Got Tin Whiskers?

The potential for “tin whisker” growths on component lead plating, solder and solder paste skyrocketed along with the industry's lead-free transition little more than a year ago. What might surprise you, though, is that tin whiskers aren't wreaking as much havoc as initially expected — yet.

What are tin whiskers? NASA has been studying tin whiskers for many years and defines them as electrically conductive crystalline structures of tin that can grow from surfaces where tin (often electroplated tin) is used as a final finish. Tin whiskers are typically shown as having the shape of a very thin, single filament or hair-like protrusion that emerges outward (Z axis) from the surface (Figure). And they can cause electronic system failures by bridging closely spaced circuit elements maintained at different electrical potentials. Most frustrating, the exact causes, conditions and time frames involving tin whisker formation remain unknown.

This image shows two tin whiskers on the tin-plated surface of the end of a lead. The end of a trimmed QFP lead is evident, with exposed copper in the center, and the tin plating on the surface is shown as the two skin layers. Whisker length was ~15 µm after 2000 hours of JEDEC temperature/ humidity testing. (Source: ASAT)

“Overall, we aren't seeing tin whisker problems yet. But that's not to say we won't,” said Leo M. Higgins III, senior vice president of engineering and technology at ASAT Inc. (Milpitas, Calif.). “The biggest whiskers we're seeing in JEDEC testing are 20-30 µm. Sometimes whiskers show up in a very short amount of time. And sometimes you have nothing at 2000 hours, but at 3000 hours, you've suddenly got a large whisker, so it's hard to anticipate. Another factor that makes the testing we do unrealistic for real world application environments is that testing is unbiased. It's also conducted on components that haven't been reflowed and exposed to flux and reflow environments. Those two factors can really make a difference.”

How about tin whisker mitigation? Companies making RoHS-compliant tin-coated leaded components can use any number of tin whisker mitigation (not elimination) strategies, according to Lasky. “These approaches include processing to minimize residual stresses in the tin, adding a small amount of bismuth [i.e., 3%], or using a flash of nickel between the tin and the base copper in the lead,” he explained. “These strategies can dramatically reduce tin whiskers to a point where they will be an acceptable risk in a product with a 10-year life. What is acceptable risk? The whisker risk is so small that the predicted predominant failure modes will be unrelated to tin whiskers.”

Other tin-related issues? Interestingly, ASAT pointed out that test socket contamination has been somewhat of a nuisance since the lead-free transition. To address the problem, they recently switched to a next-generation tin chemistry that transfers less debris to test sockets during electrical testing while providing enhanced whisker performance. “There was a 5% false failure rate with the old tin formulation, meaning that there was so much buildup on the test sockets, they started detecting 'opens.' With the new generation, the supplier claims it improves to 0.5-1.5%,” explained Serafin Pedron, ASAT's director of applications engineering. “This is roughly a 5× improvement. Thanks to the finer, more uniform grain size of the tin crystals, we have lower friction, less tin oxide buildup on the contacts, and a lower false failure rate.”

Another ongoing packaging-related challenge since going lead-free is drop testing. Over the past year, Higgins said ASAT has developed solutions for shock testing and drop tests that will enable lead-free solders to perform as well as lead in drop tests for portable electronics. Solutions exist, but these all use higher-temperature solders.

The bottom line is that there are many issues still being sorted out with the lead-free transition (including the justification for it). As far as tin whiskers go, as Lasky put it: “RoHS has been in force for more than 15 months now, and ~$400B RoHS-compliant products have been produced. With all of these products in the field, no significant numbers of tin whisker-related failures have been reported.”

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