New engineering resins meet the needs of high-density electronic connectors where thin walls, flat bodies, and tight tolerances are the norm.
Ticona Technical Polymers
Hoechst Celanese Corp.
As electronics engineers pack more functions into smaller devices, they face several major hurdles. For one, they must deal with microprocessors and similar devices that increasingly use more signal pins, pushing more contacts into their board-mounted connectors that are more closely spaced. This demands connector bodies with thinner walls and more aggressive tolerances. To make matters worse, circuit-board manufacturers are turning to surfacemount technology (SMT) to handle the higher density components. But the soldering temperatures forced on SMT devices reach 465° F and are not compatible with long, thin-walled connectors. The heat typically warps and alters critical dimensions. And last but not least, connector makers are under constant pressure to cut costs.
Although engineering resins compatible with SMT and miniature devices cost more per pound than less stable materials, the final cost per part is less. Competition in the connector market demands short market cycles. But often, molds need retooling to correct for unpredictable shrinkage. This, unavoidably, pushes out the cycle. The solution is to use plastics with proven, repeatable shrinkage rates.
Liquid-crystal polymers, linear and branched polymers, and nylons are all viable materials to solve a wide variety of circuit-board connector problems. These so-called “high-performance” connectors are distinguished by their closely spaced contacts, thin walls, tight tolerances, and the high-temperature dimensional stability needed to survive SMT soldering.
Thin walls, tight tolerances
The main purpose of molded-plastic connector housings is to retain and position metal contacts accurately and electrically insulate them from one another. Electronic connectors require resins with UL flammability ratings, Relative Thermal Indexes, and bulk electrical properties which save the connector manufacturer the cost of testing and qualifying each new individual connector housing design.
Connectors also need resins with mechanical toughness to withstand interference at pin insertion, robotic pickand- place handling during assembly, and repeated connecting and disconnecting during use. Some connectors need springlike resilience and creep resistance to grip boards or firmly mate with other connectors to maintain positive contact. Specifications for these small connectors include walls to 4 mils thick with tolerances less than ±3 mils, and bow or warp limits of less than 3 mils/in.
The choice of connector resin depends on the combined requirements of the application. For example, a manufacturer of a miniaturized two-piece power connector chose two different resins for mating parts. For the wire-end receptacle, requirements for modest wall thickness and assembly temperatures, plus long-life, spring-action latches made PBT (polybutylene terephthalate) the resin of choice. Engineers selected Celanex 4016 PBT for the latching receptacle. With contacts on 3.0-mm centers, the receptacle has 18-mil-thick walls. The molded-in latch maintains spring action without taking a set and survives more than 1,000 open-close cycles without cracking. The mating circuit-board header, which must undergo SMT soldering, is molded in Vectra L130 liquid-crystal polymer.
Smarter, faster computers and numerous other electronic devices require contacts more closely spaced than ever. The connectors commonly have contacts on 1 to 1.2-mm centers. Experts expect the trend will continue to move toward a contact spacing of 0.5 mm. In fact, engineers in Japan are already considering 0.25-mm-pitch connectors for advanced projects.
As contact pitch shrinks, connector walls get thinner and tolerances tighter. One 7-in. dual in-line memory module (DIMM) socket molded in Vectra L130 LCP (liquid-crystal polymer) has 278 contacts on 1-mm centers. The walls between contacts are only 0.012-in. thick. The longest connectors now in production with LCPs are 9-in. long, but 12 in. are under development. Wall thickness in such connectors are as thin as 10 to 12 mils.
Long connectors also demand resins can fill thin-walled mold cavities without producing flash which often breaks off and interferes with assembly or electrical continuity. The PBT polyester commonly used in less dense connectors has acceptable mechanical properties, but is typically limited to walls no thinner than 0.5 to 8 mm. But PPS (polyphenylene sulfide), PCT (polycyclohexylenedimethylene terephthalate), PPA (polyphthalamide), and other nylons have viscosities low enough to fill 0.25 to 0.30-mm walls. Moreover, high-end liquid-crystal polymers can fill walls as thin as 0.1 mm.
Today’s connector specifications typically call for dimensions accurate within about 0.08 mm. Future standards are expected to tighten tolerances to 0.0025 mm. Even more difficult, these connectors must not only emerge accurate right out of the mold, but they must maintain dimensional accuracy during assembly and service. The dimensional accuracy of as-molded, fine-pitch connectors is partly determined by allowances in resin shrinkage. Glass-filled nylon 6/6 shrinks 0.005 mm/mm in the direction of flow. PPS characteristically shrinks 0.002 to 0.003 mm/mm, while LCP has a linear shrink of only 0.0005 mm/mm, an order of magnitude less than that of other connector resins.
Stable part dimensions in service are just as critical as the initial tolerances of as-molded parts. Swelling, shrinking, and distortion in the connector housing due to temperature and moisture absorption can lead to weak or imperfect solder joints. Fortunately, the heat encountered within most electronic equipment poses no threat to the dimensional stability of high-performance connectors. However, high reflow soldering temperatures and high service temperatures can cause cardedge connectors made of some resins to creep under constant load which lets cards move. The creep resistance of linear PPS compared to other connector resins makes it suitable for sustained operating temperatures greater than 200°F. High-density, high pin-count interconnects up to 500-mm long are molded in linear PPS and maintain a springlike grip on mating daughterboards even at elevated temperatures.
