Custom Rubber Keypads

In some applications it is preferred to have a rubber keypad for the user interface rather than a membrane switch graphic. Xymox offers multiple construction options when a rubber keypad overlay is required.

Abrasion Testing
Xymox elastomers are tested in-house on a Norman abrasion tester.

Conductive Carbon Puck Elastomer Over Printed Circuit Board This construction provides tactile feedback from the elastomer and the carbon puck serves as the shorting element for traces on a printed circuit board.

Non-Conductive, Tactile Elastomer Over Sealed Membrane This construction provides tactile feedback from the elastomer. Printed shorting pads on the membrane switch serve as the shorting element for traces on a printed circuit board. The membrane switch is sealed, which is an advantage since the rubber elastomer is porous and oils can penetrate.

Non-Conductive, Non-Tactile Elastomer Over Sealed Tactile Membrane This construction provides tactile feedback from polyester domes. All of the switching is part of the sealed membrane switch with circuitry exiting via a circuit tail. Having tactile in the Poly Dome allows for a thinner construction when compared to a tactile elastomer.

Rubber Keypad Design Considerations

CONTACT MATERIAL

The contact materials used will define the life you can expect.

• Carbon puck elastomer: ≥ 3 million cycles typical
• Printed carbon elastomer: ≥ 1 million cycles typical
• Printed silver membrane: ≥ 3 million cycles typical

Typically a carbon or gold/nickel plate over copper is recommended if the circuitry is on a printed circuit board. The carbon will have a higher contact resistance with improved immunity to extremely corrosive environments.

Contact Type Resistance (ohms) Recommended Circuit Board Contact Pattern Printed carbon elastomer ≤ 1000 Carbon puck elastomer ≤ 200 Printed silver membrane ≤ 10 as a shorting pad ≤ 100 as a full switch

LEGENDS

Silk screen legend life will vary depending on key shape, style of the legend, and operating conditions. As a general guideline, a standard printed legend will meet 250,000 to 500,000 finger actuations (as simulated with a rubber test finger) before showing signs of wear.

To increase cycle life, overcoats can be added. This can increase the life to the equivalent of 1 million finger actuations.

Legend colors are typically matched to Pantone Colors with a tolerance of ± 1 shade. There are printing limitations on the distance between the legend and key edge:

• Silk screen printing: 3mm minimum from key edge
• Convex surface: 20mm minimum keytop radius
• Concave surface: 30mm minimum keytop radius

TACTILE

Sharp tactile feel occurs with the right combination of actuation force, stroke, and rubber hardness. Sharp tactile can be obtained with 60 durometer silicone and a long stroke but as these factors are increased, keypad life decreases.

Using a half membrane switch or full membrane switch with a Metal or Poly Dome for tactile feedback, can provide sharp tactile feel while allowing for a lower overall profile than if the elastomer was designed to provide the tactile feedback.

DIMENSIONS

The following dimensions will be required to design a custom rubber keypad:

• Overall Keypad Length and Width
• Key to Key Center X and Y Axes
• Key Height
• Keytop Length and Width
• Mat/Base Thickness

ELASTOMERIC KEYPAD STANDARD TOLERANCES

Legend Surface Print Tolerances: Inches +/- .012 Millimeters +/- .30

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