Understanding Silicone rubber keypad Complete Guide

What is a Silicone Rubber Keypad?

The silicone rubber keypad is made of silicone material with unique overall properties and made into an elastomer switch with webbing through different molding processes, which has design flexibility, soft touch, good elasticity, wear resistance, and weather resistance. This elastic switch usually provides a low-cost, reliable solution and is widely used in consumer and industrial electronics.

How do silicone rubber keypads work?

The center of the switch forms an angled soft webbing that utilizes the low compression set characteristics of silicone rubber to make the silicone rubber keypad work properly. Most of the time, the webbing is upright. When pressure is applied to the button, the silicone webbing deforms or collapses evenly, creating a positive tactile response. When the external force is removed, the webbing returns to its original shape. Sometimes, in order to achieve an electrical switch, conductive pills or inks of different shapes are added to the center area of the bottom of the silicone rubber keypad. When the switch is pressed, these conductive media contact the PCB/PFC to conduct the circuit.

Common design types of silicone rubber keypads

Translucent silicone rubber keypad, color silicone rubber keypads, metal conductive pills silicone rubber keypad, plastic with rubber keypad (P+R), silicone rubber membrane Keypad, silicone rubber keypad with plastic caps, silicone rubber keypad with PCB/FPC， silicone rubber keypad with LED/EL, silicone rubber keypad with contact ink.

Translucent silicone rubber keypad

Color silicone rubber keypads

Plastic with Rubber keypad (P+R)

Metal conductive pills silicone rubber keypad

Silicone rubber keypad with contact carbon pills

Siliconr rubber keypad with contact ink

Silicone rubber keypad with LED or EL

Silicone rubber keypad with FPC

Silicone rubber keypad with PCB

Silicone Rubber Membrane Keypad

What are the main materials used for manufacturing silicone rubber keypads?

The main materials for silicone rubber keypads are solid silicone rubber and liquid silicone rubber. Before molding, additional hardeners or fillers need to be added. Fillers can usually increase the hardness and stability of silicone.

Characteristics of Silicone Rubber

As we all know, silicone rubber, a member of the rubber family, is a synthetic silicone elastomer with a low compression set, good electrical insulation, excellent temperature, weather, and chemical resistance. The silicone rubber keypad after high-temperature vulcanization has excellent compression molding, mechanical properties, and resistance to extreme temperatures and environments, making it very suitable for applications that require cold resistance, heat resistance, and durability.

General physical properties of silicone materials used in silicone rubber keypads

Item	Technical index
Item	Conductive pills	No conductive pills
Density ratio（25°C）	1.18	1.17
Shore hardness(A)	65+/-5	30~80+/-5
Tensile strength(Mpa)	> 4.9	> 6.9
Elongation（%）	> 100	> 250
Tear strength(KN.m)	8.8	11.8
Linear Shrinkage(%)	3~5	3~4.5
Volume resistivity(Ω.cm)	< 5	< 1015

Common manufacturing processes for silicone rubber keypads

The original silicone rubber keypad was 100% produced by compression molding. This processing method is very flexible but has low production efficiency and high cost. With the improvement of production technology, injection molding, and liquid silicone rubber molding processes have emerged, achieving high precision, high production efficiency, and higher quality silicone rubber keypads.

Manufacturing Process of Compression Molded Silicone Rubber Keypad: Step by Step

Injection molding is the ideal choice for silicone rubber keypads. It provides silicone rubber keypads with high-precision dimensions, more complex designs, and excellent surface finish. For small to medium batches and larger sizes, silicone rubber keypads made using compression molding can provide consistent quality and performance for most industries. The process involves multiple steps.

Step 1: Prepare the tool

Compression molding molds generally include an upper mold (sometimes called a male mold) and a lower mold (sometimes called a female mold). Sometimes it is a three-layer mold including an upper, middle, and lower mold. The compression mold is usually made of high-quality metal materials and produced by internal EDM and CNC milling machines. The molding manufacturer will process the mold according to the complete keypad drawing provided by the customer or the approved internal drawing. The molding is generally surface-treated before sampling or mass production. For example, common mechanical treatment methods include sandpaper grinding, polishing, and sandblasting.

