Rubber Keyboard Actuation Force-The most important design tip

What is  actuation force of rubber keyboard?

Actuation force: the amount of pressure needed to depress a key and register a keypress. Different key switch designs allow for different levels of actuation force, which is usually defined in grams (g) or pound force (lbf).

Keyboard that use dome switches, silicone elastomer button, or tactile switches rely on actuation force as a critical feature to define how much force is required to close the normally open switch.

In this blog, we will introduce you the most import tip for designing silicone rubber keyboard.

If you have ever studied the keyboard, you should have seen the diagram, this graph is for keyboard actuation-travel. 

The horizontal axis(X) represents the travel, which is the distance of the button press. Vertical axis (Y) represents force, it says how much you need to use force to press the button.The chart can be used to say when we press a keyboard button, the button change because of force and button down travel distance.

Refer to the arrow direction and number order of the curve, When the keyboard is in the state of unstressed, the curve stays at the origin (lower left corner). When the keyboard button is pressed down with force and begin to decline, the curve starts to move to the upper right.The greater the force, the greater the stroke down, the more distance the button will fall.

When reach F1 Peak, which called the [Actuation Force] or [Peak Force], this is the maximum Force that the key webbing can withstand.

After the [point 1], the force of keyboard suddenly drops sharply. This is because the key webbing of the keyboard begins to bend and cannot withstand the press force, causing the keyboard to drop rapidly until it touches the bottom plate [F2], which called Make Force or Contact Force.

At this time, the keyboard button contacts with the circuit board to start the circuit switch.                                        

If the force is continued to be applied at this time, the force will rise rapidly, but the button can not be pressed down (the stroke is very short). This is because the stroke of the button has almost ended, and the continued force is only applied in the compression of the silicone, so the [End Force] can be infinitely large in theory.

When the force exerted on the keyboard begins to release, the force originally compressed on the rubber will be released first until [F3] which called [Rebound Force] or [Return Force] back to the place where the keyboard contact the bottom plate.

After [point 3], the keyboard will start to rebound, and you will feel a rebound force, which will get bigger and bigger until the key webbing is no longer curved. When reach [F4] which called [Return Peak Force], after this point, the rebound force will become smaller and smaller almost linearly, until it returns to the origin.

In addition, the way we feel about whether the keyboard is good or not is defined by this curve.In general, we define the Peak Force of the keyboard and the ratio of the Peak Force to the Contact Force. This ratio is known as the Click ratio or Snap ratio.The Stroke distance of the keyboard also affects.

-Peak Force Value

When F1 (Peak Force) is too strong (over 200G), the keyboard will feel hard when pressed;In contrast, when the F1 (Peak Force) Force is too small (less than 40G), the button will feel very light and soft when pressed.

Traditionally, for products with keyboard that require frequent operation, we will design a relatively small F1, so as not to make people who continuously hit keyboard feel tired because they have to exert a lot of effort.For products with less frequent use of keyboard, we will design a larger F1, because it feels better but is not easy to press.

Different products usually have different actuation force design values. For example, the F1 of Calculator was only about 30 ~ 80g, while the general telephone button was about 70 ~ 200G, and the computer keyboard was between 40 ~ 90G.

How to test keyboard actuation force?

-Click ratio or Snap ratio

The tactile feeling of keyboard is good or not depends on the Contact Force (F2), but generally we do not define F2 directly, but define the ratio of F1 and F2, called Click ratio or Snap ratio, which is defined as follows:

Note:Usually the ratio is between 0.4 and 0.65.

-So what factors affect the actuation force of the keyboard?

The key factor is the stroke/travel of the keyboard.The material hardness and resilience of the silicone keyboard will also affect the actuation force.If the silicone keyboard need to be sprayed with color coating, the thickness of the spray will also increase the actuation force.Such as laser etched silicone keyboard.We need to spray according to the color requirements. The more the coating, the more the actuation force will increase.Finally, in order to protect the symbols of keyboard, we will do protective layer, such as PU coating, which will greatly increase the actuation force.

Keyboard Travel/Stroke:

The Travel distance of the keyboard will also affect the tactile feeling of the button. If the Travel distance is too short, it means that the finger will reach the bottom immediately after touching the button (reaching the contact point), and the finger will immediately feel the impact. After pressing the button several times, the finger may feel paralysis.

And too long travel will make people feel that the button is pressed for a long time to the end.

Generally the telephone keyboard stroke/travel will be design between 0.8 ~ 1.2mm, and the computer keyboard will be designed between 2.0 ~ 4.0mm.