Silicone rubber keypads are the most widely used form of switching technology today. They offer reliability, long life and design flexibility. There are some simple rules and basic design elements to consider when designing a silicone rubber keypad.
Key design will vary with the functional and aesthetic requirements of the application. It is possible to mold a key in almost any shape and to fit almost any configuration. It is important to remember that the key shape will affect the feel of the key. While a circular shaped standard key will have a consistent feel across the entire surface a half-moon shaped key will respond different depending upon where the key is pressed.
Once you have decided upon a key shape and layout; the next item to consider is the method for marking the rubber and creating legends. There are three common methods for marking keypads; printing, laser etching, and plastic key caps.
Printing
Printing is the most common method of marking rubber. The rubber is fixtured to flatten the key top then screen printed. There is no limit to the number of colors available. The arc of the key top determines how far printing must be set back from the edge of the key.
There are currently several options available for improving legend life with printed rubber.
- Plastic Key Caps – custom molded clear plastic is adhered over the legend or molded into the rubber
- Oil or Epoxy Coating – coating is deposited on the top surface of the key over printing, available in matte, semi-gloss, or gloss finish
- Drip Coating – drip coating can be hard or flexible. It adds a glossy layer over the key top. It can not be used on some keys with sharp angles. Hard coatings are subject to cracking if the key has large surface areas.
- Parylene Coating – offers the highest level of protection for a non-plastic coating. Parylene bonds to the rubber at the molecular level.
Laser Etching
Laser etching is especially well suited for applications where the keypad is backlit. Etching typically involves three production steps.
1) Translucent rubber (any color) is sprayed with a translucent base coat ink which will be the legend color that is visible to the user.
2) Rubber is sprayed with an opaque top coat ink which will be the overall color of the keypad.
3) The top coat of ink is laser etched away using high speed etchers to reveal the base coat.
Alternatively, you can decide to use a single translucent or opaque rubber ink and laser etch the legends revealing the color of the rubber that is used.
Plastic Key Caps
The longest lasting legend type is custom molded plastic. Plastic legends will not wear out. Many cell phone keypads are designed with plastic keys over rubber.
Functional Design Considerations
Snap Ratio & Tactile Feel
The snap ratio of a keypad determines the tactile feel experienced by the user. The recommended snap ratio for designers to maintain is 40%-60%; if dropped below 40% the keys will lose tactile feel but have an increased life. Loss of tactile feel means the user will not receive a ‘click’ feedback during actuation.
Snap ratio is calculated by taking the [actuation force (F1) – contact force (F2)] / actuation force (F1).
Most customers do not design the web and rely upon the manufacturer to design and meet the requirements.
Reducing Rocking Action
A common problem with rubber keypad design is the rocking action that can occur when a key is pushed. Rocking action can reduce the life of the keypad, make actuation difficult for the user, and cause other problems. The following suggestions will assist in reducing this problem.
- Add stabilizing posts on base of key
- Keep key stroke as near 0.8mm as possible
- Keep web length to a minimum
- Keep web angle close to 40°
- Actuation force of 80-150 grams for keys 10-15 mm high and 150-175 grams for keys 15-25mm high
Return force should also be set at 30-35 grams to ensure that keys do not stick.
Switch Life
The web design and the durometer of the rubber are the two factors that affect keypad longevity most. The design should reduce stress on the rubber if long life is desired. Using higher durometer silicone, increasing the actuation force, or increasing the stroke will all decrease the keypads life.
Rubber Hardness
Rubber hardness for a keypad can vary between 30 and 70 durometer (Shore A). Typically, most keypads are built between 40 and 60 durometer.
Minimum Key Height
For any design, calculate the minimum key height as follows; Keypad Base Thickness + Bezel Thickness + Stroke of Key + 0.5mm.
Contacts
The carbon pill is the most common contact because of its long life (>10 million actuations) and low resistance (Printed Circuit Board Design
The rubber contacts need to make contact with a circuit underneath. Rubber key mats themselves are very reliable in operation. However, when considering a PCB design, the environment in which the keypads are used must be considered to ensure that the complete switching unit is reliable. imum of 1.25 times.
Flexible Printed Circuit Design
Rubber keypads are typically used with printed circuit boards. However, many rubber keypads are also used with flexible printed circuits. Flexible circuits can be made of polyester or copper (Kapton).
