Developing a Nasa Lead-Free Policy for Electronics Lessons Learned
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DEVELOPING A NASA LEAD-FREE POLICY FOR ELECTRONICS LESSONS LEARNED Michael J. Sampson National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD, 20771, USA michael.i.sarn~soniii,,nasa.aov ABSTRACT Sn60Pb40 and Sn63Pb37. It was clear as July 1, 2006 approached that there was no direct form, fit, function The National Aeronautics and Space Administration Pb-free substitute for SdPb solder alloys waiting to be (NASA) is not required by United States or international used. law to use lead-free (Pb-free) electronic systems but international pressure in the world market is making it Instead, there was an extensive array of Pb-free solder increasingly important that NASA have a Pb-free alloys, some proprietary, whose performance and long- policy. In fact, given the international nature of the term reliability in harsh environments continue to be the electronics market, all organizations need a Pb-free subject of ongoing research and technical debate. -.policy. 2. THE CHALLENGES OF LEAD-FREE Note: The chemical symbol for lead, Pb is often used in SOLDER ALLOYS this paper, to avoid confusion with termination leads. A * The symbolfor tin, Sn is sometimes used-forsymmetry. It has been known for more than ten years, that alloys based on mixtures of tin, silver and copper (SAC This paper describes the factors which must be taken alloys), possess the basic properties required to make into account in formulating the policy, the tools to aid in solder joints between electronic components and the structuring the policy and the unanticipated and difficult printed wiring boards used to make electronic circuits. challenges encountered. NASA is participating in a Other candidate Pb-free alloys include those containing number of forums and teams trying to develop effective tin-bismuth, tin-copper and tin-nickel-copper (SNiC). In approaches to controlling Pb-free adoption in high the United States, the SAC alloys have become the reliability systems. The activities and status of the work primary choice despite having higher melting points being done by these teams will be described. NASA than Sn63Pb37 and Sn60Pb40 solder. Initially the front also continues to gather information on metal whiskers, runner for reflow applications was SAC405 (4% silver, particularly tin based, and some recent examples will be 0.5% copper, balance tin), later SAC305 became shared. The current lack of a policy is resulting in industry's choice. According to McCormick et a1 of "surprises" and the need to disposition undesirable Celestica [3], this was probably driven by economics conditions on a case-by-case basis. This is inefficient, since the reduced silver content reduces the alloy cost; costly and can result in sub-optimum outcomes. there is little difference in the performance of the two alloys although SAC405 may be marginally better in 1. INTRODUCTION some respects. On July 1, 2006 the European "Restriction of the Use of The concern for the hi-reliability electronics assembler, Certain Hazardous Substances in Electrical and including NASA and other space agencies is that there Electronic Equipment" or RoHS directive went into is very limited experience with the use of Pb-free alloys, effect.[l] Even though this directive is not enforceable certainly nothing equivalent to the decades for SdPb. in the United States and is not applicable to spaceflight The higher melting point of the SAC alloys raises hardware, it was apparent that NASA would need a concerns about the risk of damage and reduced life time policy to cope with its consequences. The RoHS for electronic parts that have been designed to work requires the control of six chemicals: lead (Pb), with SdPb alloys. The limited testing that has been cadmium, mercury, hexavalent chromium, accomplished so far, has demonstrated differences in polybrominated biphenyl (PBB) and polybrominated performance between Pb and Pb-free alloys, sometimes diphenyl ether (PBDE) flame retardants. Of these, Pb the Pb is superior, sometimes it's the Pb-free. Given this has the greatest potential impact on NASA spaceflight situation, it was clear that NASA should delay adoption systems. The RoHS limits the concentration of Pb to of Pb-free solders for as long as possible, in order for 0.1% by weight [2]. Pb has been used in the solder industry to work out the problems and to select the best alloys used to assemble the electronics in every alloys and combinations for various assembly processes spaceflight system ever launched by NASA, at and application conditions. concentrations that far exceed this limit (typically -40% Pb). Decades of mission success have been founded on Pb alloys have also been used extensively to make the properties of SnPb solder alloys especially solderable finishes on component terminations and solder pads on printed wiring boards. These applications power stations in the United States and abroad have introduce a further risk in a Pb-free world. experienced