New approach to missile electronics: 3-D packaging

Aug. 1, 2004
COLLEGE PARK, Md. — Given the defense industry's reliance on commercial electronics markets for the state-of-the-art devices necessary for new weapons, the approach in the past has been either to develop very high-reliability versions for mission-critical functions or to encase the subsystems in elaborate packaging...

By John Rhea

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COLLEGE PARK, Md. — Given the defense industry's reliance on commercial electronics markets for the state-of-the-art devices necessary for new weapons, the approach in the past has been either to develop very high-reliability versions for mission-critical functions or to encase the subsystems in elaborate packaging — particularly in space applications in which they are exposed to high radiation doses.

There may be another way. Engineers at the U.S. Navy's Best Manufacturing Practices (BMP) Center of Excellence adjacent to the University of Maryland campus in College Park, Md., are working on a three-dimensional approach, in which designers embed the components inside a printed circuit-card assembly to handle higher G forces and thermal stresses than are possible with conventional technology.

BMP's in-house contractor Willcor, headed by Brian Willoughby (hence the company name), is exploring this technique, using a packaging method developed by Soldering Technology International (STI) in Madison, Ala. The prototype devices are scheduled to be tested later this year on the Navy's SM-2 (Block 3B) anti-aircraft Standard missile and could be part of the SM-3 antiballistic missile version if that program ever reaches fruition.

About 10 flyable boards are scheduled to be used in the tests of the SM-2 produced by Raytheon Systems in Tucson, Ariz. Ground tests are currently under way at the White Sands Missile Range in New Mexico, where the flights will occur. The missile program is managed by the Navy's Standard Missile Program Office (PEO IWS 3A), and the BMP is supported by the Office of Naval Research, which has funded the program at about $5 million for the past two years under a small business set-aside.

What the experimental program amounts to, Willoughby explains, is getting away from the plastic-encapsulated modules (PEMs) common to the cell-phone market and becoming increasingly common in military applications — with accompanying concerns about reliability.

The program also means tackling the problem of "tin whiskers" that can occur during operations as companies increasingly turn to lead-free solders for environmental reasons. And it circumvents the need for ball-grid-array connections.

"This is not a science program," Willoughby stresses. It is simple engineering, welding the die inside the hull cavities of the boards to connect to the outside world. "We cannot afford to solve all these problems piecemeal," he adds. "A new approach was needed."

STI was already working on this approach before the Navy started its program, says Jim Raby, founder and technical director of the company. Now he is trying to get the approach implemented across the board in military and space programs. The company calls the process Imbedded Component/Die Technology, or IC/DT.

The Navy will be the first military user, Raby says, but he thinks the method is applicable anywhere that components face heat, cold, vibration, and what he calls the "mud and slush" of Army tanks. Since STI is adjacent to NASA's Marshall Space Flight Center, that agency is also a prime candidate.

STI would license the technique to defense contractors, Raby adds, and would help set up a production line. He also envisions under-the-hood applications in the automobile industry. Raby doesn't like to talk about how much money the company invested in the technology, but an outside estimate is about $7 million.

The problem in missile applications is that missiles have to sit in storage for lengthy periods of time before they are needed. This wasn't a problem in the days before the focus on commercial-off-the-shelf (COTS) technology. In the Minuteman intercontinental ballistic missile, for example, all the electronic components were produced to rigid military specifications — with the accompanying extensive requirements for documentation.

Those days are over, but the problems are not. Willoughby and Raby estimate that a guided projectile fired from a cannon encounters forces of 20,000 Gs, and that missile applications range from 15,000 to 30,000 Gs.

To date, the program has been getting high marks from the Navy. Scott Reiter, the Standard-missile program manager, says the effort has shown promise in eliminating a myriad problems associated with current circuit-card-assembly processes, such as the tin whiskers hermeticity of PEMs, and parts obsolescence.

As he explains the situation, the Navy is troubled by current industry trends. The military's dwindling share of the electronics-component market mandates this new approach. "We no longer have the market share necessary to drive the packaging and interconnect markets of commercial electronics to the quality we need for missile systems."

Also, by eliminating secondary packaging, this embedded-component technique promises smaller form factors. STI estimates that size reductions of 80 percent can be achieved by eliminating this packaging.

There is, predictably, a down side of this technique: rework. This is particularly difficult for embedded components, and that dictates rigorous testing of the boards before the cavities are filled.

Nonetheless, this may be the answer to a number of military problems in living with COTS technology. It isn't likely the defense industry will ever again command a large share of the electronics market and therefore can no longer dictate the technologies to be used in the new generations of components now in development.

Military systems have to work right the first time — even if they have been stored in dormant status for lengthy periods. That's why missiles, for example, use solid propellants instead of the more efficient (but more volatile) liquid-propulsion methods.

What this packaging technique amounts to is matching the electronics to the weapons within today's cost constraints.

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