Mobile phone design calls for plastics change

In an ideal world, switching from one engineering thermoplastic to another wouldn't take much more effort than sifting through a few data sheets and picking a material with the right mix of properties. Engineers at Ericsson Inc., however, design mobile phones in the real world where material conversions don't come so easy.

For the company's R300Z phone, the company's mechanical designers scrapped a box-like enclosure in favor of a new design that adds a bit of aesthetic flexibility. Unlike past phones, in which front and rear housings come together to form a rigid enclosure, Ericsson's revamped phone consists of an internal structural core surrounded by a purely cosmetic shell. "With this core concept, we can easily achieve a variety of looks by simply exchanging the cosmetic shells," says Shawn Stephenson, the senior mechanical engineer responsible for the phone.

The core concept, however, upped the ante on mechanical requirements, forcing Ericsson engineers to rethink their choice of thermoplastics. During the prototyping work, they switched two of the phone's structural components from PC/ABS to a lesser-known engineering thermoplastic-a 60% glass-filled IXEF polyarylamide from Solvay Advanced Polymers Inc. (Houston, TX). "After all of the validation work on our prototypes, we found that PC/ABS just didn't give us the mechanical properties we needed," says Stephenson. "IXEF helped us get over the bar."

Mechanical design needs. More than any other property, the need for a high modulus drove the switch to IXEF. Stephenson explains that the phone derives much of its stiffness from a core component called the shield box. Assembled with the PCB and a thermoplastic carrier into a laminate structure, the shield box forms the spine of the phone. "We wanted a plastic resin that has properties approaching that of lightweight metal alloys," he says. IXEF brings them closer to that goal than PC/ABS. The 60% glass-filled material, for instance, has a tensile modulus of 24 GPa, versus 2.5 GPa for an unfilled PC/ABS that Ericsson first evaluated, according to Stephenson.

To add even more stiffness to the phone, the design team later switched a second component to IXEF. Stephenson explains that early prototypes of the completed transceiver revealed the need for increasing the cosmetic shell's stiffness. "We did that by changing the material of the battery door from PC/ABS to IXEF," he says.

Stiffness aside, the material had to meet other requirements, including enough impact strength to make sure the phone holds up to drop tests. And it had to enhance dimensional stability-by fostering flatness and resisting creep, moisture pick-up, and thermal changes-because the shield box also integrates location-sensitive mounting features like fits, screw bosses, and heatstaking sites. "IXEF was superior in all these metrics," Stephenson says.

Go with the flow. Another IXEF property, its low viscosity and associated high flow rates, proved to be a mixed blessing from a design standpoint. "Because IXEF flows so well, we found it to be more flexible in some design rules and less in others," Stephenson says, comparing the material to PC/ABS.

On the one hand, the high flow allowed the parts to meet mechanical goals in spite of wall sections averaging less than 0.5 mm. "Flow is good for thinwall parts," Stephenson says. And the material's high flow also contributed to a good appearance for the battery door, which, unlike the shield box, remains visible to the user. Because the flow promotes a 1 to 2 micron glass-free skin on the surface of the part, Stephenson discovered that the polyarylamide did not have any glass fibers intruding onto the surface. "Even with a 60% glass-load, IXEF shows no glass blooming," he says. IXEF ability to flow, as well as a mold shrinkage that averages just 0.2%, contributed to a reduction in sink marks too, Stephenson reports. "We thought we'd see sink with the cross-sections where the ribs meet perpendicular to walls," Stephenson recalls. "But we didn't." So closely did the glass-filled IXEF parts match PC/ABS prototypes that Stephenson can't tell them apart by eye. "You couldn't tell the difference between them just by looking," he says. "The only way to identify them is by twisting them."

On the downside, Stephenson found that the combination of high flow rates and high glass loadings had a tendency to worsen internal stresses to the point where some early prototype parts experienced some drop-test failures. "To eliminate any stress concentrators, we had to increase blend radii and some wall sections," he says, citing the wall-rib junctions, corners, and screw bosses as three features that needed extra design attention with IXEF. "We had to break all the inside corners," he says.

Molding matters. Ericsson's successful switch to a new material didn't stop on the drafting table. Instead, this job offers a good lesson in why it pays to bring materials suppliers and injection molders into the design process. Stephenson points out that the performance of the shield box in particular depends on how well it's molded. "Critical molding parameters must be maintained to develop a high crystallinity in the part and take full advantage of IXEF's mechanical properties," he explains. Or consider the shield box's tight flatness tolerance, less than 0.3 mm out of plane over the entire part. Meeting this spec required extensive Moldflow analyses in order to determine the best gate locations and molding conditions. Solvay and the injection molder, Hoffer Plastics, performed much of this CAE work.

The two suppliers also helped Ericsson tool up for the phone as it went from prototype to production. "IXEF did produce good parts in a prototype tool developed specifically for another material," Stephenson recalls. But the production tooling takes IXEF's low viscosity into account. "The IXEF material shows tendency to flash at parting lines, lifters, and any area in the mold in which there is moving steel and a shut off," Stephenson says. "So we needed precision tools."

Shielding it. Since the shield box provides much of the phone's RF shielding, Ericsson engineers lastly had to find a reliable way to apply conductive coating to the part's interior. Stephenson ruled out two traditional coating methods-conductive paint and vacuum metalization. "They weren't hard enough to resist any abrasion from a stamped-metal gasket that attaches to the shield box's inner surface," he explains.

Even a standard electroless finish of nickel over a copper duplex coating wouldn't have cut it in this application, according to Ken Martin, vice-president of Cybershield Inc., the Dallas-based coatings applicator that worked with Ericsson on this project. "We knew from previous experience that the coating would need to be very hard," Martin says. So hard, in fact, that Cybershield developed a custom electroless coating with base-coat layer that serves as a foundation for nickel and copper layers. According to Martin, the reformulated basecoat comes in 65-75C Rockwell Hardness after annealing, compared to just 20-30C for standard basecoats.

Cybershield did benefit from some of polyarylamide's physical properties. Martin points out that IXEF resins have very low and slow moisture absorption. "Materials with high pickup and fast rates, like nylon 66, are generally unsuitable for plating since plating requires exposure to aqueous solutions," he says. What's more, plated parts are usually cured at 140F for 60 minutes to enhance long-term adhesion. "The expansion and contraction associated with picking up and releasing moisture undermines the attachment mechanism between the coating and some nylon substrates," he says.

The new plating process, along with the other development work done for the year-old R300Z, may or may not pave the way for other IXEF components. Stephenson, who declines to talk about upcoming designs, won't say. But he does note that all the work with IXEF has made him more receptive to trying new materials. "One thing we've learned is that it helps to keep an open mind," he says.