Electronic Design

Stubble Trouble—Beating Back Those Tin Whiskers

What is a tenth of the diameter of a human hair and only 1.5 mm long, but can shut down nuclear plants, misguide Patriot missiles, and cause the recall of thousands of quartz watches? The answer is tin whiskers, those tiny singlecrystal filaments that grow from the surface of tin and subsequently occur in electronic circuitry—often with devastating effect.

Tin whiskers cause short circuits in the position they grow in or as debris, a problem that has been exacerbated by shrinking geometries (Fig. 1). They have been known about for years—65 to be precise. But since the introduction of the EU’s RoHS regulations, they have surfaced to the top of the agenda in many electronic application areas (Fig. 2).

Back in the 1950s, engineers solved the “stubble trouble” problem by adding lead to tin solder. In those days, the alloy was a mix of approximately 30% lead to 60% tin. Now, because of the RoHS lead-free dictates, whiskers are casting reliability shadows over many electronic applications. But to be fair to the electronics industry, some progress has been made in trimming them back—not an easy task when you look at the characteristics of the whiskers.

Tin whiskers are unpredictable. Some take years to develop, and unlike dendritic growths, they don’t require an applied electronic field in which to grow. Nor do they need moisture. In fact, they are perfectly happy to create themselves in a vacuum. As for their current-carrying capacity, a whisker can pass currents up to 10 mA. That equates to a carry capacity of around 1 million amps per square inch.

Because of the whisker phenomenon, mission-critical applications such as medical and military equipment were exempted from RoHS. Okay, so how can they be eliminated? We know that compressive stress, scratching, bending, and irregular inter-metallic compounds exacerbate their formation. We also know that these elements aren’t easily eradicated when it comes to some of the inherent production processes in electronics.

Many industry experts have worked long hours to come up with a number of ideas to reduce the risk, but probably won’t wipe out the whiskers. In the U.S., NASA has been particularly zealous when it comes to trimming the whiskers.

BEARD BUSTERS
Studies, and the lessons learned in the 1950s, clearly demonstrate that alloying tin with a second metal reduces the propensity for whisker growth. Also, alloys of tin and lead are generally considered to be acceptable when the alloy contains a minimum of 3% lead. Although some tests have shown whisker growth from tin-lead alloys, reports indicate that such whiskers are dramatically smaller than those from pure tin-plated surfaces.

NASA also believes it’s prudent to ensure that when a part supplier stipulates pure tin isn’t used in its products, it’s worth seeking independent verification of this fact.

Conformal coating has also been considered as a method to combat the problem. Whisker-prone surfaces are coated or encapsulated, and so far, this does appear to work. After NASA GSFC experiments, Uralane 5750 conformal apparently helps reduce the growth rate, but NASA warns that tin whiskers can grow through conformal coating. The danger is that even if all surfaces are coated, the whiskers could still create short circuits by touching each other.

Alternative coating procedures could involve spraying the components with silicone, parylene, and other materials to reduce the growth and spread of the whiskers. So although progress is being made in winning the whisker battle, it is by no means finished.

Paradoxically, for some industry experts, it remains a question of whether the reduction of lead in electronics really makes a significant contribution to our environment as demonstrably as unleaded petrol. Legacy-related arguments contend that lead-free solder could shorten product life, resulting in more scrappage and the subsequent environmental impact of providing the energy to manufacture increasing numbers of replacement product. But before that debate is resolved, we’ll have some very long whiskers.

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