Whisker (metallurgy)

Last updated

Silver whiskers growing out of surface-mount resistors SilverSulfideWhiskers1.jpg
Silver whiskers growing out of surface-mount resistors

Metal whiskering is a phenomenon which occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in the vacuum tube era of electronics early in the 20th century in equipment that used pure, or almost pure, tin solder in their production. It was noticed that small metal hairs or tendrils grew between metal solder pads causing short circuits. Metal whiskers form in the presence of compressive stress. Zinc, cadmium, and even lead whiskers have been documented. [1] Many techniques are used to mitigate the problem including changes to the annealing process (heating and cooling), addition of elements like copper and nickel, and the inclusion of conformal coatings. [2] Traditionally, lead was added to slow down whisker growth in tin-based solders.

Tin Chemical element with atomic number 50

Tin is a chemical element with the symbol Sn (from Latin: stannum) and atomic number 50. Tin is a silvery metal that characteristically has a faint yellow hue. Tin, like indium, is soft enough to be cut without much force. When a bar of tin is bent, the so-called tin cry can be heard as a result of sliding tin crystals reforming; this trait is shared by indium, cadmium, and frozen mercury. Pure tin after solidifying keeps a mirror-like appearance similar to most metals. However, in most tin alloys (such as pewter) the metal solidifies with a dull gray color. Tin is a post-transition metal in group 14 of the periodic table of elements. It is obtained chiefly from the mineral cassiterite, which contains stannic oxide, SnO2. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, and has two main oxidation states, +2 and the slightly more stable +4. Tin is the 49th most abundant element on Earth and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, thanks to its magic number of protons. It has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures, it transforms into the less dense grey α-tin, which has the diamond cubic structure. Metallic tin does not easily oxidize in air.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Short circuit Electrical circuit with negligible impedance

A short circuit is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit. The opposite of a short circuit is an "open circuit", which is an infinite resistance between two nodes. It is common to misuse "short circuit" to describe any electrical malfunction, regardless of the actual problem.

Contents

Following the Restriction of Hazardous Substances Directive (RoHS), the European Union banned the use of lead in most consumer electronic products from 2006 due to health problems associated with lead and the "high-tech trash" problem, leading to a re-focusing on the issue of whisker formation in lead-free solders.

European Union Economic and political union of European states

The European Union (EU) is a political and economic union of 28 member states that are located primarily in Europe. Its members have a combined area of 4,475,757 km2 (1,728,099 sq mi) and an estimated total population of about 513 million. The EU has developed an internal single market through a standardised system of laws that apply in all member states in those matters, and only those matters, where members have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market, enact legislation in justice and home affairs and maintain common policies on trade, agriculture, fisheries and regional development. For travel within the Schengen Area, passport controls have been abolished. A monetary union was established in 1999 and came into full force in 2002 and is composed of 19 EU member states which use the euro currency.

Mechanism

Microscopic view of tin used to solder electronic components showing a whisker Tin whisker ESA385984.jpg
Microscopic view of tin used to solder electronic components showing a whisker

Metal whiskering is a crystalline metallurgical phenomenon involving the spontaneous growth of tiny, filiform hairs from a metallic surface. The effect is primarily seen on elemental metals but also occurs with alloys.

Metallurgy domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy is used to separate metals from their ore. Metallurgy is also the technology of metals: the way in which science is applied to the production of metals, and the engineering of metal components for usage in products for consumers and manufacturers. The production of metals involves the processing of ores to extract the metal they contain, and the mixture of metals, sometimes with other elements, to produce alloys. Metallurgy is distinguished from the craft of metalworking, although metalworking relies on metallurgy, as medicine relies on medical science, for technical advancement. The science of metallurgy is subdivided into chemical metallurgy and physical metallurgy.

Crystal habit Mineralogical term for the visible shape of a mineral

In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. A single crystal's habit is a description of its general shape and its crystallographic forms, plus how well developed each form is.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

The mechanism behind metal whisker growth is not well understood, but seems to be encouraged by compressive mechanical stresses including:

Residual stress

Residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. However, unintended residual stress in a designed structure may cause it to fail prematurely.

Electroplating creation of protective or decorative metallic coating on other metal with electric current

Electroplating is a process that uses an electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode. The term is also used for electrical oxidation of anions on to a solid substrate, as in the formation of silver chloride on silver wire to make silver/silver-chloride electrodes. Electroplating is primarily used to change the surface properties of an object, but may also be used to build up thickness on undersized parts or to form objects by electroforming.

Diffusion Net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential)

Diffusion is net movement of anything from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in concentration.

Metal whiskers differ from metallic dendrites in several respects; dendrites are fern-shaped, and grow across the surface of the metal, while metal whiskers are hair-like and project perpendicularly to the surface. Dendrite growth requires moisture capable of dissolving the metal into a solution of metal ions which are then redistributed by electromigration in the presence of an electromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does not require either dissolution of the metal or the presence of an electromagnetic field.

Dendrite (metal)

A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal solidifies, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regard to material properties.

