Design Feature: February 17, 1994

Packaging problems arise simply because electronic devices are simultaneously getting smaller, faster, and more complex. Even inductor makers, such as Pico Electronics and Coiltronics, now offer surface-mount coils and transformers. These developments concentrate heat sources and ordain more connections in a smaller area.

As digital-clock speeds inexorably push beyond 50 MHz, transmission-line effects dominate the signal integrity of such systems. Higher clock speeds tax the signal integrity of pc boards, cables, and connectors.

Consumer demand plays a part, too. Users expect your new products to be more flexible and powerful than your old products were. Yet they also want your new products to fit into smaller, lighter packages and run twice as long as before on battery power.

Motorola's new Series 900 VMEbus computers provide several examples of how to reduce costs while increasing reliability. The personnel Motorola employed in the computers' development also provides a lesson for management.

Motorola's computer-system designers ran into an all-too-common marketing problem: Motorola's customers could not forecast precisely which computers they would need. So the new system needed to be as flexible as possible. "No problem," you say. "Electronic engineers have always designed modular products." But Motorola decided that, along with being modular, the new computers would use as few parts as possible. After all, every part provides a chance for error.

The resulting design (Fig 1) required innovative electronic engineering. Where the old line had 600 parts, the new computers have only 190 parts. For example, this VMEbus system has no backplane; instead the cards simply stack using conventional connectors. Unfortunately, Motorola is not disclosing how it made the stacking cards meet the VMEbus spec. (Connector manufacturers, such as Augat, happily provide you with Spice models for their connectors so that you can attempt to duplicate Motorola's feat.)

The computer also incorporates clever inputs from novel members of the design team: experts in molding plastic. If you remove the nonstructural plastic case of a conventional PC, you see several sturdy sheet-metal subframes cradling various parts of the computer. Lots of sheet-metal screws fasten this Erector-set assembly together. Wires and cables also abound, as do clamps and brackets to hold them.

In contrast, Motorola's plastics engineers made the plastic cases structural. They also provided molded-in supports wherever possible, dramatically reducing the mechanical parts count. Eschewing nuts, bolts, and screws, the designers made virtually every component snap into place—even the power supply. Simple, light-gauge sheet-metal cabinet liners serve only as RF shielding and also snap into place. Motorola's old line had 300 fasteners and 50 wires and cables. The new computers have only eight fasteners and two cables.

Reducing a chassis's parts count doesn't necessarily mean switching to plastic. Triple E Corp's Triple E card cage significantly re- duces the number of parts in a card cage without abandoning all-metal construction. The company assembles VMEbus, VXIbus, Multibus II, Futurebus, AT, and EISA card cages from just five pieces: three extrusions and two stampings.

Although some soothsayers and pundits envision all-in-one electronics products, such as a combination TV, phone, encyclopedia, and facsimile machine, users have shown themselves willing to customize products themselves. However, other than mucking out your own septic tank, it's hard to imagine a more user-hostile chore than installing a so-called "adapter" into a PC. Yet, PC users have bought an amazing number and variety of add-in pc boards.

You could make your product more flexible by designing with expensive in-circuit-programmable logic. Or you could try Free Frame Technologies' T-Modules, which comprise stacks of conventional circuit boards. Users can swap T-Modules in and out of small computers without fooling with screws or cables. Cutouts in the case locate one end of the modules, and the other end slips into a mother-board socket. Spring strips on the lid of the computer's case hold the modules in place.

Just like the designers of the T-Module, you can substitute board-to-board "mezzanine" connectors for cables. Connector manufacturers are competing to see who can make the lowest profile connector. Samtec Micro Stripe connectors consist of alternating layers of conductive and nonconductive silicon rubber. The connectors can match 0.020-in. pc-board pad spacing and connect two pc boards that are 0.125 in. apart. Specialty Electronics offers 1-mm receptacles, pin headers, and shunts that rise 0.067 in. above the pc board. Augat's Millipede surface-mount 1-mm connectors rise 0.116 in. (female) and 0.130 (male) above the pc board. Surface-mount connectors are much less noisy than through-hole connectors.

You can also mount components on board that were once chassis mounted, eliminating further wiring. Examples are Wickman's surface-mount and through-hole fuses, which eliminate the use of a fuse holder and its attendant wiring. Oxley's 2-part lamp comprises a panel-mounted lens that mates to a pc-board-mounted LED, again eliminating panel wiring.

"Thermal management"—a fancy term for keeping your product from burning up—is the bugaboo that constantly haunts shrinking designs. Never forget that every 10°C rise in operating temperature roughly doubles your product's failure rate.

Fig 2a illustrates the effects of increasing temperature. The new Pentium µP typifies today's high-speed, complex digital devices (Fig 2b). Notebook-computer makers would love to ship Pentium-equipped products. Unfortunately for them, a 66-MHz Pentium dissipates more wattage than the notebook computers' thin packages can conduct to the outside world.

Fig 3 shows a pc-board containing two heat-sinked Pentium µPs. The image renders the coolest parts of the board in blue, dropping down through the spectrum, through red and then yellow, to white for the hottest portions. Forced air flows across this board from left to right.

The two Pentiums occupy the large yellow area. Each Pentium has a pair of horizontal, equals-sign-like red bars (an artifact of their heat sinks). Between the Pentiums (and perhaps difficult to discern) is a white-hot component eclipsed by the Pentiums' tall heat sinks. In other words, the thermal management of this board needs more work.

Computers larger than notebook computers can use new products, such as Berquist's double-coated, heat-conducting tape to mount heat sinks on top of large devices, such as the Pentium µP. The electrically insulating tape readily conducts heat. Just as important, the tape eliminates the use of mounting hardware for the heat sink, reducing the pc-board's mechanical parts count. And, if a simple heat sink is not enough, IREC's tiny muffin fans can bring forced-air cooling right down to the device level.

For really tight packaging, Aavid is developing liquid-cooled heat sinks. Fig 4shows such a thermosiphon heat sink attached to a Pentium. The liquid-tight heat sink transfers heat to the Fluorinert liquid. This liquid boils at very low temperatures, absorbing considerable heat as it changes phase. The Fluorinert vapor rises to an external radiator. After condensing in the radiator, the now-liquid Fluorinert flows back into the heat sink. The company is also working on much simpler and cheaper schemes involving sealing Fluorinert in a tough, electrically insulating plastic bag. Part of such a bag would simply rest on hot devices and conduct heat to cooler parts of a system.

IC makers are not the only ones shrinking components. Motorola has shrunk devices at the lowest level of active electronics. The MUN2111/2211 series of "bias-resistor" transistors integrate a bias-resistor network into the transistors' tiny SC-59 surface-mount package. Using these devices brings down the component count and the real estate consumed, but not the heat dissipated.

Ohmite has shrunk even the humble power resistor, providing a compact but concentrated source of heat that you must somehow remove. Ohmite screens some models of its power resistors onto a thermally conductive ceramic substrate. Other models use an even more thermally conductive, enameled, porcelain-coated steel substrate. The ceramic-substrate resistors can dissipate 10W/in.², and the steel-substrate resistors can dissipate 20W/in.². The planar construction results in low inductance (50 nH at 1 MHz typ).

1. Nordwall, Bruce D, "Companies Reduce Solder to Increase Reliability," Aviation Week & Space Technology, December 6, 1993, pg 50.