Carrying portable electronics to a new level

Molded plastics face abroad range of requirements in portable electronic applications.They are most often called upon as a housing material, stylishlyprotecting all of the chips, diodes, and LEDs from impact whilesporting a contoured surface easy for the human hand to hold andoperate.

But the materials may also provide electrostaticdissipation, EMI/RFI shielding, flame retardance, and thin-wallcapabilities. Under the hood, high-temperature thermoplasticsappear as connectors, sockets, and DIMM modules. More recently,3-D molded interconnect devices are replacing switches andcircuitry with a one-piece structural component that alsofunctions as part of the electronic device.

IMM caught upwith several of the movers and shakers in this field, via phoneand at a recent SPE Retec held in Philadelphia that focused onportable and wireless electronic applications. Most industryinsiders came up with the same general consensus-these products are growing rapidly, and the enabling plastics technologies arezooming right along with them.

High-Tech Packages
Thinnerenclosure walls are a fact of life in telecommunications today.Granted, that's not the only trend to watch.Manufacturers alsowant materials that can dissipate static charges to protect thesensitive electronics within. They need greater stiffness overlong, unsupported spans. Rather than sheet metal cages, theyprefer filled plastics that will shield devices from EMI/RFI ornoise. But getting thinner wall capabilities without losingmechanical properties is often the first step.

When suchmegasuppliers as GE Plastics and Bayer start dedicating majorresources to producing high-flow grades for thin walls, it's agood sign that the technology is here to stay. Electronics OEMPhilips took advantage of this technology when it introduced theVelo 1 (Figure 1), the smallest PC on the market, made from GE'shigh-flow Lexan ML6339R. Motorola turned to Bayer's MakrolonDP1-1456, a similar grade of PC, to mold the housing for itssmallest cellular phone ever, the StarTac (Figure 2).

GE's GregTremblay has done extensive research into both the design andmolding of notebook computer housings (see "Sequential valvegating and thin walls team up to take notebooks below amillimeter," September 1997 IMM). Turning hisattention to mold design, Tremblay explained some of thechallenges. "To fill parts with wall thickness below 1.5 mmrequires multiple direct gating through a hot runner usingvalve-gated drops. This, in turn, puts constraints on the molddesign." For example, molds with length to thickness ratiosof 160:1 and higher must be designed robustly to withstand thehigh injection pressures for packing and filling without flashingor mold plate deflection. Results of Tremblay's study alsosuggest using large diameter knockout pins, and as many aspossible. And if using a hot manifold, make sure to cool directlyacross from the gate to ensure surface appearance, he adds.

Inthe world of portable electronics, OEMs are looking forfunctionality and weight reduction, rather than saving materialcost, according to LNP Engineering's Don Cianelli. "If anautomaker reduces the wall thickness of a large fascia, forexample, it is often trying to cut down on material cost. Butfunctionality seems to drive portable electronic OEMs more. Thehousings are relatively small and thin-walled, so that lessexpensive materials don't make for huge savings," he says.Instead, customers want to be able to mold thin-wall parts of 1mm or less with lower injection pressures and good surfacefinish. LNP's EP line of PC and PC/ABS materials, with high-flowcharacteristics, were developed specifically for these customers,notes Cianelli.

Most notebook computers have a short lifecycle-12 to 18 months-so early supplier involvement is anotherbig key to success. "We often get involved in critiquingdesigns for moldability and in making suggestions for maximumproductivity," he says.

Making Connections
Althoughplastics are generally put to use as electrical insulators,certain grades can be plated with an electroless nickel or coppertechnique. Two-shot molded interconnect devices make for the bestof both worlds: the first shot, a platable resin, createselectrical pathways within the 3-D board; the second shot createsa nonconductive, 3-D part that can consolidate brackets and otherfeatures.

Market-leader Molded Interconnect Device LLC (formerlyMitsui-Pathtek), who pioneered this technology more than 10 yearsago, works directly with OEMs looking to reduce part count andcosts using MIDs. According to John Rowe, general manager, mostcustomers supply the general parameters-the area allotted, howthe MID should attach to the rest of the assembly, andrequirements for getting current from point A to point B."Our designers then go to work, routing traces,consolidating parts, and consulting closely with the customerbefore finalizing designs," Rowe says.

One of the company'srecent projects, a 3-D molded interconnect shown in Figure 3incorporates a switch and replaces stamped sheet metal contactsfor a heavy-duty flashlight called the Stinger. The customer,Streamlight, guarantees its products for life, so reliability wasthe driver toward seeking out a better solution, according tomarketing manager Christine Bruner. "We wanted to eliminatesmall contacts that could degrade over time, and also save timeon the assembly line," she notes. "Also, multiple partsin the former assembly gave us a greater possibility for yieldloss."

Rowe admits that two-shot molding has been around foreons. "The innovation involves marrying the process toelectrochemistry," he says. For the flashlight application,MID designed the first shot in a catalytic grade of Radel PES(from Amoco) for platability, then a second shot in unfilled PES.

"The catalytic grade allows us to use electroless plating oneach trace or circuit," says Rowe. "Every trace israised, so we're able to plate three surfaces to carry morecurrent." After molding, the part is first annealed at 10deg F below the Tg of the lowest temperature material, thenplated in a copper or nickel bath at 170F.

Electrical connectorsrepresent another ubiquitous use of plastics inside portableelectronics. Requirements for thinner walls and dimensionalstability also apply here as the distance between contact pins inthe connector decreases to 2 mm or less (Figure 4). Unlikeenclosures, however, temperature resistance for connectorspresents a serious challenge. Connectors must remain undistortedduring vapor phase, infrared reflow, or other surface mounttechniques (SMT) that can drive short-term temperature resistanceup to 465F, according to Bob Seymour, electrical/ electronicmarket segment manager at Ticona (formerly Hoechst TechnicalPolymers). They must also stand up to assembly stresses andforces required to connect or disconnect the small parts.

Potential materials for SMT circuit board connectors, accordingto Seymour, include LCP, PPS, and PPA. Resins must meet threerequirements: filling thin-wall mold cavities down to .008 inchwithout flash; withstanding the chemical, dimensional, andthermal stresses of SMT soldering; and matching the thermalcharacteristics of epoxy circuit boards during the solderingprocess.