shutdowns as a result of tin whiskers [4]. Whiskers have been studied for more than 60 years but 3. LEAD-FREE FINISHES, TIN WHISKERS it is still not known why they grow or exactly how they AND TIN PEST grow, so the risk of any given tin-plated surface A suitable termination or solder pad finish needs to be growing whiskers is unpredictable, as is the density of readily solderable, tarnish resistant to remain solderable the growth or the distribution of lengths and growth rate during extended storage, easy to apply, economically of the whiskers. The research has tried to identify ways viable and compatible with a range of solders so as not of limiting or eliminating whisker growth and various to develop brittle intermetallics or destructive corrosion. studies have found that techniques such as use of nickel Suitable termination finishes include gold, silver, tin underplating, annealing at elevated temperatures and and tin-lead. Obviously, there is a big difference in cost reflow of the finish to be effective; unfortunately an between precious metal and base metal finishes, so for equivalent number of studies have shown these most general purpose applications the choice has been techniques not to be effective [6]. It is clear that these between tin and tin-lead. Therefore, in a Pb-free world, studies did not control for dne or more important the termination plating of choice is tin, essentially pure variables with a strong effect on whisker growth. tin. However, pure tin finishes have one unfortunate Extensive experience has shown that the most effective property, the propensity to grow tin whiskers. way to limit the risk of tin whiskers is to alloy the tin with Pb. While SnPb finishes occasionally whisker, the Tin whiskers are crystalline growths that grow whiskers are short and sparse and rarely represent a spontaneously from tin-coated surfaces. Whiskers are reliability risk. The established industry minimum Pb generally only one to five microns in diameter but over content has been set at 3%, although less than 1% is time they have been noted to grow to lengths probably effective [7,8]. There is very little evidence approaching 20 millimeters [Fig. 11. that alloying the tin with any other element than Pb is as effective at whisker suppression. For surfaces finished with pure tin, the only proven mitigation for whiskers is conformal coating with an appropriate polymer [9] but it can be impossible or undesirable to completely coat all surfaces, especially under components. It is also difficult to apply polymer coatings uniformly to avoid producing locally thin coatings. Hot dipping tin-coated surfaces into SnPb solder can also be quite effective but it is difficult to dip component leads all the way to the body without causing damage and any areas of exposed pure tin can still grow whiskers. One such example is described on the NASA Tin and Other Metal Whisker site [lo]. It is interesting to note that this particular system also employed nickel underplating which is often claimed to Figure I. 18mm whisker on Space Shuttle Card Guide inhibit tin whisker formation. They are electrically conductive and can cause circuit A large number of engineers are still unaware of malfunctions ranging from intermittent glitches to whiskers but even more have never heard of "tin pest". catastrophic short circuits. On-orbit satellite failures due Tin has two stable allotropes: beta (or "white") tin and to whiskers are well documented [4]. Whiskers do not alpha (or "grey" tin). At temperatures above 13°C the need the presence of an electrical field or humidity to familiar beta phase is the stable allotrope for tin. Beta grow and they grow best around room temperature [5]. tin is a body-centered tetragonal crystal that is ductile There is no way to accelerate or stimulate their growth and a good electrical conductor. At temperatures below that has proven effectiveness. It is a widely held belief 13"C, the alpha phase is the stable form. Alpha tin is a that whiskers are only a concern for space and other diamond cubic crystal that is brittle and has high reliability applications. This is false. Whiskers semiconductor properties. The transformation from beta have caused failures in a broad range of electronic to alpha tin is also accompanied by an expansion in equipment containing pure tin termination finishes when volume of -26%. This expansion and the brittle nature the risk of whisker growth was not effectively of alpha tin produces a wart-like, powdery conversion mitigated. For example, various military systems have of the tin commonly referred to as tin pest [Fig. 21. The been affected by tin whiskers and a number of nuclear maximum conversion rate reportedly occurs around - 40°C [ll]. There are various anecdotal, historical references to tin pest. For example the tin buttons of the for leading-edge applications, exposure to pure tin uniforms of Napoleon's army are supposed to have terminations is inevitable and in fact is already quite disintegrated because of tin pest formation in the cold common.