Silicone keypads are widely used in a number of consumer products. These keypads are used in many industrial products, and are now popular because they are a cheaper and reliable switching solution for different technologies and recent innovations . The compression molding properties of silicone are used to create the webbing around the switch center. A tactile response develops when the switch is depressed uniformly. The webbing returns to its normal state when the pressure is removed. A carbon or gold contact is used between the silicone webbing and the printed circuit board (PCB) at the base of the switch.
Flexibility of using Silicone for Keypads
Revisions to the design of the silicone webbing allow the switch to have varying tactile responses. Different key shapes can be accomplished by using varying tactile forces. The tactile force experienced by a user is measured using the snap ratio. The switch designers try to maintain a snap ratio of 40 to 60 percent. A snap ratio below 40 percent increases the life of the switch, however, the tactile feel is lost. The ‘click’ feeling cannot be experienced when the snap ratio is kept below 40 percent.
Building Multi-Key Keypads with Silicone Rubber
Typically, natural silicone rubber is mixed with pigments to create keys in different colors. These keys can be molded together to build a multi-key keypad. Printing individual tags to the keys may be used to customize the keypads. Additional options include spray painting or etching of tags into the keys. Laser technology can be used to etch legends or tags. Individual keys can be illuminated when placed on a PCB using surface mount LEDs (light emitting diodes). There are many coating materials available to protect the keypads for long periods of time. Various color effects can be produced on the keypad by applying a combination of laser etching and coating materials.
Applications of Silicone Keypads
There are many applications of silicone keypads. These include industrial control devices, remote controls for video and hi-definition devices, electronic toys and door locking controls. Industrial uses of silicone keypads are used in professional fields that include military, medical and marine. Many of the industrial applications require conformance to specific industry standards.
One of the most popular uses of silicone keypads is to build keyboards for mobile handsets. The automobile industry also depends heavily on silicone keypads to build switch mechanisms and control functions. These include window and steering wheel mounted controls. The applications in automobile and mobile handset have increased significantly in recent years.
Reasons behind the Need for Silicone
Silicone offers great feel, excellent reliability and extended life in the most rugged and tough environments. This makes the use of silicone possible in areas such as instrumentation, medical and communications. The keypad industry uses silicone in two forms, liquid and gum stock. Liquid silicone is a two-part mixed system, whereas gum stock is a one-part system. Both forms are cured by heat. Gum stock acts more like clay or putty, whereas liquid silicone acts like Vaseline. The hardness of silicone is measured using the Shore A scale. A lower Shore A score indicates that the product contains more silicone and the result will be a softer feel. A higher Shore A score indicates that the end product contains less silicone and more of other materials, making the product harder.
Molding Processes Applied for Silicone Keypads
The molding processes applied on the silicone keypads vary. These include extruding, compression, liquid and transfer. Liquid molding works under pressure and utilizes computer controls. Compression molding follows a mechanism similar to waffle iron, where the material is manually cut and measured. Transfer molding combines compression molding and insertion of gum stock into the desired mold under low pressure. Extruding uses mostly gum stock and related molding processes.
Considerations for Designing Silicone-based Keypads
There are a few considerations while making a drawing of a silicone-based keypad. These include the overall keypad size, thickness of the keypad mat, dimensions of the key-top, details of the mounting holes, details of the mounting boss, radius of the keypad and buttons, spacing of keys, actuation force in grams, specifications of the material, and finally the color of the keypad. The typical shapes used in the design process are cone, double cone, bell cone and flat cone. The actuation force for these shapes vary anywhere from 20 to 200 grams.
A positive tactile force may be obtained by designing the silicone keypads with an actuation force between 50 and 170 grams. Some designers use the method wherein a tactile layer is added between the keypad and PCB to get a good tactile feel when the keys are too small. This method increases the cost of the project. Designing keypads with a return force of at least 30 grams can eliminate the potential for a sticking key.
The actuation force is the force that collapses the rubber web in a keypad must be optimized. The air channel at the bottom of the keypads must allow for air passage when the keys are pressed. The alignment hole must be designed such that it positions the keypad in its enclosure. The base matte joins all the keys in the keypad and must be of good quality. The design must generate a tactile feel that is ideal for the user to seamlessly use the keys without distractions.
The use of silicone has been increased in recent years in many industries such as automobile, mobile handsets and medical control devices. Silicone rubber offers great durability and flexibility for the end products. Furthermore, the silicone keypads often generate the desired amount of tactile force that is necessary to function the devices properly. The success of a silicone-based keypad depends on the various design considerations applied while producing the end product. The project itself requires great technical expertise, care and geometrical knowledge. The various industry standards and measurements must be adhered to while building silicone based keypads. The design standards are more important and stringent in industries such as military and medicine.
The future of silicone keypads is that it will be extensively used in rising technologies across various industries. One such industry is the automotive, where more efficient switch mechanisms are currently being developed. There are also prospects in consumer electronics, where these keypads are a necessity in user control interfaces.