The Continuing Dangers of Tin Whiskers and Attempts to Control Them with Conformal Coating

A 1998 commercial satellite failure caused by tin whisker induced shorts prompted NASA Goddard Space Flight Center (GSFC) to issue a NASA Advisory (NA-044 and NA-044A) (1,2) to remind the NASA community of the tin whisker phenomenon and the inherent risks associated with the use of pure tin plated components. Indeed the NASA Advisory served as a “reminder” since the spontaneous growth of tin whiskers from some tin plated surfaces has been known and studied for over 50 years with dozens of technical publications and several GIDEP Alerts produced during that time. During the 1990s the US Military modified most (but not all) of their electronic component specifications to prohibit the use of pure tin finishes in order to minimize the risks of whiskering. However, as regulations and a world economy push today’s electronics industry to use environmentally friendly (Pb-free) alternatives, the prevalence of pure tin plated components is bound to increase potentially increasing NASA's risk of exposure to risks associated with tin whiskers significantly. In an effort to evaluate risk mitigation techniques, NASA GSFC initiated experiments to study the effects of Uralane 5750 (a commonly used conformal coat) on tin whisker growth. After more than two years of experimentation, we have found that conformal coat does not prevent tin whisker formation although it does appear to substantially reduce the rate of growth. We have observed that a tin whisker has grown through an area of conformal coat that is approximately 1/4 mil thick (see figure 1). Numerous tin nodules growing beneath a nominal 2 mil thick coating are also being monitored to determine if and when they will be able to penetrate this barrier. We have also observed that tin whiskers can bend in response to forces of electrostatic attraction; thus increasing the probability of tin whisker shorts either from two whiskers colliding or from one whisker bending to contact another conductor. Especially for long duration missions, use of conformal coat as a sole means of risk mitigation may not be completely effective. Research is ongoing. BACKGROUND: The growth of tin whiskers on pure tin plated electronic components and associated hardware has been documented for decades. Notable examples of pure tin plated components that have exhibited tin whisker formations include electromagnetic relays, transistors, hybrid microcircuit packages, terminal lugs and very recently ceramic chip capacitors. A few such examples are shown below in figure 2: Electromagnetic relay terminals Photo courtesy of NASA GSFC Ceramic chip capacitor termination Photo courtesy of Ericsson Hybrid microcircuit lid Photo courtesy of JPL Figure 2. Examples of Tin Whiskers Growing on Pure Tin Plated Electronic Components An article was published in the December 1998 edition of the EEE Links Newsletter that provides a basic overview of the tin whisker phenomenon and some of the inherent risks. Included in that article are explanations of the possible effects of tin whiskers in more conventional earth-based environments. Also, interim results and a detailed explanation of GSFC experiments were reported in a September 2000 paper entitled “Effect of Conformal Coat on Tin Whisker Growth”. The authors of this current article guide interested readers to these two publications for a simple primer on tin whiskers and details of the GSFC experimental process as those details will not be repeated here. In addition, the NASA Goddard Tin Whisker Homepage provides an extensive list of reference materials on the topic as well as access to our published paper: http://nepp.nasa. gov/whisker A 1998 on-orbit commercial satellite failure was reportedly caused by tin whiskers emanating from the surface of a pure tin plated relay. Over time the whiskers grew to such a length that they were capable of short circuiting the spacecraft bus. Laboratory tests dating back to the early 1990s demonstrated the potential for a tin whisker short to form a plasma in reduced barometric pressure environments (vacuum). (6,7) If sufficient energy and a low impedance path are available from the power source, this plasma can sustain an arc that is capable of carrying HUNDREDS of AMPERES! Such a short circuit is reported to have occurred on the commercial satellite opening protective fuse elements thus rendering the spacecraft non-operational. Since 1998 it has been reported that two additional commercial satellites employing a similar bus design have failed from this same mechanism. (8) Despite the extensive research performed to date by industry and academia, an accepted comprehensive description of the growth mechanism(s), effective risk mitigation practices and industry accepted tin whisker test methods still elude researchers. It is however, commonly believed that the whiskers form in order to relieve residual stresses in the plating that result from internal and/or external stresses. As