Every buyer who spec'd a metal dome from a PDF has been there: a table of numbers that look authoritative, a footnote no one reads, and a sample that feels nothing like the datasheet promised. The gap between a good spec and a bad sample usually comes down to three numbers that buyers underestimate - actuation force, snap ratio (travel), and rated life. This guide explains what each one actually means, how suppliers measure it, and where the common loopholes hide.
1. Actuation force (F1): the number everyone quotes but few verify
Actuation force is the peak force required to push the dome from its stable "up" position into its inverted "down" position. Suppliers quote it in grams (g) or Newtons (N). A typical four-leg dome for a steering wheel switch might be quoted at 250g; a mouse dome at 80g; an industrial push button at 500g.
The catch: there is no universal test standard. One supplier might measure F1 at the peak of the force-displacement curve; another at 0.2mm of travel; a third with a 0.5mm radius probe instead of a flat probe. A 250g dome measured with a sharp probe can feel like 320g under a real keycap. When you compare quotes, ask for the test method: probe diameter, probe speed, pre-load, and whether the number is average or minimum. Our metal dome technical specifications guide lists the test set-up we use, so you can compare apples to apples.
2. Snap ratio and tactile feedback: the "click" that buyers actually feel
The snap ratio is the difference between F1 (actuation force) and F2 (reset force when the dome pops back up), expressed as a percentage: (F1 - F2) / F1 × 100. A typical dome has a 40–60% snap ratio. Below 30%, the dome feels mushy - no clear "click." Above 70%, it feels crisp but can be harsh on long-press applications.
This is the number that determines whether a gamepad shoulder button feels responsive or numb, and whether a headphone call button satisfies the user. A high snap ratio is great for gaming; a lower one is better for medical devices where accidental actuations are dangerous. Spec the snap ratio, not just F1.
3. Rated life: 500,000 cycles means what, exactly?
Datasheets quote cycle life as "500,000 actuations" or "1,000,000 cycles." The fine print matters. Life is typically defined as the point at which actuation force drops by 30% OR contact resistance rises above a threshold (often 100Ω). But who is cycling the dome? At what speed? With what probe? In what temperature?
A dome rated at 500,000 cycles at room temperature with a 1mm/s probe may fail at 80,000 cycles under a fast typist's finger in a -20°C car cabin. For automotive applications, ask for life data at your actual temperature range - our automotive snap dome line is tested at -40°C to +85°C. For consumer keyboards, 100,000 cycles at 25°C is often sufficient; for industrial equipment, demand 1,000,000 at +60°C.
4. Dome size, shape, and the four-leg vs round decision
Round domes (6mm, 8mm, 12mm) are the default: cheap, reliable, and available everywhere. Four-leg domes (also called "4L" or "cross" domes) offer better alignment on narrow PCBs and easier PET dome sheet array lamination, but they cost slightly more and have a smaller contact footprint. Cross tactile domes sit between the two - they align like a 4L dome but have the contact area of a round dome.
The shape decision is driven by your PCB layout, not by cost. If your contact pad is under 4mm, use a 4L or cross dome; if you have 6mm or more, a round dome is the simpler choice. Our engineering team reviews Gerber files free of charge - send the pad layout and we will recommend the shape.
5. Material and plating: SUS301 vs SUS304, and why gold matters
Almost all snap domes are made from SUS301 stainless steel (a high-margin, work-hardened spring steel). SUS304 is softer and used only for low-force, low-life applications. The plating on the contact side determines contact resistance: unplated SUS301 oxidizes and contacts drift; tin-plated domes are cheap but develop oxide layers; gold-plated domes (usually 0.1–0.5µm over nickel) hold a stable contact resistance of under 50Ω for the rated life.
For consumer devices where cost matters, tin is fine. For automotive airbag switches, medical devices, and industrial controls, demand gold flash. Our stainless steel grade metal dome article goes deeper on material selection.
6. The questions to ask before you place a PO
When you request a quote, ask these six questions in writing. A supplier who answers them confidently is one you can trust; a supplier who says "our domes are standard" is one you should qualify more carefully:
- What is F1 at my target snap ratio (e.g. 50%)? (Not just the average.)
- What is the minimum life at my operating temperature?
- What is the contact resistance spec, and how is it tested?
- What is the flatness of the dome when laminated on a PET carrier?
- Can you provide a PPAP or IATF 16949 certificate if I need it?
- What is the MOQ, and what is the price break at 50k / 100k / 500k pieces?
SUNAURA Machinery has been making metal domes, dome sheet arrays, and silicone keypads from Qingdao for over a decade. We publish our test methods on purpose, because we would rather lose a quote than argue about a spec later. If you are qualifying a new dome for a medical device, automotive interior, or consumer product, send us your drawing and we will send back a datasheet that actually matches the sample.
