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Introduction

Thank you for your interest in Rubber keypad products. This booklet has been designed to offer basic

information on rubber technology and to provide some basic design constraints to ease the flow of your new

project.

Conductive silicone rubber keypads were originally developed for the electronics industry as an economical

design alternative to discrete switches. 30 years on they are the most widely used form of switch technology

mainly due to their reliability, long life and multitude of design opportunities.

Kingley Rubber Industrial Co. Ltd has over 25 years experience in silicone rubber keypad design and

manufacture. With five factories spread throughout Asia employing over 1000 people they have the resources to

supply the needs of the most diverse customer base.

Diamond Electronics formed a relationship with Kingley Rubber some 3 years ago as part of a strategy to offer

keyboard solutions using many different technologies and offering design support to small to medium size

businesses in the electronics market. With field sales engineers available throughout the UK we can respond to

enquiries with speed and support your in-house design team with Rubber, Membrane, Piezo electric and

discrete switch solutions.

For additional information contact:

Diamond Electronics Ltd

Fourways Technology Park

London Road

Smallwood

Nr. Sandbach

Cheshire

CW11 2US

Tel: 01477 500450

Fax: 01477 500656

E-mail sales@diamondelec.co.uk

WWW http://www.diamondelec.co.uk

DIAMOND

Rubber Keypads

INGLEY

Terminology

Actuation Force

. . . . . . . The force required to collapse the membrane of a rubber switch (identified as Fl

on the force/stroke curve).

Air Channel

. . . . . . . . . . . Air path(s) on the bottom of rubber keypads and switches that allows for air

passage (venting) when switch is actuated. Switches must be vented on at least

two sides.

Alignment Hole

. . . . . . . . Through hole in rubber keypad that is used to position keypad in enclosure when

overall keypad size exceeds three inches in either length or width.

Base

. . . . . . . . . . . . . . . . . Silicone sheet material that joins all keys/switches on a rubber keypad. Also known as

apron.

Bezel

. . . . . . . . . . . . . . . . The faceplate or cover, typically either plastic or metal, used to secure a key pad

to a printed circuit board. The bezel also aligns the keypad during the final

assembly and protects keypad-base material from contact with human hands.

Breakdown Voltage

. . . . Voltage at which an insulator or dielectric ruptures. Also known as dielectric

strength.

Compression Set

. . . . . . The measurement of a material’s ability to recover it’s original size and shape after

compression under prescribed conditions. It is usually expressed as a recovery

percentage (fraction) of the compression condition.

Conductive Rubber Switch

Mechanical switch made of silicone rubber, with either direct or indirect contact.

Contact

. . . . . . . . . . . . . . The current-carrying area/surface under each rubber switch (conductive pill or

carbon-inked surface) that makes an electrical connection with the electrode on a

printed circuit board when the switch is actuated.

Contact Force

. . . . . . . . . The force required to maintain rubber-switch contact closure (F2) force/stroke

curve with a printed circuit board.

Contact Rating

. . . . . . . . The electric power handling capability for rubber contacts under strictly controlled

laboratory conditions.

Dual Durometer

. . . . . . . Silicone-rubber keypads manufactured using a two-shot moulding process and

two-material hardnesses.

Electrode

. . . . . . . . . . . . . Contact surface/design on a printed circuit board that conducts current when

rubber switch is actuated and switch closure occurs.

Key Height

. . . . . . . . . . . The measured distance from the bottom of a keypad (base) to the top surface of a

key.

Legend

. . . . . . . . . . . . . . Some type of printed graphic (symbol, letter or number) on the top of the key

surface.

Life

. . . . . . . . . . . . . . . . . . The number of switch actuations realised before the switch membrane ruptures or

over stresses.

Membrane

. . . . . . . . . . . . The non-conductive hinge that permits a rubber key to flex, and is responsible for

the tactile feel realised.

Negative-Image Graphics

Graphics that allow switch colour or switch masking colour to be seen through

top-surface printing on keypad.

Overstroke

. . . . . . . . . . . Additional travel experienced with a rubber switch after initial switch closure has

been realised. Rubber switches with overstroke require a double-cone or double-

bell shaped membrane.

Positive-Image Graphics

Single or multi-color printing on top of key surface.

Return Force

. . . . . . . . . . Force created by switch membrane as it returns the key to a non-actuated

position.

