Understanding Metal Dome Switch Force Curves: A Buyer Guide to SUS301 Snap Dome Selection

If you have ever specified a snap dome for a keypad project, you have probably seen a force-displacement curve: a line that rises steeply, snaps back sharply, and settles into a lower contact force. That curve is the single most important piece of engineering data about a dome, yet it is also the most misunderstood. This guide explains how to read it, what the numbers mean, and how to translate a curve into the right part number for your application.

Shanyo has been stamping SUS301 stainless steel domes for keypad manufacturers since 2004. In that time, we have seen engineers over-specify force for no reason, under-specify it and get double-tap complaints, and misread curves that looked identical on paper but felt different in production. The notes below distill what our application engineers actually do when a customer sends us a keypad spec.

What a Force Curve Actually Shows

A force curve plots downward force (in grams-force, gf) against plunger displacement (in millimeters). When you press a dome, the force rises as the dome crown deforms. At the snap point, the dome inverts: the force drops sharply and the dome collapses. The contact closes at the bottom of that drop. When you release, the dome pops back, but at a lower force than the snap point.

Three numbers define the curve: actuation force (F1), the peak force at snap; contact force (F2), the force when the dome is fully inverted and touching the circuit; and return force (F3), the force at which the dome pops back on release. The ratio (F1-F2)/F1 is the tactile ratio, usually expressed as a percentage. A well-designed dome has a tactile ratio of 40-60%. Below 30%, the feel is mushy; above 70%, the dome is noisy and may rattle.

Choosing the Right Actuation Force

The most common mistake is picking F1 from habit rather than application. Here is how Shanyo engineers think about it:

• 60-160gf: Feather-light, for consumer remotes, TV keypads and wearable buttons. Best for ungloved fingers. Too light for gloved or industrial use.
• 180-250gf: General-purpose, for game controllers, calculators and office equipment. The most popular range for consumer-electronics domes.
• 250-350gf: Industrial and automotive range. Used for steering-wheel switches, door controls and machinery HMI. This is the sweet spot for gloved operation.
• 400-500gf: Heavy-duty, for safety interlocks, EV charge-port buttons and outdoor equipment. These domes have shorter travel and a firmer feel.

For medical devices, the choice depends on whether the user wears gloves. Ungloved glucose meters and thermopanes use 160-250gf. Infusion pumps and surgical equipment, where clinicians wear gloves, need 280-350gf. See our article on medical keypad design for the full comparison.

Diameter and Travel: Matching the Plunger

Dome diameter and plunger travel are linked, but not in the way most designers assume. A 4mm dome travels about 0.15mm; a 12mm dome travels about 0.30mm. The plunger (the rubber or silicone nub that pushes the dome) must be sized to contact the crown without extending past the dome legs. If the plunger is too large, it contacts the legs and changes the force; if too small, it wanders off-center and causes inconsistent actuation.

General rule: plunger diameter should be 50-70% of the dome diameter. For a 6mm dome, use a 3-4mm plunger. For a 12mm dome, use a 6-8mm plunger. The plunger height should be set so that the dome is 50-70% of the way to snap at rest, leaving room for the click.

For dense keypads where plunger spacing is tight, dome sheet arrays with four-lead cross or triangular domes are often better than circular domes, because the legs provide more stable positioning. See automotive snap domes for examples of leg configurations.

Plating: Gold vs Nickel vs Bare

The dome material is almost always SUS301 stainless steel. The plating on the contact side determines contact resistance and corrosion resistance:

• Bare (no plating): Lowest cost, contact resistance 50-200 ohms. Fine for low-voltage, low-signal consumer applications. Not recommended for wet or corrosive environments.
• Nickel-plated: Contact resistance 20-50 ohms, better corrosion resistance. Standard for automotive and industrial use.
• Gold-plated: Contact resistance under 10 ohms, best corrosion and wear resistance. Required for medical, aerospace and low-signal applications (glucose meters, ECG electrodes, low-level sensors).

Gold plating adds cost, but for medical devices it is almost always worth it. The difference in contact resistance between a bare dome and a gold-plated dome can mean the difference between a stable sensor reading and a noisy one. For a deeper comparison, read gold vs nickel plated metal dome guide.

Tolerance: How Tight is Tight Enough?

Domestic-grade domes are usually held to +/-30gf on F1. Automotive-grade is +/-15gf. Medical and aerospace specs can go to +/-10gf. Tighter tolerance costs more, but it matters more than most designers think. If your dome F1 varies by +/-30gf on a 250gf nominal, the actual range is 220-280gf. A user pressing a 280gf dome on a cold morning will notice the difference, and may think the keypad is defective.

Shanyo holds automotive-grade domes to +/-15gf on F1 across full production runs, with 100% force-curve sampling on every reel. We provide force-curve charts with every shipment so customers can verify the lot.

Common Application Mistakes

After 22 years of stamping domes, we see the same mistakes repeatedly:

• Using a 12mm dome where a 6mm would do: Larger domes have more travel and lower F1 per gram of material. If your keypad has room, a smaller dome with higher F1 often feels crisper than a large, soft one.
• Ignoring temperature: SUS301 force rises about 5% at -40C and drops about 8% at 85C. Automotive specs should be validated at both extremes.
• Overlooking leg coplanarity: If the dome legs are not flat on the circuit, the dome rocks and the click feels uneven. This is a stamping quality issue, not a design issue, but it shows up as inconsistent feel in production.
• Specifying a dome without sampling: A force curve on a datasheet is not the same as a feel in the hand. Always request samples and run your own actuation test before approving a part.

Working with Shanyo

If you are selecting a snap dome for a new keypad, send us your application requirements: target F1, travel, environment (temperature, humidity, washdown), circuit type (FPC vs PET vs rigid PCB), and expected volume. Our engineering team will recommend three dome options with force-curve charts and samples within 48 hours. We supply dome arrays, loose domes and silicone keypads integrated with domes for turnkey programs.

Request samples through the contact page. We typically ship samples within three business days from our Qingdao facility.

About Shanyo: Shanyo Precision Electronic Technology Co., Ltd. is a manufacturer of metal domes, dome arrays, FPC keypads, silicone keypads and copper bus bars for automotive, medical and industrial customers. Visit www.yo-shan.com.