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Selecting a handheld enclosure

Whether designing a cell phone, PDA, or handheld DMM, engineers at some point need to specify a housing for their product. Choosing the right enclosure for a design is dependent upon a variety of features that include size, shielding, materials and construction, ergonomics, and customability.

Plastic Box , Plastic Box , Plastic Waterproof Enclosure , Hand-held Enclosure

 

Knowing what to look for can help design engineers get a grip on the right enclosure for portable/handheld design.

However, the first step is finding the right enclosure manufacturer. "They need to select a partner," said Randall Dennis, president of San Luis Obispo, CA-based injection-molding firm Alltec. "They need to select a molder that isn't just a hang and bang shop."

The manufacturer should also be asking some qualifying questions. "Are they, the molder, asking in-depth questions about the usage so that they can identify the environment and the application?" Dennis asked. "If the engineer is not seeing that demonstrated, get out early."

How small is your portable device?
When it comes to enclosures for portable and handheld devices, there are two basic sizes: pocket and handheld. Pocket boxes typically house a single board and components for devices such as remote controls in security and safety applications, and measure no more than about 2 to 3 in.

Typically, pocket enclosures offer optional body clips�molded either in the case material or from spring steel�that enable the device to be more portable by freeing up the user's hands. On these smaller enclosures, manufacturers can also configure keypads, key rings, and buttons for applications such as key fobs or remote heart-rate monitors.

Ranging from about 6 to 11 in., handheld enclosures accommodate larger components, multiple boards, and a host of accessories. Unlike the pocket-sized parts, larger handhelds can more readily accommodate cable glands, display areas, and separate battery compartments. Typical built-in compartments accommodate 9-V, AAA, and AA cells.

"Almost all handheld boxes will come with a recess in the top for membrane keypads or product labels," said Robert Cox, director of sales and marketing for OKW Enclosures (Bridgeville, PA). These types of accessories allow the end product to retain a brand or identity, which may be a selling point for the design company trying to market its device.

Construction and design
In both pocket and handheld sizes, the de facto standard is plastic, more specifically ABS (acrylonitrile-butadiene-styrene). Used in a multitude of consumer products from children's toys to automobile accessories, ABS is considered a middle-ground commodity as opposed to more rarely used, specialized materials that can drive costs up.

"That's just the norm now," Cox said. ABS has "better stability from a UV point of view" than older polystyrenes or polycarbonates.

The need for lightweight as well as durable attributes has driven out the need for metal enclosures in all but the most specialized handheld and portable applications. "We find very little [metal use] even in the heavy-duty industrial applications," Dennis said. "Composites are as durable as metal now."

While Cox agrees, he does point out that in RF-sensitive applications, metal boxes accommodate "higher attenuation" than shielded plastic enclosures. Unlike plastic, however, metal enclosures cannot be sealed as well to meet IP 65 or NEMA 4X specifications.

Many manufacturers offer enclosures that are partially or completely made with infrared materials to accommodate IR control systems.

Protection
Whether it's to meet standards such as IP 65 and NEMA 4, or thwart static dissipation and EMI/RFI, enclosure manufacturers offer a variety of shielding and sealing techniques. As mentioned previously, metal enclosures offer the highest attenuation and are highly suitable for extremely sensitive RF conditions.

However, if metal is out the question because of aesthetic or weight concerns, plastic enclosures can be protected in several ways. Vacuum deposition is a process in which the enclosure is vaporized with aluminum. "That's really the best solution for a plastic box for shielding against EMI/RFI," Cox said. An alternative to more expensive impregnatation of resins with noise-canceling polymers is a spray lab, where generic plastic enclosures are sprayed with anti-EMI/RFI materials.

Enclosures that are typically going to be used outdoors should be sealed to protect components from water, contaminants, and corrosion. Some companies accessorize their enclosure products with PVC cradles that protect the device from impacts.

Customization vs. off-the-shelf
The clear line between customized and off-the-shelf products has become somewhat blurred in recent years, with manufacturers attempting to provide off-the-shelf solutions that are highly customized. "The designs of the standard are getting sexier and sexier, so they're looking more and more custom," said Dennis.

If customization and support needs are minimal, a designer can buy the enclosure from a distributor. However, Cox said working with the manufacturer also has its advantages. "The other way is to come direct to us and say, 'Look, I've got all these parts and I need to get them in a box and I want you to ship the box complete to me so all I have to do in my factory is plug it all in.'"

Dennis recommends that the designer choose the enclosure first and retain flexibility with board sizes. "If the designer will look for the box first, it's less expensive to change the size of your circuit board."

Application in mind
Just as is done during the selection of an audio amplifier or temperature sensor, when selecting the enclosure, the engineer should always have the application in mind�and should be able to communicate that to the manufacturer. This isn't always the case, Dennis said.

For instance, he contends that many manufacturers continue to produce enclosures that may be too wide for use in areas like the medical market, which comprises a large population of women workers. "They need to take into consideration the end user," he said. "They don't realize, especially in medical, how many of the nurses are actually running these machines."

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