Step 2: Prepare materials

Solid silicone rubber is a high-viscosity, dough-like elastomer. Before molding, silicone of appropriate specifications is mixed with a strict proportion of curing agent and other additives, then stirred evenly to obtain a slightly cross-linked silicone rubber roll. This process can add multiple colors according to the requirements of the drawing or personal preference. Next, the silicone roll is cut into specific sizes by accurately weighing the silicone. Sometimes it is also called a silicone preform.

Step 3: Molding

The correct mold is placed on the appropriate plate vulcanizer and preheated. When the mold temperature reaches the preset temperature, the mold is opened and the silicone rubber preform is manually placed into the cavity surface. Sometimes this action is preceded by placing conductive carbon or metal pills on the mold. After the preparation is completed, the mold is closed. The mold must be kept at high pressure before it is ejected so that the solid silicone rubber can fill the required details and features.

Step 4: Curing

The silicone rubber is cured into the shape of the mold cavity through a chemical reaction at high temperature and high pressure. time of vulcanization, temperature, and pressure are the keys to making high-quality silicone rubber keypads. During this process, the mold must be heated to a specific temperature maintaining for a while until it becomes a solid elastomer. Depending on different design requirements, the curing time may take from a few seconds to a few minutes.

Step 5: Demolding

The mold is cooled to a suitable temperature after the curing process and then ejected from the vulcanizer machine. After the upper mold is opened, the operator uses an air gun to assist in removing the silicone rubber keypad from the mold cavity.

Step 6: Post-processing

Post-processing is a key step to achieving the perfect appearance and performance of silicone rubber keypads. These procedures may be trimming rough edges, printing, laser marking, assembly of different parts, surface spraying, glue dripping, etc.

Step 7: Reliability Testing

Depending on the design requirements, your silicone rubber keypad may need to undergo various tests during the manufacturing process to ensure the complete function, durability, tolerance, and reliability of these keypads. These tests include trigger force test, strike life test, electrical test, and alcohol wear test.

Advantages of silicone rubber keypads

Silicone rubber keypads offer significant advantages due to their reliability, low cost, and durability. They are often used as components of input interfaces and provide functionality. They are popular in the medical, industrial, automotive, and consumer electronics industries because you can customize them to your unique functional or performance requirements, making them ideal for your specific application.

Here are some of its advantages:

Temperature, weather, and chemical resistance

Made of silicone rubber with many excellent properties, the silicone rubber keypad has excellent temperature and aging resistance. It is very suitable for applications exposed to extreme temperatures and environments including construction, industrial control panels, mobile equipment, etc. It also has good chemical and water resistance and can withstand daily disinfection, various liquids, and other chemicals. You can often see them on medical equipment and feel free to touch them.

Design Flexibility

One of the biggest advantages of silicone rubber keypads is their design flexibility. Silicone rubber keypads are usually composed of different material components, which allow them to provide customized tactile feedback, and multiple aesthetics and appearance options. It can meet the tactile feedback requirements of your specific application by adjusting its travel, actuation force, and stainless steel dome. The metal dome inside the button makes an audible click to let the user know that the button is working properly. Silicone rubber materials of different hardness and surface processing can be used to achieve multiple colors, and shapes and enhance its durability. Keypads with plastic keycaps can provide designs, images, shapes, and transparency that cannot be achieved with rubber to meet a wide range of uses and required aesthetics. Electronic functions are generated by contacting the gold-plated layer using carbon pills, metal pills, or conductive carbon ink. Its design flexibility also lies in backlighting using LEDs, ELs, etc.

Low-cost and reliable

Silicone rubber keypads are usually made of lower-cost silicone materials and molded tools, which means that their unit cost of production is lower. Although plastic keyboards made by injection molding have the advantage of lower cost, they are less durable and therefore not suitable for working in harsh environments. Silicone rubber keypads have excellent durability and long shelf life, making them more suitable for equipment applications that require frequent maintenance or replacement of parts. It can be chosen for high-volume, low-cost budget projects.

