Medical Device Link.

Originally Published MEM Spring 2002

KEYBOARDS

Key to an application's success are the right components and the right design—and, to ensure those things, a good supplier.

Paul E. Knupke

An environmentally sealed rigid metal dome keyboard with embedded LEDs.

Both suppliers of input devices and original-equipment manufacturers (OEMs) that specify input devices have called keyboards many things. Some use the term keypad, others say front panel or user-interface panel, whereas others refer to them as electropanels (a rare and unusual stretch). Industry suppliers favor terminology that reflects the technology used in their own products. For example, to the conductive-rubber-boot suppliers, a keyboard is a rubber boot or a keypad, whereas flexible-membrane keyboard suppliers refer to keyboards as membranes.

Whatever the terminology by which they are known, and whatever the technology that underlies their function, these interface mechanisms are all keyboards and share an essential characteristic. They are electromechanical devices, and like any other electromechanical device, keyboards can and do fail.

When keyboards fail, the cause can be attributed to many things. Some, such as general abuse, breakage, and vandalism, are obvious; however, other possible causes are not so obvious and are often overlooked. They include invasion of the switch cavity by airborne contaminants, and the formation of nonconductive metallic oxides on switch contact points. These less-conspicuous sources of keyboard failure are the focus of this article.

Airborne contaminants typically gain access to the switch mechanism in the form of nonconductive pieces of dirt, lint, dust, and hair. A lot of this contamination comes from simple, everyday materials, like writing paper, and consists of particles too small to see. Like the metallic oxides that can form quite naturally on non-precious-metal contacts, these nonconducting contaminants act as insulation between the switch contact points, causing intermittent operation, excessive switch bounce, or complete switch failure. If a switch cannot provide reliable electrical function, it makes no difference how sleek or inexpensive the equipment is, its keyboard is not going to work, and the performance capability of the equipment will be severely limited.

Varieties of keyboard materials and switch types are available that can minimize the chances of keyboard failure in a particular product application. An equipment manufacturer must be able to specify the right components and get them from the right type of keyboard supplier to be sure that a keyboard will function properly throughout the life of the equipment.

External Contamination

Computer or terminal keyboards do not generally fail because of dirt and dust falling between the keys. The real damage usually occurs inside the keyboard, in the layers beneath the keys that cannot ordinarily be seen. Contaminants lodge in the switch cavity where the contacts reside and the electrical switching takes place. If they come to rest between two switch contact points, the contacts may be prevented from closing.

Industrial control rigid metal dome keyboards intended for capital equipment applications.

If the switch-contact cavity is kept relatively clean and environmentally sealed, the introduction of outside contamination in the form of dust or lint should not be a worry. Some keyboard designs are inherently sealed from airborne contaminants and others are not. Use of a nonsealed keyboard in an environment with an abundance of even microscopic dirt and lint will mean questionable and unpredictable switch longevity and integrity. Sooner or later, that contamination will work its way into the switch cavity and prevent contacts from closing properly.

Switch failure due to external contamination usually appears in unsealed keyboards in the switches that are used most often. Pressing on a switch pushes air that is in the switch cavity out of the unsealed keyboard. Releasing the switch then creates a partial vacuum in the cavity that acts as a vehicle to pull external air with its burden of contaminants back into the space. The time necessary for failure to occur will vary with the amount of ambient potential contamination and the length of the path from the switch cavity to the point of ingress in the keyboard.

Any keyboard construction can be unsealed, but certain keyboard technologies cannot be sealed. Conductive-rubber-boot keyboards are unsealed by design and are the style most prone to external contamination failures.

