carbon pills, non-conductive rubber actuators, or stainless steel tactile domes.
• Unusual keypad shapes and sizes can be used, as well as different rubber durometers (hardness).
• Multiple colors can be achieved by flow molding the color into the compression molding process.
• Rubber keypad graphics can be further customized by screen printing the keypad top surface.
• Rubber keypad switches can be spray coated with polyurethane for enhanced durability.
• Rubber keypads can be impervious to liquids, dust and gases by utilizing creative designs such as a wrap-around design.
• Back Lighting Flexibility: Rubber keypads can be backlit using LED’s, fiber optic lamps, and EL lighting. Laser-etching the rubber keypad can enhance the effects of back lighting. The use of light pipes in the individual keypads is also a way to customize back lighting and preventing light scatter.

What are Some Design Considerations for Silicone Rubber Keypads? 

• Tactile Response: Varying the tactile response is accomplished by a number of factors such as changing the web geometry and the durometer of the silicone rubber. Durometer can range from 30 – 90 shore A. Many key shape sizes can be designed, and the keypad travel can be as much as 3mm. Actuation force can be as high as 500 grams with certain keypad shapes and sizes.
• Snap Ratio: Altering the snap ratio of the keypad will also affect the tactile feedback of your rubber keypad. Snap ratios of 40% – 60% are recommended for the optimum combination of feel and maximizing the keypad life. Once the snap ratio goes below 40%, the keypad snap-action feel is diminished, although the life of the switch is enhanced.
• Flow Molding: A process by which custom colors are introduced into the compression process so that the colors are molded into the actual silicone rubber. Further customization can be achieved by screen printing custom graphics onto the top surface of the keypads.
• Laser Etching: The process of removing the top coat layer of a painted keypad (usually black in color) to reveal lighter colored layer below (usually white). This way the back lighting only shines through the areas that have been etched away. By combining laser etching with fiber optic, LED, or EL back lighting, there is no limit to the range of creative back lighting effects you can achieve.

Contact us now to speak with a Flexkys Imaging engineer about rubber keypad switch assemblies.

Why Consider Flexkys for Your Silicone Rubber Keypad Assembly?

A rubber keypad by itself is just that… it cannot work without a number of other critical components. Flexkys specializes in the integration of all the necessary components into one complex, feature-rich rubber keypad switch assembly. Our silicone rubber keypad assemblies could include any or all of the following components:

• Silicone rubber keypad
• Printed circuit board
• Bezel with our without additional graphics
• Backlighting components such as fiber optic lamp or LEDs
• Touch screen or LCD display
• Rigid support back panels (aluminum, stainless steel, acrylic, FR4, etc.) with PEM mounting hardware
• Enclosure or molded housing/case

How Do You Design a Silicone Rubber Keypad?

Silicone rubber keypads are the most widely used form of switching technology today. They offer reliability, long life and design flexibility. There are some simple rules and basic design elements to consider when designing a silicone rubber keypad.

Key design will vary with the functional and aesthetic requirements of the application. It is possible to mold a key in almost any shape and to fit almost any configuration. It is important to remember that the key shape will affect the feel of the key. While a circular shaped standard key will have a consistent feel across the entire surface a half moon shaped key will respond different depending upon where the key is pressed.

Once you have decided upon a key shape and layout; the next item to consider is the method for marking the rubber and creating legends. There are three common methods for marking keypads; printing, laser etching, and plastic key caps. 

Printing

Printing is the most common method of marking rubber. The rubber is fixtured to flatten the key top then screen printed. There is no limit to the number of colors available. The arc of the key top determines how far printing must be set back from the edge of the key.

There are currently several options available for improving legend life with printed rubber.

  Plastic Key Caps - custom molded clear plastic is adhered over the legend or molded into the rubber 
  Oil or Epoxy Coating - coating is deposited on the top surface of the key over printing, available in matte, semi-gloss, or gloss finish 
  Drip Coating - drip coating can be hard or flexible. It adds a glossy layer over the key top. It can not be used on some keys with sharp angles. Hard coatings are subject to cracking if the key has large surface areas. 
  Parylene Coating - offers the highest level of protection for a non-plastic coating. Parylene bonds to the rubber at the molecular level.

Laser Etching

Laser etching is especially well suited for applications where the keypad is backlit. Etching typically involves three production steps.

1) Translucent rubber (any color) is sprayed with a translucent base coat ink which will be the legend color that is visible to the user. 
2) Rubber is sprayed with an opaque top coat ink which will be the overall color of the keypad. 
3) The top coat of ink is laser etched away using high speed etchers to reveal the base coat.

Alternatively, you can decide to use a single translucent or opaque rubber ink and laser etch the legends revealing the color of the rubber that is used.

Plastic Key Caps

The longest lasting legend type is custom molded plastic. Plastic legends will not wear out. Many cell phone keypads are designed with plastic keys over rubber.

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