Silicone Keypad Design Guide

03 Tactile Feel

Force Introduction

Intro: Why Tactile Feedback?

Silicone keypads are designed to provide a mechanical, tactile feedback when pressed. The feedback provides tactile clues that help us confirm if we have fully pressed a key or helps us prevent accidental presses.

Abatek has spent 20 years manufacturing input devices, such as silicone keypads, with mechanical tactile feedback. We have extensively studied and quantified the behavior of keys presses, and created processes so the key presses can meet spec.

This page first discusses how a key acts almost like a mechanical spring. The force vs. travel will be reviewed with respect to actuation force, snap and stroke. Longevity of the web is discussed in terms of Key Web life.

Next a brief section will discuss some force/snap/stroke suggestions depending on the type of keypad. A 2D drawing notes and performance section follow after that. The force tolerance, which depends on factors like number of keys and force amount, is listed next. Since Abatek will ultimately design the web, a section is devoted to how our customers should initially design the web.

The last sections of this page will discuss how to prevent key rocking as well as a brief over view of other types of actuations including teeter keys, silicone keypads with plastic key-tops and silicone Keypads with metal domes or tact switches.

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Spring and Key Comparision
Spring Like Mechanics

When a spring is pressed down, it pushes back with a certain force. When the spring is released, it pops back into the neutral position.

Abatek is able to design the thin skirt, or web, around a the bottom perimeter of a key to act just like a spring. The amazing properties of silicone allows the web to push up with a force when a key is pressed. The web can even be constructed to provide a varying force, which characterizes the mechanical tactile feedback a user feels.

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Force vs Displacement Graph
Actuation Force, Snap and Stroke

The key forces felt during a key's 1mm or so travel are:
1. actuation force
2. snap
3. touch force
4. contact force
5. return force

Each is dependant of the other. An example of a typical keypad force curve is shown at left. While key webs can be carefully designed to offer a variety of different actuation forces and snap, not all combinations are possible.

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Key Web Life

Over time, continually pressing the key will slowly begin to fatigue the key's web. During the life of the key web, the actuation force will lower and the snap will decrease slightly. If the key is pressed past its rated life threshold, the key web will first fall out of force specification, and then break.

Normal specification for a keypad is 100,000 presses or less. Some applications require one million or more presses.

Ask your Abatek engineer on how to best validate the life of your keypad design.

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Difference Forces for Different Keypads
Different Force Spec for Different Keypads

Abatek can design key webs with a variety of tactile feedbacks. Customers can spec the force to match the type of keypad for their application.

Critical Buttons Keys that have critical functions should provide a high tactile feedback. The keys should have a high actuation force (400g), a high Snap (35% minimum) and a long stroke (1.5mm) - note that key web life is limited due to the high force and snap.

High Frequency Keypads Keypads that are often pressed, such as numeric keypads, should have low tactile feedback for easy, quick presses. The keys should have a low actuation force (125g), a low snap (15% - 5%) and a short stroke (0.8mm) - this force specs helps increase the life of the web.

Typical Keypads Applications that are neither critical nor high frequency (most keypads) should have medium actuation force (250g - 150g), a standard snap (35% - 25%) and a standard stroke (1mm) - these keypads are the best compromise between good tactile feedback and long key web life.

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2D Drawing Specification Notes

specification element


2D drawing note example

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Force Tolerance Table
Actuation Force Tolerance

Abatek tool engineers carefully design the web to meet a very specific actuation force and snap spec for the key. In an ideal world, the performance of the keypad will always meet nominal specification. However, variations in material and process will change the performance of the key web. In addition, over each production shot, the tool will begin to wear down, further affecting the range for the force performance of the key.

In addition, to meet the force spec during the life of the web, the performance of a key must begin at the higher tolerance. During its life, the force will lower.

Finally, it is more difficult to design and adjust the tool to meet multiple force requirements on a keypad. In this case, the tolerance must also be wider. See chart at right.

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Web Design
How to Design the Web

A key's web can be designed to achieve an actuation force, snap and stroke spec by varying its thickness, angle and length. To determine these three variables, Abatek considers many factors, including key size, shape, material and performance specs.