A good lesson in how keyboard failure occurs is provided by the typical television remote control with its rubber keyboard, which typically experiences intermittent operation after 12 to 18 months of use. One failure symptom is the need for excessive force to close the switch and activate the function. In some cases, a switch may even be completely nonresponsive. Most remote-control users blame weak batteries for these problems but find that new batteries do no good. The problem lies elsewhere. If a failing remote is taken apart and the conductive-rubber keyboard boot is removed, dirt or sticky material can be observed on the printed-circuit-board (PCB) contacts, and on the back side of the boot. Such contamination buildup is virtually inevitable even in the relatively clean environment of a well-kept home. Imagine the difficulty of maintaining the performance of an unsealed keyboard in an industrial setting or other busy workplace.

Rigid keyboards with metal domes can be environmentally sealed, but most vendors purposely do not seal them in order to provide an attractively tactile switch. Usually, sealing a switch cavity traps a fixed volume of air within it. Pressing on the switch compresses the trapped air, which pushes back because its pressure has been increased. This resistance reduces the tactile feedback that suggests that a switch has been successfully engaged. The easy way to preserve tactile feel is to unseal the switch cavity; however, this leads to failure from external contamination somewhere down the road. The best solution is to control where the air squeezed from a switch cavity goes, or, more to the point, where it does not go.

Rigid metal-dome keyboards can be environmentally sealed and still retain crisp tactile snap. However, some keyboard vendors cannot satisfy both of these requirements, and some will not unless the OEM insists on them.

Internal Contamination

The second category of switch contamination is entirely internal and is a product of time rather than air currents. The growth of various nonconductive oxides on the surfaces of switch contacts has the same effect on switches as airborne contaminants, in that it prevents two switch poles from coming together and closing.

Such oxides are familiar from everyday life: the rust that forms on iron and steel, the aluminum oxide that keeps aluminum from rapidly corroding, and the copper oxide that distinguishes a penny that is a few years old from a shiny new one.

It is true that stainless steel does not appear to rust, but that is because chromium in the steel alloy forms a layer of chromium oxide on the metallic surface that seals the iron molecules away from the outside environment and retards the visible corrosion process. This thin, transparent oxide layer on stainless steel makes it nonconductive, however, and therefore a very poor switch contact. If a contact point on a PCB is bare copper, or is solder or tin over copper, its surface will oxidize over time and it will not be suitable for switching.

Although most metallic oxides are nonconductive, silver oxide is an exception. Use of silver-polymer conductors will not cause a switch problem. Gold-plating a PCB switch-contact surface is another good idea, but it does add some cost. Most keyboard vendors using rigid circuits buy raw fabricated PCBs from vendors who offer only the usual, and therefore limited, choices of switch-surface finishes. However, reasonably inexpensive PCB finishes that do not form nonconductive oxides can be obtained from some keyboard suppliers.

Switch Configurations

A rigid metal dome keyboard with an integrated electroluminescent lamp.

Understanding what switch configurations are available is the first step in selecting the right one for a particular application. In general, keyboards are either of the membrane or the rigid type. Membrane keyboards are flexible because they are printed and manufactured from alternating layers of thin polyester plastic sheet and adhesive, die-cut and laminated into fairly thin sandwiches. Rigid keyboards are made from a rigid PCB-based assembly.

The construction of membrane keyboards is still about the same as it originally was, but there have been many advances in production methods in the past 10 or 15 years. The materials used have gotten a little better and manufacturing techniques have improved so as to overcome some of the early limitations. A membrane switch is still a labor-intensive product, however. It is neither cheap enough nor reliable enough to suit every keyboard application.

When very high volumes of product are to be manufactured, it is sometimes better to go to a rigid PCB with a switch element other than the membrane type. Economies of scale available with other types of switches, but not membranes, cause the alternatives to be more economical to produce in high volume. In addition, membrane switches are not appropriate for very-high-reliability applications. Furthermore, if other components need to be assembled to the unit, such as soldered components, electronics, or displays, then membrane switches are definitely not suitable.