Good elasticity and wear resistance

Silicone rubber is a highly flexible elastomer that can be molded into unusual shapes and dimensions. Custom silicone rubber keypads have good elasticity and can maintain their original shape for a long time. Silicone rubber keypads with added surface treatment also have good wear resistance and will not wear out even if they are clicked continuously for a long time.

Limitations of Silicone rubber Keypads

Although silicone rubber keypads have many advantages such as softness, waterproofness, and flexibility, there are also some limitations to using these keypads. Here are some common silicone rubber keypad problems:

Failure

Loose, worn, or damaged elastomers inside the keypads may cause no response or failure when pressed.

Sticking

After long-term use, silicone keypads may become sticky due to dust, dirt, or liquid infiltration, making them unable to bounce normally.

Deformation or Looseness

Silicone keypads may deform or loosen due to long-term use or external force, causing the keys to shake, irregular shape, or inconsistent height.

Wear of Surface Coating

In addition, the surface coating of the keypads may wear over time and after repeated use, causing their appearance to deteriorate or even lose their touch.

Design Guidelines of Silicone Rubber Keypads

Classic structure of silicone rubber keypad

The correct thickness of the base pad is the key to the dimensional accuracy of the silicone rubber keypad. The base pad thickness is generally 0.8-1.2mm. The recommended stroke is 0.8-1.5mm for designs with conductive pills or ink. If the bottom is a tactile switch, its stroke depends on the stroke of the switch itself.

The thickness and slope of the inclined wall are crucial to obtaining a clear sense of rhythm and touch. Usually, the inclined wall is designed with a slope of 45C°. The designed thickness of the inclined wall is 0.2-0.6mm.

The hardness of silicone rubber is also an important factor affecting the feel of the keypads. The commonly used silicone hardness is 50+/-5 degrees.

Classic structure of silicone rubber keypad

Air path design

The silicone rubber keypad's base pad and circuit board must be designed with an air path. If there is no air path, negative pressure will be formed between the PCB/FPC during pressing, causing the key to fail to rebound. The design depth of the air path is generally 0.3mm, and the width (1.0-2.0mm) can be adjusted according to the size of the key.

The pressing force-travel curve of the silicone rubber keypad

The tactile feedback depends on the snap ratio. According to experience, a 40%-60% snap ratio can obtain better tactile feedback.

Force-stroke curve of silicone rubber keypad

Force
FP:Peak Forcer(FMAX)
FC:Contact force
FR:Min,Return Force(FMIN)
FM:Max.Return Force
FD:Drop Force(FD=FP-FC)
FG:Gap Force(FG=FP-FM)

Stroke
S1:Peak Stroke
S2:Contact Stroke

Location
O:Roinginal Point
P:Peak Point
C:Contact Point
R:Return Point
M:Max Return Point

Travel
O-P:Prelod Travel
P-C:Travel To Contact
C-S: Post Travel
S-R-M-O: Return Travel

Force-travel curves of different structural forms of silicone rubber keypads

Due to structural morphology, space limitations, or based on functional requirements, silicone keypads can be designed into a variety of cross-sectional shapes. These classic cross-sections include flat cone type, top cylindrical type, flat dome type, double cone, ring dome, and ring dome top.

The picture below shows the force-travel curves of different structural forms.

Flat Dome type

Top Cylindrical Type

Flat Dome type

Double Cone

Ring Dome

Ring Dome Top

Some key dimensions of silicone rubber keypad design

Designing the right dimension ensures that the silicone rubber keypad maintains a good feel, no resistance, and no sluggish feeling, and produces positive feedback during pressing and resetting. So what specific dimensions should be paid attention to when designing?

