A customer sent us a membrane switch back in April. They had designed it for an outdoor parking payment terminal, and they asked us to manufacture it exactly as drawn. We looked at the drawing and told them it would fail within a year. They said the design was approved by their engineering team, and they did not want to change anything. We manufactured it, and shipped it in May.
They called us in September. The switches were bubbling up at the edges, the print was fading, and the buttons were sticking. The terminal was in a parking lot in Arizona, where the surface temperature reaches 70C in summer, and the UV exposure is extreme. The membrane switch was made from standard polyester (PET) film, which is not UV-stabilized. After 4 months in the sun, the film had degraded, and the adhesive had failed.
We redesigned the switch with UV-stabilized polycarbonate film, a silicone gasket around the edge, and a -40C to +85C operating temperature range. The new switches have been running for 6 months, and they are still in good shape.
If you are designing a membrane switch for outdoor use, here is what you need to change from a standard indoor design.
Why indoor switches fail outdoors
A standard indoor membrane switch is made from 0.175mm PET film, with a pressure-sensitive adhesive (PSA) on the back. It is designed for office environments, where the temperature is between 15C and 30C, the humidity is below 60%, and there is no direct sunlight.
Outdoors, three things kill the switch:
UV radiation. Standard PET film degrades under UV exposure. The film turns yellow, becomes brittle, and cracks. The printed ink also fades, because the ink is not UV-resistant. In Arizona, the UV index can reach 11, which is extreme.
Temperature cycling. Outdoors, the temperature can swing from -20C in winter to 70C in summer. The materials in the switch expand and contract at different rates. The adhesive can delaminate, the dome can shift, and the contacts can oxidize.
Moisture. Rain, dew, and humidity get into the edges of the switch. The PSA absorbs moisture, and the adhesive weakens. The silver traces underneath oxidize, and the switch becomes intermittent.
Outdoor membrane switch with gasket sealing and UV-stabilized film.
What to change in the design
1. Use UV-stabilized film. Instead of standard PET, use a UV-stabilized polycarbonate (PC) film, which has UV inhibitors built into the material. The PC film also has better impact resistance, which is important for outdoor terminals that get vandalized. The tradeoff is that PC is more expensive than PET, about 30% more per square meter.
2. Seal the edges with a gasket. Do not rely on the PSA to keep moisture out. Add a silicone gasket around the perimeter of the switch, which compresses against the enclosure when the switch is mounted. The gasket is 1mm thick, and it compresses to 0.5mm when the screws are tightened. This creates a watertight seal, which meets IP65.
3. Use UV-resistant ink. The printed graphics must be made from UV-cured ink, which has UV inhibitors. Standard solvent-based ink will fade in 6 months. UV-cured ink will last 5 years outdoors. We print the graphics with a silk-screen process, using 4 layers of ink, to make sure the colors are opaque.
4. Specify the right operating temperature. The switch must work from -40C to +85C. At -40C, the silicone rubber under the dome must still be flexible. At +85C, the adhesive must not melt. We test every batch in a thermal chamber, cycling from -40C to +85C 100 times, to make sure the switch survives.
Four-legged stainless steel domes, plated with gold, for outdoor applications where corrosion resistance is critical.
What about the domes?
The domes inside the switch must also be outdoor-rated. Standard stainless steel domes will rust in coastal environments, where there is salt in the air. We recommend using gold-plated domes for outdoor applications, because gold does not corrode. The gold plating is 0.5um thick, which is enough to protect the stainless steel underneath from salt spray.
We also recommend using dome sheet arrays instead of loose domes, because the array holds the domes in place during temperature cycling. Loose domes can shift when the adhesive expands and contracts, which causes intermittent contact. The dome sheet array is attached to the PCB with PSA, which is also UV-stabilized.
Common mistakes we see
The most common mistake is using an indoor design for an outdoor application. The customer saves 30% on the switch cost, but then spends 10x more on warranty repairs and replacement. We always recommend the outdoor-rated design, even if it costs more upfront.
The second most common mistake is forgetting to specify the IP rating. The customer says "outdoor," but they do not specify IP65 or IP67. We manufacture the switch to IP54, which is splash-resistant but not waterproof. Then rain gets in, and the switch fails. We always ask the customer what IP rating they need, and we design to that spec.
If you are designing an outdoor membrane switch, send us your enclosure drawing and your operating environment. We will recommend the right materials, the right gasket, and the right IP rating. We can also build a prototype in 5 days, so you can test it in your real environment before you commit to production.
We also produce the metal domes, silicone buttons, and FPC circuits that go inside the switch, so we can deliver the complete assembly as one part. Talk to our engineering team about your outdoor project.
- John Smith, Senior Application Engineer, Shanyo
