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Tin whisker debate roars on
Rob Spiegel -- Design News, January 8, 2006
The Electronic Components, Assemblies and Materials (ECA) sector of the Electronic Industries Alliance (EIA) recently released a statement on tin whiskers that offers tolerable sizes of tin whiskers while also stating the danger of lead in the environment outweighs the danger of tin whiskers. Engineers in the defense and aerospace industries, however, still insist that tin whiskers will become a significant problem if defense and aerospace manufacturers continue buying commercial off-the-shelf (COTS) components once those components become RoHS compliant.
The ECA/EIA report, Comments on Test Methods and Criteria for Tin Whiskers in Surface Mount devices (http://www.ec-central.org/engineering/bulletin/CB22.pdf) claims that “the problem of tin whiskers was limited to some very specific applications.” The report notes that there has been anecdotal reports of weapon and satellite systems failing because of tin whiskers, but no scientific data exists in the public domain to support those claims. Engineers who claim tin whiskers have caused system malfunctions insist that you can’t prove a tin whisker caused a failure because you can’t retrieve a satellite and if you could, the whisker would not be apparent when you opened the system.The ECA/EIA report goes on to note that tin whiskers of a certain length and breakage strength (or robustness) are not vulnerable to breaking. “Some of our member companies have performed various shock, vibration and other tests of varying severity (as high as required by space launching agencies) on parts with grown whiskers and have found no evidence of whiskers breaking,” says the report. The ECA goes on to suggest that these tests should be standardized and used to determine whether parts may be used in critical applications.The report also proposes that whisker length be linked to size rather than application, noting that if the whiskers are short enough, they will present no danger, no matter what application they are used in. The report concludes that for almost all commercial applications, tin whiskers are not a problem, while lead in the environment is a problem. “It is more important at this point to eliminate harmful products like lead from the environment,” says the report in its final comment.
A Lockheed Martin engineer insists the report overlooks potential problems associated with commercial lead-free solder. “What the article does not mention is the real testing of lead alternatives in the tin solder have not yet been proven to stop the tin whiskers in the long term, nor have the lead alternatives been proven not to replace the tin whiskers with another set of problems,” says the engineer. “The mechanical and thermal fatigue properties and tensile properties of the AgCuSn (silver/copper/tin) or SbSn (antimony/tin) solders are in question and not yet proven for all environments.”