Key dimensions of silicone rubber keypad design

1. Excircle diameter ≥2.0
2. Diameter ≥0.6mm
3. Inner circle diameter ≥0.4mm
4. From the edge of the inclined web to the positioning hole ≥1.0mm
5. Positioning hole diameter ≥2.0mm
6. Width of the inclined web‘ bottom - A ≥2.0mm
7. Round corner diameter of the key surface ≥0.50mm
8. Round corner diameter of the top edge of the key surface ≥0.4mm

Minimum dimensions that should be followed in silicone rubber keypad's design

What are the minimum dimensions that should be followed in the structural design of silicone rubber keypads? The following are the minimum dimensions of standard silicone rubber keypads:

Standard silicone rubber keypads minimum dimension

A≥1mm
B＞1.5mm
C≥0.5mm
D≥0.3mm
E＞1.5mm
F＞0.8mm
G≥1.0mm
H≥1.5mm
I≥1mm
J≥1mm

Dimensions and tolerances of silicone rubber keypads

After molding, the dimensional accuracy of silicone rubber keypad is limited by the shrinkage rate of silicone.

General dimensional tolerances:

Length(mm)	＜10	＜10≤20	＜10≤20	＜30≤40	＜40≤49	＜40≤49	＜49≤100	＞100
Tolerance(mm)	+/-0.08	+/-0.10	+/-0.15	+/-0.20	+/-0.25	+/-0.30	+/-0.40	0.6%

The silicone rubber keypad's design of the draft angle

The draft angle is crucial for the smooth demolding of molded silicone rubber keypads. When the height along the demolding direction is above 10mm, the recommended draft angle of the key is 1~1.5°. When the key height is less than 10mm, it can be made into zero draft.

Silicone Rubber Keypad's design of Stable Column

For higher silicone rubber keypads, stabilizing columns are designed around the keys to reduce the swing amplitude of the keys. If space permits, the diameter of the stabilizing column can be appropriately enlarged during the design. Common stabilizing column designs are as follows:

Common stabilizing column designs

Silicone rubber keypad's assembly design

Design rules for the combination of silicone rubber keys and plastic shells

A&B: Dimension of plastic shell keys
a&b: Dimension of silicone rubber keypads
A-a≥0.3mm, B-a＞0.2mm

R: Corner radius of plastic key
r: Corner radius of silicone rubber keypad
R:1mm~1.25mm
r:0.75~1.0mm

H: The height of the plastic shell which the silicone keys exceed
S: Keypad stroke
H-S≥1.5mm

D-d=1.5mm~2.0mm
1.5mm≤D-d≤2.0mm

P: Diameter of post
t: The gap between the post&conductive part
P=1.0mm
0.1mm≤t≤0.15mm

What are the common locating column designs?

The positioning column helps production assembly positioning and can quickly fix the substrate on the PCB board. Common positioning columns include tensile type, push type, and inverted hook type.

Push-type

Stretch type

Inverted hook type

Common key surface forms of silicone rubber keypads

Flat

Concave

Rocking

Angled

Convex

Flat with blind spots

Silicone rubber keypad's anti-misoperation design

To prevent accidental operation of the keypads, the key surface is designed to be lower than the panel. For the silicone rubber keypad with a tactile switch, an idle travel of 0.5~1.0mm is designed between the bottom and the switch.

The difference between non-conductive and conductive silicone rubber keypad

The bottom of the non-conductive silicone rubber keypad is pure silicone. Pressing forces the metal dome to collapse and contact the copper foil of the circuit board to achieve a conductive circuit. Compared with the conductive silicone rubber keypad, this type of keypad allows the use of higher hardness silicone materials, which are usually designed together with membrane switches to form a waterproof and more durable electrical switch.

Conductive pills (carbon/metal) or conductive ink are added to the bottom of the conductive silicone rubber keypad. When the mesh structure is deformed, the bottom contacts the copper foil of the circuit board to achieve loop conduction.

Conductive pills design and specifications

Conductive pills are usually made of carbon powder or other metals mixed with silicone rubber. Generally, its hardness is about 70 degrees and its shape is mainly round. Choose low-resistance black pills or metal pills according to the design requirements of the drawing. The structural dimension of the conductive pills is recommended to be more than 2mm smaller than the top size of the key.

