The Silicone Rubber Keypad and Its Design Specifications.
Silicone rubber keypad has been around for decades, but it has gained rising importance in the past few years. From their utilization in industrial equipment to household remote controls and children toys, these keypads have varying applications.
A Guide to Silicone-Rubber Keypad
Working of Silicone Rubber Keypad
A silicone rubber keypad is provided with a top key. This key can be of either plastic or silicone. A silicone webbing supports this key and contains the conductive pill beneath it.
When the user presses a key, the webbing deforms, and causes the conductive pill to connect with a circuit board to complete the circuit. Which then carries the command over to the device.
There is an air channel between the conductive pill and the PCB that provides insulation for when the force on the key lifts and the circuit breaks. The dimension of this air channel ranges from 1.5-2.0 mm.
Features of Silicone Rubber Keypad.
Silicone rubber keypad has become increasingly sought-after owing to its dynamic features. With the boom in the tech and construction industry, the need for durable and long-lasting material has increased. Hence, the demand for silicone keypad shot up directly increasing its popularity.
Tactile Response
Silicone rubber keypad comes with a tactile feedback system. A tactile feedback system is the one that enables the user to know that the key has been pressed and the function carried to the processor.
This is mostly due to the tiny metal hemispheres which are present beneath every key. The pressure applied on the keys flattens these hemispheres, hence giving that “feel” to the user.
Ability to Withstand Rough working conditions.
The reason silicone rubber keypad is so popular is due to its their ability to withstand rough working conditions. Being made of silicone, these keypads can bear extreme temperatures.
They are durable and stay in service for a longer period without wearing out. Their durability can be further increased by coating them with polyurethane.
Backlight Feature
Owing to its application in a wide range of industries, silicone rubber keypad allows backlight connections. This enhances visibility in low-light conditions. These backlights can be light-emitting diodes (LEDs), Electroluminescent (EL) lamps or a fibre optic lighting system.
Highly Resistant
Being made with silicone, the silicone rubber keypad is highly resistant to moisture and harsh chemicals. This feature enables their use in chemical industries and in hospitals where exposure of equipment to harsh chemicals and moisture can be frequent.
Conveniently Customizable
Another feature that sets the silicone rubber keypads apart is its customizability. This feature allows wide-range applications of these keypads as they can be moulded into any shape, size, and design and can be made in any user-required colour.
Standard Design Guide for Silicone Rubber Keypads
General specifications of a silicone rubber keypad
The key travel of this keypad, that is the distance between the conductive pill and the circuit board normally ranges between 0.8-1.5 mm. The rubber keypad can work at temperatures ranging from -40 ˚C to 150 ˚C making them very good candidates for applications in high-temperature conditions.
These keypads can withstand force ranging from 60 grams to 300 grams. A single switch in the keypad has a life of about 1 million keystrokes. The life of the keypad is the number of strokes that the keys can bear before the membrane switch damages.
Design Features
Silicone rubber keypads, because of their dynamic applications, need to follow some standard dimensions and design features for them to be applicable in user-defined conditions.
Following are some other design features that the engineers and manufacturers of silicone rubber keypad take into consideration while designing and manufacturing a silicone keypad.
Standard Physical Characteristics for Silicone Rubber Keypad.
The hardness of the material used as an insulator in the silicone rubber keypad ranges between 40 to 80 Shore A with a tolerance of +/- 5. While, for the conductor in the keypad, the hardness should be 60 Shore A.
The tensile strength for standard material should be 70 kg/cm for conductor and between 55-75 kg/cm for insulator. And a tear strength of 10 kg/cm for conductor and 8-12 kg/cm for insulator.
Button Travel
The button travel of a keypad is the distance between the inside of the key and the circuit board beneath it. The ideal button travel distance for a silicone rubber keypad would be between 0.8 mm and 1.5 mm.
Increasing or decreasing the button travel comes with certain trade-offs between tactile response and life cycle of the keypad.
Increasing the travel beyond 1.5 mm would mean increased tactile response but decreased life. While, decreasing the button travel below 0.8 mm would result in reduced tactile feel but increased keypad life cycle.
Actuation Tolerance.
Tolerance is an allowable range within which a certain value can fluctuate without incurring any consequences on the machine or its part.
Actuation is the force with which the user must press the key for it to complete the circuit and allow the PCB to transfer information to the processor.
The design of silicone rubber keypad requires it to be within certain limits with regards to actuation specifications.
Ideally, the optimal actuation force for enhanced performance ranges between 80 grams to 150 grams but many keypads and keyboards have been designed with much wider actuation ranges.
The tolerance for different actuation forces is different. For 50 grams design actuation force, the tolerance is around +/- 15 grams. Similarly, for 75 grams of design actuation force it is +/- 20 grams and for 100 grams design force, the tolerance is +/- 25 grams. This defines a relationship that for every 25-gram increase in design actuation force, the tolerance changes by +/- 5 grams.
Tactile Characteristics of Silicone Rubber Keypad Keys.
A keypad is said to have a good tactile feel if it’s click ratio is greater than 50 %. A keypad has a poor tactile feel if it’s click ratio is less than 50%. Designers of the keypad calculate the click ratio by dividing the difference of actuation force and the contact force by actuation force.
Dimensional Requirements
All the silicone rubber keypads are required to follow a set of standard dimensional requirements. Such as, key spacing on a rubber keypad should be 1 mm minimum.
The edge of the key to the guide hole distance must be at least 1 mm. The radius of inside corners of cut-outs should be minimum 5 mm. While, the inside radius of internal cut-out should be 1 mm.
The minimum key pitch, that is the distance between two adjacent keys should be 3 mm minimum. And the clearance between the key and the bezel should be 0.3 mm minimum.
A keyboard bezel is the hard surface on the keyboard on which the keys are supported. It shields the keyboard interior from human touch.
Dimensional Tolerance
Dimensions for a silicone rubber keypad range from 0.1 mm to over 100 mm. In this case, the dimensional tolerance is different for different dimensions.
For a dimension range of 0-10 mm, the dimensional tolerance is +/- 0.10 mm. While for a range of 10-20 mm, the dimensional tolerance is +/- 0.15 mm and for a range of 20-30 mm, the dimensional tolerance is +/- 0.20 mm. For a dimensional range of 30-50 mm the tolerance changes by +/- 0.25 mm.
Beyond this dimensional range, the tolerance changes by +/- 0.05 mm for every 10 mm change in dimensional range.
Legend Printing
The graphics (letters, numbers or symbols) are printed on the surface of the key using silicone ink. These graphics are known as legends. Keeping the legends-printed keypads at high-temperature makes the legends more durable. The legends can be made resistant to wear and tear by coating them with translucent ink.
The legend size should be 1 mm minimum with the width of the lines of the legend at least 0.2 mm. The minimum spacing between two adjacent legends should be 0.3 mm minimum.
Conclusion
The Silicone rubber keypad finds numerous applications in a wide range of settings We, Keypad-FD offer top quality keybads, keyboards membrane, membrane overlays, etc. This is due to its easy adaptability as per the requirements. There are many features of a rubber keypad that set it apart from its predecessors.
The silicone rubber keypad has many fundamental design standards that a design engineer should consider before designing a rubber keypad. From standard material properties to keypad graphics, all the basic design parameters of the silicone keypad require attention during it’s designing process.
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