How to Read a Metal Dome Force Curve: Snap Ratio, Travel and Contact Points Explained

Every metal dome has a force curve, and every force curve tells a story about how the key will feel. Most buyers look at one number - the actuation force - and move on. The problem is that a dome with the right actuation force can still feel dead, mushy or harsh, depending on the rest of the curve. This guide explains how to read the curve so the feel is a decision, not a surprise.

1. What the Curve Plots

The force curve plots force (in grams or newtons) against travel (in millimeters). The stylus presses the dome center, and the measurement records the force needed at each point of the press. The curve has four landmarks worth understanding:

• Pre-load: the small force where the dome first makes contact with the test probe - the start of the curve.
• Peak force (actuation force): the highest force before the dome snaps. This is the number in the datasheet.
• Snap point: where the force drops sharply as the dome buckles. The location and steepness of this drop define the "click".
• Bottoming: the end of travel, where the dome rests on the contact ring. The shape here affects over-travel feel.

2. Snap Ratio: The Number That Defines the Click

Snap ratio is the drop from peak force to the minimum force after the snap, expressed as a percentage of the peak. A dome with a 250gf peak that drops to 150gf after snap has a snap ratio of 40 percent.

• High snap ratio (40-60 percent): a pronounced, crisp click. Users feel the snap clearly; good for products where tactile confirmation matters.
• Low snap ratio (10-25 percent): a softer, more gradual transition. Better for light-touch or quiet keypads, but the feedback is subtler.
• Very low snap ratio (under 10 percent): the dome is close to a "soft feel" switch - it still clicks electrically but the tactile snap is weak.

Shanyo publishes snap ratio in the dome datasheet because it predicts the user experience better than peak force alone.

3. Travel and Over-Travel

Total travel is the distance from pre-load to bottoming. Over-travel is the distance the dome continues to move after the snap - the cushion after the click.

• Short travel (0.2-0.3mm): a fast, direct feel. Common in compact devices where space is tight.
• Standard travel (0.3-0.45mm): the classic keypad feel, comfortable for frequent typing and pressing.
• Long travel (0.45mm+): a deliberate, deep press; used where accidental actuation must be avoided.

Over-travel protects the dome from hard pressing: a dome that bottoms abruptly on the contact ring can fatigue faster. Designers specify travel with the housing and key cap in mind - the dome travel must match the available key movement.

Reading the force curve: peak, snap ratio and travel define the tactile feel.

4. Contact Point and Contact Resistance

The curve also reveals where the electrical contact happens. The contact point - where the dome touches the contact ring - appears as a change in the curve near bottoming. If the contact happens too early or too late relative to the snap, the user can feel a click but get no electrical result, or get an actuation with no click.

• Contact should close at or just before the end of travel, reliably under the specified force.
• Contact resistance is measured separately (typically below 10 ohms for gold-plated contacts) but the curve shows where the contact is designed to land.

A mismatch between snap point and contact point is a classic cause of "clicking but not registering" complaints - a failure mode that force-curve review catches before tooling.

5. Force Tolerance and Lot Consistency

Datasheets quote a force window (for example, 200 +/- 30gf). The real question is how tight the distribution is across the lot and across lots:

• Domes from one stamping tool and plating lot should hold a tight spread, typically +/-10 percent of nominal.
• Force drift over life (after 100k actuations) is a separate curve - a fresh dome feels different from a cycled dome, and the datasheet should state the drift.
• Shanyo samples force curves from every production lot and records the mean and spread in the inspection report.

6. How the Curve Changes with Plating and Thickness

Plating and material thickness shift the curve in predictable ways:

• Material thickness: thicker SUS301 raises peak force and snap ratio together; the dome gets stiffer and snappier.
• Dome height: a taller dome increases travel before bottoming.
• Plating: adds negligible mechanical change, but gold plating keeps contact resistance stable over life, preserving the electrical behavior at the contact point.

These relationships let the design team dial the feel without trial and error - change one parameter, read the curve shift, iterate.

7. Reading Curves for Array Programs

In a dome array, the curve per position is what matters. Shanyo verifies arrays with per-position force-curve sampling, so the customer sees not just a dome datasheet but a position map of actuation force across the keypad. This catches tooling issues - a worn die in one corner shows up as a force outlier in one position.

8. Practical Steps for Design Teams

• Start with the feel: peak force and snap ratio from the target user experience, then travel from the housing constraints.
• Request the full curve, not just the datasheet number, when evaluating sample domes.
• Test the dome with the actual key cap and overlay - the assembled curve differs from the bare dome curve.
• Confirm the force window across lot samples before tooling, not after.

For the specification side of the same topic, the force curve selection guide covers how to write the specification, and the contact page reaches the Shanyo engineering team for a curve review on your application.

About Shanyo: Qingdao Shanyuan Precision Electronic Technology Co., Ltd. manufactures metal domes, dome arrays, FPC keypads and stamped components. Visit www.yo-shan.com.