In a membrane switch project, the earliest decision is also the most expensive one to reverse: what goes under the key — a metal dome or a poly dome? The choice is not just about "click feel". It simultaneously determines total panel thickness, cycle life, operating temperature range, the debounce window in your firmware, and the unit cost. Below we compare both technologies with measurable parameters and give concrete thresholds for choosing between them.
What Are Metal Dome and Poly Dome? Structural Differences
Metal dome
A metal dome is a separate mechanical part stamped from stainless steel foil, typically SUS301 or SUS304, and usually nickel or gold plated. Common diameters are 4, 5, 6, 8.4, 10 and 12 mm, available in round, triangular, four-leg and five-leg geometries. Domes are not placed individually into the panel; they arrive arrayed on a polyester retainer tape that is laminated over the circuit layer. When pressed, the dome inverts abruptly and shorts two silver pads on the circuit. That abrupt drop in the force-travel curve is exactly what the finger registers as a click.
Poly dome
A poly dome is not a separate part. It is formed by thermally embossing the circuit layer or an intermediate layer into a dome shape. Dome height is typically 0.5-1.5 mm with a base diameter of 8-16 mm, and a carbon or silver contact pad sits on the inner surface of the dome. Because the structure relies on the elasticity of the polyester film, the return force comes from the material itself. No separate component, carrier tape or placement operation is required.
How Is Tactile Feel Measured? Snap Ratio, Force and Travel
Tactile feel is not a subjective opinion; it is a measurable quantity, expressed in the industry as snap ratio (click ratio):
Snap ratio (%) = (F1 − F2) / F1 × 100
F1 is the peak force at the moment the dome collapses and F2 is the base force immediately after collapse. Below roughly 35%, the finger struggles to identify a distinct click; values around 50% produce a sharp, unmistakable snap. A metal dome keeps this ratio high by geometry. In a poly dome the material behaves elastically, so the force curve is rounder and the feel is softer.
| Parameter | Metal dome | Poly dome |
| Actuation force | 150-550 gf (1.5-5.4 N) | 100-350 gf (1.0-3.4 N) |
| Travel | 0.20-0.45 mm | 0.40-1.00 mm |
| Snap ratio | 40-60% | 25-45% |
| Dome height | 0.25-0.60 mm | 0.50-1.50 mm |
| Cycle life | 1-5 million | 100,000-500,000 |
| Added stack thickness | 0.15-0.30 mm | Negligible |
| Closed-circuit resistance | Typically 10-100 Ω | Typically 100-500 Ω (carbon pad) |
| Alignment sensitivity | ±0.15-0.20 mm | More tolerant |
| Continuous operating temp. | -40 / +85 °C | -20 / +60 °C |
These are typical ranges. Final values depend on dome diameter, the material and thickness of the graphic overlay (PET 0.125-0.2 mm, PC 0.175-1.0 mm) and the total thickness of the layer stack. Rim or pillow embossing on the overlay does not change the snap ratio directly, but by guiding the fingertip toward the dome centre it noticeably improves perceived consistency.
Service Life and Environment: 1 Million Cycles or 200,000?
The numbers alone are misleading; what matters is how the life ends. A metal dome largely retains its snap ratio through 1-5 million cycles and then fails abruptly through metal fatigue. A poly dome operates in the 100,000-500,000 cycle range but usually does not "fail" — it loses feel. As the polyester film takes on permanent deformation, dome height decreases, snap ratio drops, and although the key still switches, the operator can no longer tell whether the press registered. This is a common misdiagnosis behind field reports of "the key doesn't work".
Temperature drives this directly. The glass transition region of polyester film sits around 70-80 °C; above a continuous 60 °C and under sustained load, creep in a poly dome accelerates and dome height drops permanently. A metal dome largely preserves its force characteristic across -40 / +85 °C. For oven controls, welding equipment and panels mounted near motors, this alone can be the deciding factor.
One frequent misunderstanding deserves correction: IP65/IP67 sealing has nothing to do with dome type. Sealing is achieved by the uninterrupted graphic overlay and the continuity of the perimeter mounting adhesive. Both dome types can reach the same ingress protection class.
Cost: Unit Price or Tooling Investment?
Framing the comparison as "which is cheaper" is a mistake; the two technologies accumulate cost in different places.
- Metal dome: Every key needs a separate part, a carrier tape and a placement operation. Cost rises linearly with key count multiplied by production quantity. In exchange, standard diameters require no additional tooling investment.
- Poly dome: Requires a one-time embossing tool. Once the tool exists, unit cost is almost unchanged whether the panel carries 8 keys or 40.
The practical threshold: for low key counts and low volumes (prototypes, a few hundred pieces), metal dome usually wins on total cost. Once key count passes about 20 and volumes reach several thousand, poly dome takes the lead on unit cost. Cutting follows similar logic — die cutting holds ±0.2 mm and is economical at volume, while laser cutting works to ±0.3 mm but eliminates the tooling investment, making it the usual choice for low-volume prototypes.
The hidden cost item is alignment. A metal dome must sit over the centre of the circuit pad within ±0.15-0.20 mm; beyond that, intermittent contact and scrap rate climb. A poly dome has a much larger contact area and therefore a more forgiving manufacturing window.
Which One Should You Specify? Decision Checklist
If three or more of the following apply to your application, look at metal dome; if not, poly dome is likely the better fit:
- Will the key exceed 300,000 actuations over product life?
- Will the panel operate continuously above 60 °C?
- Will operators wear gloves? (Thick gloves mask travel; 350-500 gf domes at 10-12 mm diameter give far more reliable feedback.)
- Is key count below 15 and volume in the low hundreds?
- Must the force characteristic stay stable for years? (Metering, dosing and medical interfaces where consistency matters.)
- Do you need low closed-circuit resistance?
Conversely, if the panel has 25 or more keys, volumes are high, stack thickness is tight, a low actuation force of 100-200 gf is required, or usage frequency is low, poly dome is the better answer on both cost and design flexibility.
Three Details Engineers Often Miss
1. Debounce time depends on dome type. The hard contact of a metal dome typically produces a few milliseconds of bounce, and a 15-20 ms software debounce is sufficient. The softer contact of a poly dome can extend bounce; 20-30 ms is safer. Remember also that closed-circuit resistance in a carbon-pad poly dome can reach 500 Ω: if the pull-up is specified too low (1 kΩ, for example), the divider formed with the switch resistance lifts the logic LOW level and narrows the threshold margin. With a 10 kΩ or higher pull-up, 500 Ω stays comfortably on the safe side of the input threshold.
2. Stack height and connector thickness must be calculated together. A metal dome layer adds 0.15-0.30 mm to the stack — less than the dome height itself, because the dome seats inside the spacer cut-out. A typical six-layer membrane switch lands between 0.9 and 1.4 mm. If the tail enters a 1.0 mm pitch ZIF connector, tail-area thickness must be planned separately and a stiffener added where required.
3. LED and backlight placement can collide with the dome. Where in-key LEDs or a backlit panel are used, neither the LED nor its conductive traces may intrude into the dome seating area. That area is rigid and fixed with metal domes; with poly domes the dome base is wider, so the clearance for the light diffusion layer must be settled early. LED life of roughly 50,000-100,000 hours should be planned independently of the panel's mechanical life.
We can define actuation force, snap ratio and layer stack together, based on your application. Send your drawing or an existing sample to the Tuşhan team for a quotation.