The standard diameters of round conductive pills are Φ2.0, Φ2.5, Φ3.0, Φ3.5, Φ4.0, Φ4.5, Φ5, Φ5.5, Φ6, Φ7, Φ8, Φ9, Φ10. They can also be customized according to the customer's unique application. The thickness of the conductive layer is usually 0.2-0.6mm. The shape of the conductive pills is usually 0.5 times the perfect circle.

Common shapes and specifications of conductive inks

The bottom of the silicone rubber keypad is printed with conductive ink of different shapes and sizes by silkscreen or pad printing. The thickness of this layer of ink is usually 25-40 microns. The classic forms of conductive ink include ring, oval, square, rectangle, square ring, triangle, semi-ring, semi-square ring.

Ring

Oval

Rectangle

Square ring

Square

Half square ring

Semi-ring

Triangle

Comparison of various conductive solutions for silicone keypads

Types	Carbon pill	Printed conductive ink	nickel pill	Gold pill	Low Resistance Carbon Pill	Metal dome
Contact Surface of PCB	Gold	Gold	Gold	Gold/Carbon	Gold	Gold
Design shape	Circle	No limited	No limited	No limited	Circle	No limited
Contact resistance	< 100Ω	< 200Ω	< 1Ω	< 1Ω	< 20Ω	< 10Ω
Contact pressure	< 1.5N	< 1.5N	< 0.5N	< 0.5N	< 1.5N	2.5~7N
Click life	>5 Million	1~3 Million	1~5 Million	> 5Million	> 1Million	> 500,000
Advantage	Low cost, front and back sides being available, stable service life	Shape no limited, large area printing, special shape printing, lower cost	No dust problem,Lower resistance	Resistance reaching nearly 0Ω	Small resistance, low cost	0.3~0.6mm stroke, strong feel
Disadvantage	High resistance, slow response	Higher resistance	High cost	Dust problem,Low production efficiency, high cost, noisy contact	Short lifespan	Shorter lifespan

Backlight design of silicone rubber keypad

To make the key easier for users to read and operate accurately, LED or EL backlighting with indicator functions is usually combined with laser etching to achieve a uniform and infinitely creative backlighting effect. Backlighting involves correctly placing the light in the switch structure. Common backlighting methods include LED (light emitting diode), and EL ( Electroluminescence).

Design and advantages of LED backlight

The LED is located at the bottom of the switch center

The LED bottom design is suitable for keypads that need to be illuminated and have limited space. Therefore, when designing, pay attention to arranging the number and position of the lights according to the dimensions of the buttons, and reserve space for conductive pills, springs, and stabilizing columns. Therefore, metal domes with holes and semicircular or circular conductive carbon pills are usually used.

Side-mounted LED

Placing a metering light or forward-facing light is a popular low-cost lighting method. The light from the lamp hits the inherently colored (translucent) silicone on the side and shines through the surface of the key.

Advantages of ED backlighting

Reliable and cost-effective
Suitable for small spaces
Various types of backlight sources
Various colors, intensities, and bulb packages
Low heat and low voltage operation
Long service life, no less than 100,000 hours

Design and advantages of EL backlight

Since EL backlight emits a single color, requires a high-power drive, and has a service life of 2000-3000 hours, it is only suitable for low-light and no-light applications where the backlight is turned on at a low frequency, such as logos and fonts that require large areas to emit light simultaneously.

Advantages of EL backlight

Low power consumption
Provided a more uniform backlight
More suitable for most membrane switch structures

Technical requirements of silicone rubber keypads

The technical requirements of silicone rubber keypads used in remote controls are as follows:

Item	Technical requirements
Operating temperature(℃)	-40~150
Storage temperature(℃)	-50~180
Travel(mm)	0.8~1.5
Force(g)	60~300
Life cycles(Times)	100000
Electric strength(MV/m)	20
ESD backdown(V/min)	＞500
Contact bounce(ms)	＜5
Contact resistance(Ω)	＜200

Applications of silicone rubber keypad

Communication equipment

Typical applications of silicone rubber keypads include mobile phones, landline phones, and other communication equipment.