Long-term moisture absorption can swell and soften some plastics commonly used in interconnects. For example, a nylon 6 or 6/6 formulated for thin-walled molding absorbs 3% moisture with 24-hr immersion. Only 2% moisture absorption can cause dimensional changes. High-temperature nylons like PPA and 4/6 are less sensitive to moisture absorption, but even little moisture makes nylon less rigid and allows card edge connectors to creep under load. PPS absorbs only 0.02 to 0.03% moisture, and LCP about 0.02 to 0.04%, minimizing dimensional variations in environments with changing humidity. For the connector designer, dimensional stability means more reliable electrical contact in service as well as during board assembly.
Compared to traditional throughhole soldering, SMT allows for increased density of components on circuit boards. However, automated vapor-phase soldering at 419°F and infrared reflow soldering at 445 to 465°F can warp long connector bodies. Excessive warp and bow can break solder joints and deform circuit boards. SMT connector specifications commonly require coplanarity of the connector body and contacts within ±2-mil bow or warp over the length.
Connector designers match the thermal expansion of connectors with that of circuit boards during design. Careful material selection, part design, and orientation of parts via board layout lets disparate materials move together and protect solder joints from excessive stress. The heat deflection temperatures of high-temperature nylon, polyester, and PPS fall within 10° of one another. However, each resin behaves differently during soldering. Coefficients of thermal expansion (CTE) can also vary greatly in flow and transverse directions within the same part made from a glass-filled resin. Ideally, board designers want to minimize thermal expansion or match it to that of the circuit board. The CTE of the typical FR4 epoxy board is 12 to 13 ppm/°C. Connector resins such as PPS, have a CTE of about 18 to 20 ppm/°C, and liquid-crystal polymers have about 6 ppm/°C. Both can maintain tight molding tolerances through SMT processing while reducing part cost.
High-density connector designers pay a higher price per pound for resins with high-dimensional stability to withstand soldering temperatures for only a few minutes during assembly. Consequently, SMT-compatible resins generally cost more than 2 to $3/lb. PCT and high-temperature nylon are typically priced from 2.50 to $3.50/lb. A wide variety of Fortron linear PPS grades are available in a similar price range. Liquid-crystal polymers, with high mold and exceptional dimensional stability have been the most expensive connector resins, priced up to $9.00/ lb. However, new polymerization technology has reduced the cost of certain LCPs by about 25%, compared to earlier, widely used LCP electronic resins. Price per pound is nevertheless a misleading indicator for cost-conscious connector makers. In a high-density connector, the true cost of a part is in the total cost of the manufactured connector, not just the cost per pound of the raw materials.
With most connectors containing only a few grams of resin, material price is only part of the total cost. The cost/part equation includes equipment, processing costs, molding cycle time, startup/ shutdown scrap, usable yield, secondary operations, after molding, and maintenance on tools and machinery. A resin which is less costly per pound can lead to higher costs elsewhere in production. For example, high-temperature nylons are relatively expensive resins, but their high melt viscosity limits their use in very thin-walled parts. The high capital cost of precision connector molds makes cycle time a critical factor in the finished part cost. High-priced, high-flow resins including branched and linear PPS, LCP, and high-temperature nylons pay off in more parts per hour. The higher thermal stability of linear PPS and LCP, however, means they are less prone to degrading in the molding machine and less likely to produce brittle rejects.
Secondary operations are another significant contributor to finished-part cost. One major connector manufacturer, for example, introduced a DIMM socket with contacts on 1.25-mm centers, and walls 0.3-mm thick. Designers of the high-density connectors originally chose a PCT polyester priced less than $3/lb, and didn’t expect a need in the design stage for a secondary deflashing operation. Unfortunately, flash appeared on actual parts which slowed assembly and caused customer rejects. Furthermore, the brittle PCT fractured on too many parts.
The manufacturer switched from the relatively inexpensive polyester, PCT, to a more readily moldable LCP priced at $7/lb. LCP molds essentially flash-free. It made a tougher connector, and the high mold flow and fast solidification rate cut molding cycle time by half. Productivity improvements, thanks to fewer rejects and faster processing, let the LCP connector match the total connector costs incurred with cheaper PCT material. The payoffs were enough to justify an investment in new tooling with a resin about three times more expensive than PCT.
Depending on the size of the part, deflashing adds from 15 to 50% to the cost of molded connector resins. The ratio of deflashing cost to total part cost is higher with a small, intricate part which contains little resin but requires significant secondary cleanup. For long connectors using more material, deflashing is a smaller portion of the price per part. A new linear PPS, Fortron 1140E7, is formulated to reduce cycle time and minimize flash. It reduces molding cycle times 20 to 30% more compared with other PPS resins, and it can eliminate deflashing operations altogether. The combined effect can be a 15 to 60% reduction in part cost, depending on size, compared to earlier PPS formulations.
Easy ways around corners
One busbar system from Erico, Solon, Ohio, is different from most solid busbars. It is called Flexibar, and contains several thin, solid-copper laminations that maintain uniform thickness when bent, eliminating hot spots. The laminations slide within the insulation to fit the inner and outer bend radii of a corner. In addition, this lets installers use smaller bend radii while handling more current for the same radius as other types, as much as 50 to 2,500 A more.