TechArchive

This article was originally written in the period 1995-2000

Silicone rubber keypads feel good and last well. Here’s an extract from an excellent design guide sent to us by Pat Duffy of ARC Turmkey Technology’s office in Stirling.

Conductive rubber switches and keypads were originally developed for the electronics industry to meet the increasing demand for inexpensive momentary switches. Conductive rubber keypads became desirable because they look and feel like traditional push-button style switches, but do not have their inherent disadvantages. Since their introduction almost 30 years ago, conductive rubber switches and keypads have gained enormous popularity and wide-range acceptance because of their reliability, long operational life and excellent tactile feel.

Silicone rubber is a synthetic rubber compound that possesses the most desirable properties of both organic and inorganic materials.

All of ARC’s conductive rubber keypads and switches are compression-moulded in precise carbon steel tools using highly elastic, non-toxic silicone rubber compounds.

After moulding, all keypads are subjected to a two-hour post-curing cycle at a temperature of 200degC. Post-curing is essential to the manufacturing process because it removes catalyst and oxidant residue retained by the keypad during the moulding process, stabilises the physical properties of the silicone and increases thermal stability.

Great care should be exercised when a keypad is designed using silicone rubber because it is a highly elastic material subject to manufacturing variations. Tolerances for silicone rubber, by nature, need to be considerably larger than those for less elastic materials because silicone tolerances are affected by variances in shrink rates, moulding conditions and material compounds.

The snap ratio (or click ratio) of any conductive rubber keypad directly affects the tactile feel realised by the operator. Keypads with snap ratios of 40-60% have excellent tactile feel and relatively long life, while keypads with snap ratios below 40% have relatively weak tactile feel, but longer life. Dual-durometer keypads also improve tactile feel.

It is very difficult to recommend specific guidelines for creating best tactile feel. However, if actuation force and stroke are identified for a given application, it is possible to design a keypad’s switch membranes precisely to realise the identified parameters. A very general guideline that can be followed for developing good tactile feel is to specify higher actuation forces for keypads with large keys than those with small keys. This rule also applies to key heights: tall keys require higher actuation forces than short keys.

Another typical guideline for actuation force is to specify a minimum actuation force of 80-100 grams for keys with heights of 10-15 mm and a minimum actuation force of 150-175 grams for keys with heights of 15-25 mm.

Care should be taken when designing tactile feel so a minimum return force of 30 grams is realised. This minimum return force will help greatly to eliminate the potential problem of sticking keys, as will proper bezel design.

There are several different types of contacts available for rubber switches, each with its own unique electrical characteristics.

• The carbon pill is the contact most frequently used in conductive rubber keypads and switches, primarily because of its long life and low contact resistance. The average life for carbon pills exceeds 5 million actuations, and contact resistance is typically less than 200 ohms. Carbon pills are usually circular shaped and available in one-half sizes ranging from 2.0-8.0 mm. The typical thickness of a carbon pill is 0.4-0.5 mm; this should be taken into consideration when the stroke of the switch is determined. ARC also offers several different sizes of oval-shaped pills which are also considered ‘standard’ because they do not require any type of special tooling charge.
• The second most commonly used contact type is silk screened or conductive ink. Screened contacts are available in any shape or size and allow great design flexibility because of the manner in which they are printed on switch-contact areas. Their biggest drawbacks, however, are shorter life and higher contact resistance compared to carbon pills. Screened contacts are typically only 10-20um thick, hence the shorter life and eventual climbing contact resistance. It is not uncommon for screened contacts to realise contact resistance of approximately 1,000 ohms over the life of the keypad. Careful attention must be given to the keypad’s electrical requirements when this contact type is selected.
• A third contact type, GRSP/OYPS, is unique because it combines the advantages of the carbon pill and silk-screened contacts. GRSP/OYPS contacts utilise a special low-resistance conductive ink that is not applied in the conventional silk-screen manner. As a result, the ink layer on the switch-contact area is thicker than silk-screened contacts (15-30um); therefore, it has a lower contact resistance (less than 200 ohms) over an excess of 5 million actuations. Contact ARC for more information about GRSP/OYPS contacts.

Switch reliability and life depend on the membrane style chosen and the durometer and quality of the material selected. Actuation force, snap ratio and stroke also influence life, as does a proper post-curing cycle. All other things being equal, switch life is reduced when higher durometer silicone is selected for the base material of the keypad, actuation force is increased or stroke is elongated.

A minimum membrane thickness of 0.40 mm is usually required to realise an average life cycle of 1 million actuations. Refer to the graph below for other typical switch characteristics as one or more of the previously cited parameters are changed.