Home appliances

A silicone rubber keypad is a widely used component. For example, it is commonly used in remote controls for TVs, air conditioners, fans, and other home appliances. It can also be used for control panels of daily home appliances, including washing machines and refrigerators.

Automotive

It performs well in high temperature environments and is very suitable for car audio control, air conditioning control, and steering wheel. Use them to provide users with more accurate feedback during driving.

Industrial control equipment

Silicone rubber keypads can withstand harsh working environments. Therefore, they are often used in the operating panels of various control equipment in the industrial field.

Medical devices

Since silicone rubber keypads are corrosion-resistant, stable in harsh environments, and antibacterial, the medical industry uses them in blood pressure monitors and electrocardiographs to ensure the accuracy and safety of operations.

What are the common testing types of equipment for silicone rubber keypads?

Various professional testing equipment enables us to provide high-quality silicone rubber keypads for your project. Commonly used equipment includes a key life tester, tension tester, key elastic curve meter, two-dimensional image measuring instrument, alcohol wear tester.

Key life tensile test instrument
The life test equipment is designed to simulate operations such as long-term continuous or rapid pressing in actual use scenarios, helping manufacturers accurately evaluate silicone rubber keypads' life and reliability.

Key elasticity curve meter
Designers and engineers use key force curve meters on silicone rubber keypads to obtain accurate data on key travel, resistance, rebound, etc. They base on these reliable data to optimize the keypad to improve the user experience and the service life of the keypads.

Tensile test instrument
Apply to test the elongation at break and tensile strength of silicone rubber raw materials.

Two-dimensional image measuring instrument
Two-dimensional image measuring instruments can efficiently detect the contour, surface shape, size, and angle of different silicone rubber keypads.

Alcohol wear test instrument
The alcohol wear test instrument is suitable for testing the wear life of characters on the surface of silicone rubber keypads. This process generally uses 98.7% pure alcohol. The load is 500g. The test standard is 1000-3000 times.

How do you quickly customize and design your silicone rubber keypad?

Generally speaking, a complete 3D drawing is required for custom silicone rubber keypads. Some customers only have physical samples for us to copy. A few customers only provide concepts. Our professional engineering team will confirm the keypad structure and surface treatment with your engineers before starting the drawing design. It usually takes 3-5 days to finish drawings. Here are the forms you need to fill out:

Estimated batch order quantity - choose the most suitable molding process for your product
Product application - to recommend the silicone material usage environment suitable for your project.
Use LED or EL backlight?
What cross-section form of silicone rubber keypad?
Add conductive pills or conductive ink?
Key surface treatment?
Service life requirements?
Dimensional tolerance of silicone rubber keypads?
Is the silicone material transparent or provides RAL or Pantone?
Legend's RAL or Pantone

By fully understanding your needs, we provide complete STP drawings that can be directly used to creat the mold. Next, forming mold making will finish within 10-20 days after your approved drawings. Sometimes, according to your requirements, we provide rapid prototypes to verify the feasibility of your concept.

How do you custom high-quality silicone rubber keypad molds?

Using high-quality mold materials is the key to mold durability and meeting the requirements of high-precision silicone rubber keypads. To ensure high production efficiency, high precision, and long service life of silicone rubber molds, the following details should be noted when selecting mold steel:

High hardness and high toughness: The higher the hardness of the steel used, the higher the precision of the silicone mold produced, and the longer the service life.

Corrosion resistance: Since some chemicals are often used to clean the mold during the production of silicone molds and subsequent production, corrosion-resistant steel must be used.

Wear resistance: Silicone molds will constantly rub against external substances during use, so materials with good wear resistance must be used.

What are the common types of mold steel?
50#
P20
NAK80
S136
SUS420

P20 has higher hardness, good wear resistance, and longer service life, and is more suitable for long-term production.