USC doctoral student unravels ‘tin whisker’ mystery(sc.edu)
sc.edu
USC doctoral student unravels ‘tin whisker’ mystery
http://www.sc.edu/news/newsarticle.php?nid=5371#.UMQOcOTAdnE
7 comments
Congratulations, now can you actually link to the paper instead of summarizing it in half a sentence as is "caused by high-strain gradient built up inside the device"? This is useless reporting.
It is a bit hard to find one that isn't behind a paywall, but here's the actual paper: http://www.osti.gov/bridge/servlets/purl/1013351-BXJitV/1013...
From my reading it sounds like we've known for sometime that it was due to strain on the board, but the work provides some insight by mapping stresses, and SEM imaging.
From the conclusion:
"In summary, a methodology has been developed to incorporate DIC technique with SEM images for mapping the local strain fields during whisker growth. By processing SEM images taken in consecutive time intervals over the area of interest on Sn-Cu finishes, the local strain evolution due to whisker growth can be obtained. This methodology provides an alternative way for exploring the complicated process of whisker growth through the page 9 of 13 morphology changes. From the DIC results, it is proposed that strain or stress gradient, instead of an overall compressive stress field is the key for whisker growth. Results from SEM and DIC analysis also indicate that the whisker growth is a continuously dynamic process, during which the subsequent whisker is triggered by the redistribution of strain or stress field after local strain relaxation. The findings have advanced the understanding of whisker growth mechanisms and may provide insight for developing whisker mitigation technology for lead-free solder alloys."
From the conclusion:
"In summary, a methodology has been developed to incorporate DIC technique with SEM images for mapping the local strain fields during whisker growth. By processing SEM images taken in consecutive time intervals over the area of interest on Sn-Cu finishes, the local strain evolution due to whisker growth can be obtained. This methodology provides an alternative way for exploring the complicated process of whisker growth through the page 9 of 13 morphology changes. From the DIC results, it is proposed that strain or stress gradient, instead of an overall compressive stress field is the key for whisker growth. Results from SEM and DIC analysis also indicate that the whisker growth is a continuously dynamic process, during which the subsequent whisker is triggered by the redistribution of strain or stress field after local strain relaxation. The findings have advanced the understanding of whisker growth mechanisms and may provide insight for developing whisker mitigation technology for lead-free solder alloys."
I assume whiskers cause a short circuit. Why can't the circuit board be coated with plastic, epoxy, non-conductive paint, or something to prevent the whiskers from causing a short?
You can't just spray anything on the board. Most coatings would separate from the board over time with due to thermal or mechanical stresses. There are many coatings that do work for this, but cause additional expense due to additional labor in finishing, and possibly design to make sure the components that need protected most can actually be coated.
Conformal coating is often used for protection in harsh environments.
http://en.m.wikipedia.org/wiki/Conformal_coating
http://en.m.wikipedia.org/wiki/Conformal_coating
Tin whiskers can break through conformal coating in some situations and the coating process does not always cover the entire surface evenly leaving thin areas or even entirely uncovered places in which whiskers can still form and escape. http://www.freelists.org/post/tinwhiskers/NASA-experiments-c...
Since the growth is caused my material stress, coating the solder with a hard plastic will shift the stress elsewhere, might cause another suite of problems.
This is the University of South Carolina, not Southern California which most people probably associate with “USC”.
"Historically lead (Pb) has been added to the solders to prevent whisker growth. Due to the harmful effects on human health and environments, it is no
longer a valid solution to prevent whisker growth with lead. With the shift to whisker- prone “Pb-free” solders, controlling whisker growth from low melting point metals has again become technically challenging."
> While manufactures had been able to control some whiskers by mixing small amounts of lead into tin solder, the 2006 European Union ban on lead in most electronic equipment had ignited a debate among scientists about whether whiskers would remain a perpetual problem.
Oh Europe. Yes, let's ban the use of minuscule amounts of lead so that people will have to replace entire devices when they fail.
Oh Europe. Yes, let's ban the use of minuscule amounts of lead so that people will have to replace entire devices when they fail.
that's not necessarily true.
As a mechanical engineer I used to work in an electronics factory. there is still a lot of soldering done by hand. not to mention that the operators of the automated soldering machines. during this soldering process there is a considerable amount of fumes, even though you're using blowers and filters, that the workers are exposed to. Lead is known to be hazardous for long exposes. So I guess keeping the manufacture-workers safety is as important as the end user's one.
As a mechanical engineer I used to work in an electronics factory. there is still a lot of soldering done by hand. not to mention that the operators of the automated soldering machines. during this soldering process there is a considerable amount of fumes, even though you're using blowers and filters, that the workers are exposed to. Lead is known to be hazardous for long exposes. So I guess keeping the manufacture-workers safety is as important as the end user's one.
This is an amazing discovery. I'm surprised it isn't more prevalent in the news. This whisker issue has been a thorn in the side of many since ROHS was implemented. The biggest impact with this I see is safer, more reliable planes. How exciting.
If the functionality loss is caused by these whiskers shorting circuits, why can't one simply restore functionality by lightly scraping the soldering to knock off the whiskers?
The whiskers are often far too short for that to work, and that assumes the whiskers are even exposed such that you could "scrape" them. Furthermore, there'd be a risk you'd end up depositing the whiskers elsewhere on the circuit...