Lead-free solder: A train wreck in the making

John Keller, Editor in Chief

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A slow-motion train wreck in military and aerospace electronics design is taking place right in front of us. Everyone seems powerless to do anything to head off the catastrophe, yet no one can tear his eyes away from the impending crash that we all know is virtually certain to happen.

The wreck-in-progress revolves around the evolving switch in the electronics industries in the U.S., Europe, and throughout the world from conventional lead solders to the new lead-free solders.

The specific threat is tin whiskers, which are physical abnormalities that grow in nonlead solders that lead to unpredictable shorting and failures of electronic parts. This phenomenon will compromise the reliability and reputation of most, if not all, commercial off-the-shelf (COTS) electronic parts and subsystems.

The move to nonlead solders stems from the Reduction of Hazardous Substances rules-better known as RoHS-that will apply to electronic products sold in Europe as of next July.

What that means is no one who wants to sell into the lucrative European electronics market will continue using leaded solders on a large-scale basis. The electronics industry throughout the world is starting to default to nonlead solders in the interests of maintaining business in Europe.

What we see happening now with lead solders is similar to what we saw a decade or more ago with military-specific integrated-circuit fabrication. The military’s share of the electronics market relentlessly shrank, the Defense Department’s COTS edict was issued, and integrated-circuit companies no longer had any incentive to produce military-grade devices.

One-by-one, the military chip lines closed, except for the rare small boutique fab. Everyone else went commercial because that’s where the money was.

Those in the military and the defense industry, of course, sought to keep the military IC industrial base intact through a series of weak, half-hearted measures, but ultimately the chip companies followed the money, and who could blame them?

Military officials then were living in a past in which the military was the largest consumer of electronic components and could call the shots in industry. It was a rude awakening to them when military chip lines folded up their tents with barely a glance behind them.

It looks like the military and defense industry failed to learn the lesson then, because the same thing is happening now with leaded solder. Companies are walking away from leaded solders because they see their economic futures elsewhere, driven primarily by the European program to limit the use of lead.

So where does this leave the military-particularly high-reliability, life- and mission-critical systems such as missiles, navigation systems, aircraft avionics, and communications satellites?

Where this trend places the military, at least in the short term, is in a lot of trouble.

The industry-accepted alternative to leaded solders is lead-free tin as a final finish, which spontaneously can sprout single crystal hair-like growths-the so-called “tin whiskers.”

These growths are electrically conductive, can grow in days or years, and can easily bridge between contacts, can touch each other to cause electrical problems, and can break off to bridge board traces and foul optics.

Although there are some programs that are starting to address the problem, the military now has no effective and accepted tests to determine the susceptibility of platings to whiskering, and no mitigation technique guarantees the protection that the Defense Department requires for high-reliability systems except the addition of 3 percent or more lead to the tin.

In addition, no quantifiable means of predicting tin-whisker-related problems exist. In short, there is no one solution for all tin plate applications.

Eventually, scientists and engineers will come up with ways to mitigate the forming and the effects of tin whiskers, but the electronics industry isn’t waiting. Nonlead solders are already in the marketplace, and military leaders are already seeing tin-whisker-related system failures.

The U.S. Naval Air Warfare Center (NAWC) at China Lake, Calif., for example, supports about 20 air- and surface-launched weapons, such as the Tomahawk cruise missile, Sidewinder air-to-air missile, and Joint Direct Attack Munition (JDAM).

Of those weapons programs, NAWC officials report that five-that’s 25 percent, mind you-built from 1985 to 1992 have had documented tin-whisker failures.

Reports indicate that six satellites sustained partial or complete loss due to tin whiskers. These involved Galaxy-3, Solidaridad 1, Direct TV3, and HS 601 satellites built between 1998 and 2002. Problems also have been reported with the F-15 jet fighter radar, the Patriot missile, and the Airborne Warning and Control System (AWACS) aircraft.

The period during which those systems were built was just the beginning of the semiconductor industry shift to pure tin solders. We ain’t seen nothin’ yet.

It’s probably going to take a lot more failures-some likely involving more than a few human deaths-before this issue gets the attention it deserves.

In some corners of the military, officials are trying to come to grips with this problem. Near-term attempts to date, however, promise to be ineffective. Most military leaders are simply ignoring the problem and hoping it goes away.

The U.S. Defense Logistics Agency, for example, has provided a sample list of defense specifications that preclude the use of pure tin. These include MIL-PRF-38535 paragraph A., MIL-PRF-55182 paragraph, MIL-PRF-55342 paragraph 3.5.3, and several others.

These days, however, trying to control private industry by issuing a series of government specifications is a lot like herding cats or teaching a pig to sing; it wastes your time, and only annoys the animals.

Last May officials of the Air Force Aeronautical Systems Center at Wright-Patterson Air Force Base, Ohio, issued Airworthiness Advisory AA-05-01, Lead-Free Solder. The directive points out what most in industry already know about the problems and systems failures that tin whiskers threaten.

The advisory concludes with this guidance: “Though there are many alternative solder alloys available to replace traditional tin-lead, none of them has passed the reliability testing required of aerospace-quality hardware.”

The directive continues, “Until such a time that a suitable, reliable, lead-free solder replacement is identified, all program managers should ensure their electronic equipment suppliers continue to provide items which meet all performance, compatibility, and reliability requirements. Failure to do so could adversely affect the reliability of weapons systems.”

You hear that, all you program managers? It’s your job to keep industry from switching away from leaded solders.

Before you do that, a word of advice: make sure you know what songs that pig likes best.

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Mil & Aero Magazine

August 2015
Volume 26, Issue 8

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