Snap Ratio

. . . . . . . . . . . (F1-F2) divided by F1. The difference between the actuation force (F1) and the

contact force (F2) of a switch divided by the actuation force.

Stroke

. . . . . . . . . . . . . . . Distance from the contact surface on a rubber switch to an electrode pattern on a

printed circuit board.

DIAMOND

Rubber Keypads

Technical Component Distribution

Silicone Rubber

Silicone rubber is a synthetic material which has the structure shown in Fig 1.

Fig 1 – Structure of silicone rubber

The “Silicone-Oxygen” molecular structure provides characteristics that allow the final material to withstand a

wide variety of chemical and mechanical degradation as well as serving as a strong dielectric.

Silicone rubber possesses the following characteristics:

Environmental durability with excellent resistance to both heat and low temperature (-55

C - 250

Minimum abrasion and high resistance to SO

and oxidisation even in heavy humidity

Minimum chattering or noise generation due to soft and elastic contact structure.

Features of Silicone Rubber Keypads:

Multicolour designs easily accommodated

Design-in of both Tactile and Linear feedback

Translucent materials available

Water and contamination proofable

Cost effective

DIAMOND

Rubber Keypads

INGLEY

Specific Characteristics

The tables below show data relating to the standard material used by Kingley in the manufacture of conductive

silicone keypads. Additional information on materials with different characteristics to suit specific applications

can be obtained from Diamond Electronics Ltd.

Characteristics

Conductor

Insulator

Hardness (Shore A)

65±5

30-80±

Tensile strength (Kg/cm

)

65-85

Tear strength (Kg/cm)

10-15

Compression set (%)

11-22

After 22 Hrs at 175

Specific gravity at 25

1.18

1.11-1.18

Contact resistance

<200

at 12V dc 30mA

Insulation resistance

>100m

at 250V dc

Max contact loading

24V dc 100mA

Strength

20-25kv/mm

Constant

26-35 MHz

Volume resistance

>2x10

(

-cm

)

DIAMOND

Rubber Keypads

Technical Component Distribution

Basic Key Design

INDIVIDUAL KEYS

Key design will vary with the functional and aesthetic requirements of the application. A designer may consider

the options detailed in this section which show alternative key styles and the new possibilities for adding

legends and backlighting to the design.

The table on page 13 details some traditional applications of rubber keymats with outline specifications. These

are meant for guidance only but will offer you an insight into the possibilties available to a designer.

Below is a diagram of a basic key structure:

Fig 2 – Basic key structure

NEW TECHNOLOGIES

Over recent years advances have been made in many areas of keymat construction. Most recently the area that

has been influenced has been

improved legend life

This has been achieved by secondary operations to treat the top surface of the keymat after the printing

process:

P/R keytops – clear plastic keytops adhered to the keymat base material

L/C keytops – laser etched legending

E/R keytops – epoxy coating deposited on the top surface of the key over printing

Ink coating – full coverage of keypad top surface with PC link.

DIAMOND

Rubber Keypads

INGLEY

Key Top

Optional Colour Insertion Key

Conductive Pill (Contact Area)

Web (Membrane)

Air Vent

Stroke (Travel)

Design Considerations

SNAP RATIO

The snap ratio of a keypad is directly linked to the tactile feel experienced by the user. Designers should attempt

to maintain ratios of around 40-60% only dropping below this if they are prepared to compromise tactile to

ensure longer life.

Snap ratio is measured as F1-F2 divided by F1, where F1 is the actuation force and F2 is the contact force.

See figure 3.

F1 : Actuation Force

F2 : Contact Force

FR : Min Return Force

FD : Drop Force (F1-F2)

S1 : Peak Stroke

S2 : Contact Stroke

Push Curve

Return Curve

Fig 3 – Snap ratio

TACTILE FEEDBACK

The membrane shape and size of any rubber keymat can be designed to achieve almost any combination of

actuation force and tactile response. Most applications simply require a positive tactile feel with a long life and

as such an actuation force of 125-150grms, with an accompanying snap ratio of 40-60% is a good

recommendation.

Variations in tactile response can be achieved with various combinations of contact stroke, actuation force,

keyshape and material hardness. As a simple rule it should be remembered that the higher the force ,the longer

the life but the poorer the tactile response. More specific guidelines are difficult to lay down. However, if a

customer specifies keysize, actuation force and stroke, Diamond can assist in the membrane design to achieve

the required parameters. Always remember to specify higher actuation force for wider or taller keys.

