Sponsored By

December 1, 2001

6 Min Read
Beyond mobile phone design



Figure 1. Telecom enclosure designs from Lucent Technologies include the Pico (top), a 1-sq-ft unit molded from EXL9330 PC/siloxane (GE Plastics); the Hydra (bottom left), a self-contained radio unit for aerial applications made from PBT (Valox from GE Plastics); and the TTAA (bottom right), a 6-ft-tall PVC module.

The ubiquitous mobile phone is certainly one product in the telecommunications market that has taken a hit recently. Demand continues to plunge, production has shifted to Asia, and several major producers have announced layoffs as a result (see "Market Snapshot: Electrical/Electronics," August 2001 IMM, pp. 38-42). 

However, other products in this industry are actually predicted to grow in the short term, according to a recent e-seminar on telecom design issues conducted by GE Plastics. Seminar presenters explained that Internet and computing infrastructure products such as base stations are on the rise. These often require enclosures (Figure 1), many of which are made with injection molded plastics. 

Concurring with the GE statistics is Jack Dispenza, technical manager for Bell Labs/Lucent Technologies' Design and Engineering Center of Excellence. "Smaller plastic modules will replace metal cabinets and frames," he says, "because plastics help us to reduce volume, weight, and cost." For example, a 30- to 40-lb steel cabinet can be replaced by a 6- to 10-lb plastic version with a cost reduction of $150 to $350. "We are averaging a 50 percent volume and weight reduction along with a 60 percent cost reduction for a select group of radio products," he adds. 

An explosion in Internet-based electronics is driving the predicted growth in plastic enclosures for telecom. Broadband refers to internet access via DSL, cable, and satellite modems. These options offer greater bandwidth, and therefore faster speeds, to users. Internet home devices, many of which are still in development, also will probably take advantage of this evolving technology. 

Pushing Material Limits 
Dispenza stresses that telecom enclosures present their own set of design challenges. "New requirements are pushing materials and design processes to the limit," he says. "Few materials can handle structural targets for large parts as well as temperatures of ­40C. We are evaluating some candidate materials that appear to work, including PVCs from PolyOne, ASA from BASF, and a PC/siloxane blend from GE Plastics." 

Other requirements for materials that will enclose expensive equipment outdoors include agency compliance specs such as high and low temperature impact, low solar gain, chemical resistance, recyclability, EMI/RFI shielding, and resistance to shotgun loads. For the latter requirement Lucent developed a ballistic cloth preform for overmolding. It is a recyclable system using 3-D fiberglass and a base thermoplastic resin (not a thermoset), which enables the use of injection molded products in high-risk environments. 

Although plastics are lighter and lower-cost than metals, designers still have to address the issue of dissipating heat in these enclosures because of the sensitive electronics within (see sidebar). At Lucent, heat-dissipating devices such as manifolds and heat spreaders are produced using MIM (metal injection molding) or TXM (thixotropic semisolid molding). 

"We are also using thermally conductive resins from PolyOne to replace metal board-mounted components with plastic ones," Dispenza explains. "We replaced a $75 machined metal filter with a $17 IM assembly in this way." In the past, plastics could not be used in this application because the coefficient of thermal expansion was not low enough. However, Lucent found the CTE to be so low in these new materials that it was able to produce functional first prototypes that exhibited only minor tolerance problems. 

Another material innovation created by Lucent and PolyOne is an electrically conductive TPE. "Our telecom enclosures traditionally use miles of thermoset gaskets with metal fillers or metal screen covers," Dispenza says. "We wanted to replace the thermoset with a thermoplastic for recyclability." While electrically conductive TPEs existed, none could meet stringent electrical conductivity requirements and a UL94 V-0 fire rating. So Lucent and partner developed their own version called CTPEs (conductive thermoplastic elastomers). The material is now patented and expected to be compounded and distributed through PolyOne. 

Taking the Heat 

Managing and dissipating heat in telecom devices is critical. Failure rates for electronic equipment increase exponentially as temperature goes up. Containing this potential means getting a handle on several portions of the design: electronics, enclosure, materials, and ambient conditions. 

During a portion of GE's telecom e-seminar, presenters explained the three modes of heat transfer—conduction, convection, and radiation. There are also interactions between each of these modes. For instance, an increase in conduction (or heat spreading) can affect both radiation and convection heat transfer. 

To aid enclosure designers, most products are specified by the OEM or by datasheet with a maximum case temperature, which is the highest temperature the electronics within can endure. Most designs require a heat sink or spreader, typically manufactured from aluminum, to dissipate heat efficiently. Ambient conditions are also specified by the OEM. 

Detailed studies of the thermal management of enclosures show that plastic enclosures combined with metal heat sinks can offer a better cost performance than metal alone. One example is a comparison GE conducted between a full metal cover and a full plastic cover, with a single heat source mounted in both boxes. At first, the metal box would appear to be better suited to minimize heat buildup because it is inherently conductive. However, the study showed that the location and mounting of the heat source was of much greater importance.

Cost Out, Performance In 
Designing products to meet cost targets is an overall goal in this market niche. Traditionally, enclosures have been made from sheet metal, which requires die work, grinding, and rework. Plastics, on the other hand, generally have lower production costs, more design freedom, fewer secondary operations, no corrosion, and higher dent resistance. In fact, as part complexity goes up, the cost of a metal enclosure skyrockets compared to one made of plastic. "Plastic also eliminates punching, welding, plating, and painting operations," explains Dispenza, which also can lead to dramatic reductions in overall cost. 

The total cost of ownership will also decline, thanks to plastic enclosures. "Besides a reduced materials and processing cost, plastics reduce assembly labor, installation time, service calls, energy consumption, and recycling difficulties," he says. In addition, customers can now mount the lighter-weight enclosures in less costly places. For example, a cellular radio unit used to require space on a tower or a downtown urban location, both of which cost about $14,000/year to lease. Modular plastic units can be installed on a building, roof, utility pole, or other less expensive option. 

Today, thermal management may be at the top of the list of what's important, but overall system cost is a close second. Other issues include low-temperature impact, weather-ability, shielding, and chemical resistance. 

Contact information
Lucent Technologies, Bell Labs
Whippany, NJ
Jack Dispenza
(973) 386-3232

GE Plastics
Pittsfield, MA
(800) 845-0600

PolyOne Corp.
Cleveland, OH
(216) 589-4000

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like