To maximum the web design flexibility for Abatek, customers should use the following initial web values shown at left. These initial web geometries should be applied to the 3D and referenced on the 2D.

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2D Drawing Specification Notes

specification element


2D drawing note example

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Preventing Key Rocking

The best key shape for good actuation are those that are round or square with large radius corners. These provide a direct up/down actuation feel.

Long, rectangular keys are prone to rocking - which gives an un-steady motion of the key or can affect electrical contact.

Key Rocking 1
Problem: Key Rocking

Keys that are rectangular or very long will most likely rock if a user does not press the key exactly in it center.

When the key press is un-centered, the side of key which is presses will actuate but the opposite side will most likely not move. This provides an un-natural tactile feel and even worst, can prevent electrical contact from being made because the pill does not lay flat against the PCB trace.

Key Rocking Solution 1
Good Solution: Use Anti-Rocking Pins

Adding anti-rocking pins will prevent the keys from missing electrical actuation. Here, if the anti-rocking pin hits first, it will leverage the key so that its center, where the pill is, presses onto the PCB. The anti-rocking pin MUST be shorter (recommend 0.2mm) than the carbon pill otherwise electrical contact cannot occur.

Key Rocking Solution 2
Best Solution: Use Multiple Pills

A better solution is to use redundant pills at the corners of the key. This way, even if one pill makes contact, electrical switching is assured.

Note: the circuit or electronics should be designed to register a single key press, even with multiple contacts under the same key.

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Other Types of Key Actuations

There are a number of different types of possible actuations beyond the straight up/down motion of a rubber key.

Teeter Actuation
Teeter Keys

Some keys, such as volume keys, are purposely designed to rock (or teeter). Here a single button will have two sides that can be pressed independently.

As a general rule, the key web will resemble that of a regular key. To make the key teeter, add a pivot feature to the bottom of the key, centered between the two contacts. Keys such as this must also be long enough to assure flat contact of the carbon pill.

Rocker keys will not have a high snap. To help improve tactile feel, the rocker pivot feature should be approximately 0.2mm from the PCB and be at least 1.5mm wide with a rounded tip.

Preload Image 1
Pre-Loading Plastic Key-Tops

The best way to use plastic keytops on sub-layer keypads is to pre-load the plastic keys and constrain the keys to an up/down movement by the housing.

Doing this has two advantages: 1) the plastic keytop provides a direct, linear tactile feel to the user, and 2) this configuration allows for a higher positioning tolerance of the keypad, preventing stretched keys. (see Designing with Plastic Keys)

To prevent the key from rattling, the plastic key-top can be pre-loaded by the key. This means that the key pushes up against the plastic keytop, which is then secured against the housing.

Preload 2

Here the key is pushing up against the plastic key. A lip retention system prevents the plastic-keytop from coming out of the housing.

The key is pushed down 0.2mm - but this provides enough force to push up against the key-top and prevent rattles.

Preload 2

While the key itself is designed around a 1.0mm stroke, the pre-load reduces the useable stroke of the assembled keypad to 0.8mm. As long as the preload is 20% or less of the total travel, the user will feel the typical actuation force and snap of a standard silicone keypad.

Metal Dome & Silicone Actuation
Silicone Keypads with Metal Domes or Tact Switches

Some silicone keypad designs use separate components, such as metal domes and tact switches, to provide the actuation feel and electrical function for the key. Tact switches and metal domes offer a different actuation feel from conventional silicone web keys. The stroke is typically lower (0.8 - 0.4mm) and the snap is higher (70% - 30%). Additionally the life of metal domes and tact switches is usually longer (500,000+ actuations) than those of silicone key webs.

Dead Web
Dead Webs

To prevent double clicks a dead web must be used instead of a conventional (angled) silicone web. The dead web (or flat web) is designed to not provide any snap feedback and add minimal force to the stroke of the key.

Pestle Design
Pestle Design

When using metal domes or tact switch, the force from the silicone key press must be transferred using a pestle to the separate component underneath. If the pestle is too thick, a mushy feeling might be felt. If it is too thin, it might collapse or be off center to the metal dome or tact switch.

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2D Drawing Specification Notes

specification element


2D drawing note example