Rubber Membrane Keypad Design Considerations
Know Your Options
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)
Printed carbon elastomer ≤ 1000
Carbon puck elastomer ≤ 200
Printed silver membrane ≤ 10 as a shorting pad
≤ 100 as a full switch
Recommended Design Pattern:
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
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.
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
Keytop Length and Width
ELASTOMERIC KEYPAD STANDARD TOLERANCES
Legend Surface Print Tolerances: Inches +/- .012 Millimeters +/- .30