Conductive rubber keypads are very reliable, but the environment in which they are used should be considered very carefully when the printed circuit board is designed. In order for any keypad to provide trouble-free operation, it is imperative that all components be designed properly, particularly the printed circuit board.

Printed circuit boards can be supplied with several different types of plating; the only type that is specifically not suitable for use with conductive rubber switches is tin-lead solder boards. Gold plating over nickel on the printed circuit board offers the lowest possible contact resistance (less than 100 ohms) for any keypad application, and a minimum layer of 30-50uin of gold over 100-200uin of nickel is recommended for best switch performance. The width of gold traces typically ranges from 0.25-0.40 mm, while the minimum distance between them is typically 0.30mm and the maximum is usually 0.40mm.

Nickel plating, like gold, is extremely reliable and relatively inexpensive, when compared to the cost of gold-plated boards. Contact resistance for nickel-plated boards is typically less than 100 ohms, and nickel has an excellent track record in even the most severe environmental conditions. If nickel plating is used without gold, a minimum plating thickness of 200uin is recommended for best overall performance. Most keypad applications utilise nickel-plated boards because of their high reliability and low cost.

Silk-screened carbon boards can also be used with conductive rubber switches, but should only be selected when contact resistance between 500-1,000 ohms can be tolerated. If screened carbon boards are used, the minimum distance between the traces should be 0.50 mm, and the overall size of the electrode should be greater than 5.0 mm.

It should be noted that there is not a single recommended pad pattern for use with rubber keypads. Printed circuit board electrode design should be developed carefully taking all switch characteristics into consideration. The most important single objective to be considered in designing any pad pattern is to provide as many shorting paths as possible so best switch operation can be realised when the button is actuated. Several common contact patterns are shown below for reference purposes only.

Pads on printed circuit boards should never be smaller than the conductive pill or contact area on the bottom of the rubber switch. It is strongly recommended that the electrode (pad) on the printed circuit board be 1.25 times the diameter of the conductive pill, or at least 1.0 mm larger than the overall size of the contact on the bottom of the switch surface.

Almost all keypad graphics, either positive or negative-image, are surface printed using a special silicone ink that is actually bonded to the keypad during the manufacturing process. Graphics are permanently applied to the top surfaces of rubber switches by curing all keypads in high-temperature ovens after printing. Special attention must be given to keytop design if negative-image graphics are desired because printing is difficult on concave and convex keytops. Each graphic colour requires its own individual screen, and represents an additional step in the manufacturing process. Pantone numbers are normally used for specifying graphic colours, but colour chips can be matched if Pantone numbers are not suitable for a given application.

If the keypad graphics are going to be exposed to conditions more sever than normal abrasion (i.e. operator wearing gloves) of subjected to an excessive number of actuations, they can be coated with translucent ink (matte or shiny finish) to enhance legend life. Overcoating typically doubles the life of silk-screened graphics, and two overcoats of translucent ink can be applied if necessary. However, overcoating does increase the price of the keypad because additional steps re required in the manufacturing process.

The following checklist should be reviewed when connectors are going to be used.

• The cost of a connector is directly linked to the amount of conductive material it contains.
• Elastomeric connectors usually require bezels of some type to promote easy assembly and eliminate alignment problems.
• Bezels, or holders, are also used with elastomeric connectors to prevent over compression of the connector and buckling problems.
• The more ‘square’ (H x W : 1 to 1) a connector is, the more likely it will not require a bezel, or holder, when assembled.
• Connectors can be colour-coded for easy identification when side insulation is not identical on both sides of the connector.
• Elastomeric connectors utilising carbon conductors are much cheaper than those using silver. In addition, carbon connectors remain oxidation-free over the life of the connector.
• The conductive areas of elastomeric connectors should never be handled by human hands because the oils and contaminants inherent in the human body can cause contact problems after the product is assembled.
• High-quality connectors can only be supplied if the manufacturing process is completely sanitary and the raw materials utilised are of exceptional quality.
• ARC connectors go through two curings – the second being six hours at 200degC. The second curing is critical because it eliminates siloxanes which cause contact problems as connectors are left out in the field.
• The most popular connector currently being used in the United States is the (GL) style.
• (GL) style connectors, with a pitch of 0.25, are suitable for most LCD applications.
• If the dimensions of a connector are not known, the pitch for the same can be determined by obtaining the dimensions for the pads on the printed circuit board. The height of the connector can be determined by finding the distance between the LCD and the printed circuit board, then adjusting for connector deflection (approximately 15%).

• ARC Turnkey Technology
• Tel: 01786 446300
• Fax: 01786 446624