A common problem with rubber keypad design is ensuring that the rocking action that can be a feature of a

switch design is minimised. The following suggestions will assist in reducing the problem.

Keep stroke as near 0.8mm as possible

Add stabilising posts on base of key ( see diagram in assembly section)

Keep web length to a minimum

Keep web angle as close to 40

Actuation force 80-150grms for keys 10-15mm high and 150-175grms for keys 15-25mm high

Return force should also be set at around 30-35 grms to ensure that keys do not stick.

DIAMOND

Rubber Keypads

Technical Component Distribution

SWITCH LIFE

Membrane style and the durometer of the material are the factors that most effect switch life. Using a higher

durometer silicone, increasing the actuation force or by increasing the stroke will all decrease life. See fig 4.

Fig 4 – Switch life

MINIMUM KEY HEIGHT

This can be calculated for any design from:

Keypad base thickness + bezel thickness + stroke of key + 0.5mm

CONTACTS

Kingley offer three different types of contact solutions, each with it’s own unique characteristics. The carbon pill

is most commonly used due to it’s long life (>10 million ops) and low resistance (<100

). The pills are usually

circular with diameters ranging from 1.5-10mm and thickness from 0.4-0.6mm. Oval shaped pills are also

available in the sizes shown in fig 5.

Fig 5 – Oval shaped conductive pills

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Rubber Keypads

INGLEY

Printed carbon contacts are available in any shape however thickness is typically 10-20 microns and resistance

around 800

. Care must be exercised with electrical design if specifying this type of contact.

Dipped carbon contacts offer a compromise with any shape being available and contact resistance of <300

PRINTED CIRCUIT BOARD DESIGN

Rubber keymats themselves are very reliable in operation but when considering PCB design, the environment

that the keypads are to be used in must be taken into consideration to ensure the complete switching unit

remains reliable.

The choice of plating for the board is probably the most important factor with the cheaper tin/lead solder boards

not being recommended for use with rubber keymats.

Gold plating over nickel is the preferred choice with a recommended layer of 30-50 microns of gold on 100-200

microns of nickel giving contact resistance of <100

Nickel plating is the next best option again with good reliability but far more cost effective. A plating level of

>200 microns is recommended for best overall performance. This is the most common solution for rubber

keymat PCBs.

Silk screen carbon boards should only be used when contact resistances of over 800

can be tolerated. Also

the minimum track separation should be 0.5mm and the overall pad size should be greater than 5mm.

When designing shorting pads always attempt to get as many shorting paths as possible to increase switch

reliability and ensure that the pad size is never smaller than the carbon pill by a minimum of 1.25 times.

DIAMOND

Rubber Keypads

Technical Component Distribution

Dimensional tolerances

Due to the fact that silicone is a highly elastic material subject to manufacturing variations in moulding

conditions and material compounding, careful consideration to should be given to the tolerance tables below.

DIMENSION TOLERANCE

Dimension (mm)

General (±mm)

Precise (±mm)

<10

0.1

0.10

10-19.9

0.15

20-29.9

0.20

0.15

30-49.9

0.30

0.25

>50

0.6%

0.4%

ACTUATION FORCE

Design force (g)

General (±g)

Precise (±g)

40-59

60-79

80-99

100-150

>150

25%

20%

10 DIAMOND

Rubber Keypads

INGLEY

Mechanical Drawings

A sample mechanical drawing is shown below. To assist our designers please ensure that the following

information is included in the drawing:

Overall keypad size

Keypad/switch colours

Base thickness

Stroke/travel

Keytop outside dimensions

Actuation force

Overall key heights

Snap ratio

Contact size

Electrical specs

Mounting hole details

Material specs

Mounting boss details

Graphic colour(s)

Dimensions (keypad and buttons)

Printing artwork

DIAMOND

Rubber Keypads

Technical Component Distribution

SPECIAL DESIGNS FOR RUBBER KEYS

1. Different shorehardness and colour in the basic keypad and key

2. Construction idea for application on LED

3. Square key top design with LED window

4. LED on same PCB

5. Control of stroke distance

14 DIAMOND

Rubber Keypads

INGLEY

Conductive Pill

Different hardnesses and
colours available

PCB

LED

Tube of LED

Conductive Pill

PCB

LED

a -b >= 0.2

Plastic key top

PCB