Both membrane and rigid keyboards will function properly when new. Any evaluation or life test performed on any keyboard must take into account oxidation and external contamination as potential mechanisms of failure. Without such testing, many keyboard types will falsely appear to be suitable for an application. A membrane keyboard generally does not form nonconductive oxide because it often uses silver-polymer conductors. This type of keyboard may or may not be sealed, depending on its construction and design. If it is sealed and has silver contact points, it will be free of both nonconductive-oxide growth and external contamination, and can serve as a long-term keyboard. However, an unsealed membrane keyboard allows dirt, dust, and lint from the environment to enter, which may lead to switch failure. The same applies to metal-dome or conductive-rubber keyboards: Both internal and external sources of contamination must be eliminated if an application calls for long-term reliability.

A keyboard evaluation or life-testing regimen should include tests that expose the keyboard assembly to simulated real-world environments. If the assembly is not sealed, then life testing should involve airborne dirt circulating in the test chamber.

Internal nonconductive-oxide growth can be simulated by means of a high-humidity and high-temperature soak, during which time the keyboard assembly switches are not cycled. A suitable test condition for growth of nonconductive oxide is 1000 hours at 80°C.

After the humidity and temperature soak, each switch should be tested for contact bounce and resistance on the first switch closure—before any nonconductive oxides can be scraped away by cycling of the switches. A simple life-cycle test may not adequately separate the reliable keyboard constructions from those that are unreliable. Conducting the humidity and temperature soak test, as well as performing life testing with airborne contamination, provides the OEM with a complete evaluation, and thus all the tools necessary to identify the best keyboard construction for the application and focus on the best potential component supplier.

Today, the great majority of rigid-keyboard suppliers employ stainless-steel domes as the tactile switch element, and most of them use nonplated stainless steel because they believe that stainless steel does not oxidize and therefore is a good switch element. However, the passage of time, along with the presence of certain temperature and humidity conditions, will cause chromium oxide to form. Consequently, the stainless steel must be plated to inhibit oxide growth.

Instead of unplated stainless-steel domes, a select few keyboard suppliers are using gold-plated domes. Gold has a low tendency to form oxides, thus freeing the keyboard from oxide-buildup problems, even if it is used in a high-temperature and high-humidity environment, or is specified for a long-term application. Manufacturers should consider building gold-plated domes into standard rigid-switch construction on any keyboard, whether specified or not. Certain economies of scale can be realized, and a longer-lasting, better-functioning keyboard can be produced than is possible with other materials.

Conclusion

A custom keyboard can be manufactured exactly to specification and yet not perform as expected. It may even fail. Why? Because the specifications provided were inappropriate for the application. Any OEM without its own keyboard design expert needs the engineering assistance of an experienced keyboard supplier who can act as an application adviser.

To evaluate promising suppliers for their potential to be such an adviser, the OEM must ask a multitude of empirical questions. Does the vendor have experience with only one type of product, or with a range of technologies to suit possible future as well as current needs? How many custom keyboards has the company produced, and for what kinds of applications? Has it supplied keyboards for capital-equipment applications—medical equipment, test instrumentation, or communications equipment—that have longer-term life requirements, or only for the throwaway markets? Are keyboards its principal business? What is the supplier's reputation for freedom from field failures, for delivery of customer service, and for things besides unit cost that affect the end installed price of an application, such as noncompliant components, late shipments, and the like?

Prospective suppliers who are skilled and conscientious should ask the OEM questions about expected product service life, the keyboard's likely theater of operation and use environment, the temperatures to which it will be exposed, and other performance criteria. The supplier whose answers and questions suggest experience, professionalism, and a genuine concern for the success of the OEM's application is a good candidate for handling a custom project.

The equipment manufacturer should try to find an experienced keyboard supplier who inspires confidence early in the product design stage. From such a vendor will likely come a cost-effective and elegant custom-engineered solution that will meet the specifications of both the OEM and the equipment end-user.

Paul Knupke is the executive vice president of sales and marketing for Suncoast Digital Technology Inc. (Largo, FL). He can be reached at pknupke@suncoastdigital.com.

Copyright © 2002 Medical Electronics Manufacturing