Up Time Magazine - February/March 2009 - (Page 42) precision maintenance upload Tin Whiskers When Making Things Better Makes Things Worse by Derek Burley, CMRP D espite our best efforts it seems that there are occasions when our attempts to improve or ‘make things better’ actually backfire and result in the opposite effect – things can get worse in an unexpected way. What this article seeks to achieve is to raise awareness of a condition that was first identified in the 1940s, but has only recently become an issue that warrants a more concerted research effort. (Sn) and has been used for many years to join metal components in everything from water pipes to computers. The problem is that when you remove the Pb from the solder, strange things can start to happen. “Tin whiskers” can form and the consequences can be serious. • The Millstone nuclear generating plant in Connecticut unexpectedly shut down when a circuit board monitoring pressure in a steam line failed (April 17, 2005) • During 2006, multiple batches of a Swiss company’s watches were recalled – the estimated cost was $1bn • The failure of the Galaxy IV communications satellite in 1998 • The FDA recalled a number of pacemakers in 1986 • Relays in AT&T telephone switching centers failed during the 1950s In all the above cases, tin whiskers were identified as the cause. What Are We Trying To Make Better And Why? Over the last couple of decades, there has been an increasing push to become more ‘green’. Environmental awareness and the momentum to change have led to the introduction of legislation or incentives in many western countries. This article examines an example of legislation introduced by the European Union and its emerging effects. This legislation is the “Restriction of certain Hazardous Substances” (RoHS) and “Waste Electrical and Electronic Equipment (WEEE) Directives”. These new directives set deadlines for electronics suppliers and manufacturers to eliminate many of the common uses of lead (Pb) from their products – including solder. A major driver for ‘cleaner’ solder stems from the huge amount of electronic equipment that is dumped into landfills. Pb was considered likely to eventually leach into ground water and was the motivation to change current practices. Put simply, there is a growing legal mandate and moral drive to remove Pb from solder. To add a little perspective, in 2002, the US Environmental Protection Agency (EPA) estimated that 80,000,000 kilograms of Pb solder was used globally. The RoHS and WEEE legislation set a deadline of June 2006 to achieve the goal of Pb free solder. It is worthy of note that some types of electronics were exempted from the law since its inception. Among these exceptions were military and other national security equipment and certain medical devices, among others. Currently, the United States has not made Pb-free solder mandatory, but does offer tax benefits for reducing or eliminating its use. There was no problem removing Pb from gasoline. We took the Pb out of paint. So, it seems very reasonable that we could remove Pb from solder - what could possibly go wrong? What Are Tin Whiskers? Firstly, the whisker phenomenon is not new and it is not confined exclusively to solder. Other metals that can form whiskers include zinc, cadmium, indium and antimony. Whisker events have been observed since WWII, when military radio failures occurred often enough to initiate investigations into the cause. Although the problem was recognized over 60 years ago, it has only been the subject of serious research over the last few years. Tin whiskers are crystalline growths that can grow spontaneously – crystals of tin that can cause short circuits in electrical and electronic equipment. An applied electrical field is not required to enable their growth – they appear randomly. Whiskers can grow at ambient temperature and humidity, in a vacuum, in air, in continual or varying temperatures and, in time, they will eventually force their way through any protective coatings that have been applied to contain them. A single whisker can conduct about 30mA. This is certainly a high february/march 2009 What Happens When You Remove The Lead From Solder? Traditional solder typically contains 37% Pb and 63% tin 42
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