such, "bright" tin finishes which have a high residual stress after plating are more prone to whiskering. Numerous other factors also affect whiskering propensity such as substrate material, plating chemistry and process, plating thickness and grain size. The studies to date have not conclusively demonstrated the relative importance of these factors nor combinations of these factors and as such a "proven" whisker-free pure tin plating process that is adaptable to all types of components is not yet available. Some studies suggest that alloying as little as 0.9% lead (Pb) with the tin dramatically reduces the size and growth rate of whiskers to a low enough level that is safe for current microelectronic geometries. As such, NASA Advisory NA-044 reports that the most effective risk mitigation technique against tin whisker induced short circuits is to prohibit the use of electronic components and associated hardware that employ pure tin plating as a final surface finish (common practice is to require a minimum of 3% Pb). The current worldwide initiative to reduce the use of potentially hazardous materials such as Pb is driving the electronics industry to consider pure tin plating as an alternative to tin-lead plating. With respect to factors such as solderability, ease of manufacture and compatibility with existing assembly methods pure tin plating is a viable alternative. However, the current lack of an industry accepted understanding of tin whisker growth factors and/or test methods to identify whisker-prone products makes a blanket acceptance of pure tin plating a risky proposition for high reliability systems. Knowing that simple prohibition of pure tin plating will become more and more difficult, NASA GSFC decided to conduct experiments to study the effectiveness of other tin whisker risk mitigation techniques. Several mitigation approaches have been suggested in the past including annealing or reflowing the plated surface with high temperature to relax internal stresses, covering solderable surfaces with a Pb-containing solder or coating the surface with a protective barrier of conformal coat. All of these practices have benefits and limitations. The conformal coat approach seemed to be the most practical and least invasive technique for high reliability systems. GSFC review of the available literature found limited information related to the benefits of conformal coating as a risk mitigation technique. Therefore, in December 1998 NASA GSFC began experiments to evaluate the effectiveness of using conformal coat to mitigate the risk of tin whisker growth. Experimental Approach: The objective of the GSFC experiments was to determine if conformal coat could be used as an effective risk mitigator, when whisker-prone (or unknown whisker propensity) components are used in electronic systems. Uralane 5750 was selected as the conformal coat material for these experiments because of its common use in NASA flight hardware. Experiments were devised to evaluate the effectiveness of Uralane 5750 at: • Delaying the onset of tin whisker formation (incubation period) • Affecting the growth rate of tin whiskers • Affecting the growth density of tin whiskers • Preventing tin whiskers from growing through the coating As such, for the purpose of these experiments, test specimens with extremely high propensity to form tin whiskers were intentionally produced, so that the effects of the coating on whiskering could be observed and documented. A literature search found that brass substrates with “bright” tin electroplate of approximately 200 microinches were highly prone to whisker formation. The specimens for our experiments were procured from a commercial plating shop. • Substrate Material: Brass Type 260 (test coupons were 4" x 1" x 0.032") • Underplate: 50% of samples have a copper strike and copper plate to 0.0001" min 50% of samples have NO underplate (i.e., tin plate direct on brass) • Plating Process: "Bright" tin bath • Tin Plate Thickness: 200 ± 50 microinches To further promote whisker formation, portions of the test specimens were intentionally scratched using a knife blade. Such surface defects are reported to create localized stresses in the plating that may promote whisker formation. Samples were then coated over half their surface using Uralane 5750 to a nominal thickness of 2 mils; the other half was left uncoated as a control. Seven (7) samples were stored under ambient laboratory conditions (approximately 22°C and 30% to 70% relative humidity) while eight (8) samples were stored at 50°C which our literature review found to be commonly reported as the optimal temperature for whisker formation. Experimental Results/Observations After 2 1⁄2 Years: For the last 2 1⁄2 years the test specimens have been examined periodically, using both optical techniques and scanning electron microscopy (SEM). Incubation Period and Whisker Density: Our experiments to date have shown that Uralane 5750 conformal coat applied over top of whisker-prone test specimens does NOT PREVENT