Fern group of plants, usually a class

A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissues that conduct water and nutrients and in having life cycles in which the sporophyte is the dominant phase. Ferns have complex leaves called megaphylls, that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns, sometimes referred to as true ferns. They produce coiled fiddleheads that uncoil and expand into fronds. The group includes about 10,560 known extant species.

Electromigration transport of material caused by the gradual movement of the ions in a conductor

Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in microelectronics and related structures. As the structure size in electronics such as integrated circuits (ICs) decreases, the practical significance of this effect increases.

Effects

Several mm long zinc whiskers on zinc coated steel Zinc whiskers.jpg
Several mm long zinc whiskers on zinc coated steel

Whiskers can cause short circuits and arcing in electrical equipment. The phenomenon was discovered by telephone companies in the late 1940s and it was later found that the addition of lead to tin solder provided mitigation. [4] The European Restriction of Hazardous Substances Directive (RoHS), that took effect on July 1, 2006, restricted the use of lead in various types of electronic and electrical equipment. This has driven the use of lead-free alloys with a focus on preventing whisker formation, see § Mitigation and elimination. Others have focused on the development of oxygen-barrier coatings to prevent whisker formation. [5]

Airborne zinc whiskers have been responsible for increased system failure rates in computer server rooms. [6] Zinc whiskers grow from galvanized (electroplated) metal surfaces at a rate of up to a millimeter per year with a diameter of a few micrometres. Whiskers can form on the underside of zinc electroplated floor tiles on raised floors due to stresses applied when walking over them; these whiskers can then become airborne within the floor plenum when the tiles are disturbed, usually during maintenance. Whiskers can be small enough to pass through air filters and can settle inside equipment, resulting in short circuits and system failure.

Tin whiskers don't have to be airborne to damage equipment, as they are typically already growing in an environment where they can produce short circuits. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature antennas, affecting the circuit impedance and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures. Tin whiskers often cause failures in relays, and have been found upon examination of failed relays in nuclear power facilities. [7] Pacemakers have been recalled due to tin whiskers. [8] Research has also identified a particular failure mode for tin whiskers in vacuum (such as in space), where in high-power components a short-circuiting tin whisker is ionized into a plasma that is capable of conducting hundreds of amperes of current, massively increasing the damaging effect of the short circuit. [9] The possible increase in the use of pure tin in electronics due to the RoHS directive drove JEDEC and IPC to release a tin whisker acceptance testing standard and mitigation practices guideline intended to help manufacturers reduce the risk of tin whiskers in lead-free products. [10]

Silver whiskers often appear in conjunction with a layer of silver sulfide which forms on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulfide and high humidity. Such atmospheres can exist in sewage treatment and paper mills.

Whiskers over 20 µm in length were observed on gold-plated surfaces and noted in a 2003 NASA internal memorandum. [11]

The effects of metal whiskering were chronicled on History Channel's program Engineering Disasters 19.

Mitigation and elimination

Several approaches are used to reduce or eliminate whisker growth with ongoing research in the area.

Conformal coatings
Conformal compound coatings stop the whiskers from penetrating a barrier and reaching a nearby termination and forming a short. These include barriers made of a ceramic or polymeric compound. Polymeric compounds tend to deflect the whisker away while ceramic chemistries prevent puncturing of the coating. [12]
Altering plating chemistry
Termination finishes of nickel, gold or palladium have been shown to eliminate whiskering in controlled trials. [12] [13]

Tin whisker examples and incidents

Galaxy IV

Galaxy IV was a telecommunications satellite that was disabled and lost due to short circuits caused by tin whiskers in 1998. It was initially thought that space weather contributed to the failure, but later it was discovered that a conformal coating had been mis-applied, allowing whiskers formed in the pure tin plating to find their way through a missing coating area, causing a failure of |to|100|kg||||}} per payload. [14]

Millstone Nuclear Power Plant

On April 17, 2005, the Millstone Nuclear Power Plant in Connecticut was shut down due to a "false alarm" that indicated an unsafe pressure drop in the reactor's steam system when the steam pressure was actually nominal. The false alarm was caused by a tin whisker that short circuited the logic board that was responsible for monitoring the steam pressure lines in the power plant. [15]

Toyota accelerator position sensors false positive

In September 2011, three NASA investigators claimed that the tin whiskers they identified on the Accelerator Position Sensors [16] of sampled models of Toyota Camry could contribute to the "stuck accelerator" crashes affecting certain Toyota models during 2005–2010. [17] This contradicted an earlier 10-month joint investigation by The National Highway Traffic Safety Administration (NHTSA) and a large group of other NASA researchers that found no electronic defects. [18]

However, in 2012 NHTSA maintained: "We do not believe that tin whiskers are a plausible explanation for these incidents...[the likely cause was] pedal misapplication." [19]

Toyota also maintains that tin whiskers were not the cause of any stuck accelerator issues: "In the words of U.S. Transportation Secretary Ray LaHood, 'The verdict is in. There is no electronic-based cause for unintended high-speed acceleration in Toyotas. Period.'" According to a Toyota press release, "no data indicates that tin whiskers are more prone to occur in Toyota vehicles than any other vehicle in the marketplace." Toyota also states that "their systems are designed to reduce the risk that tin whiskers will form in the first place." [20]

See also

Related Research Articles

Solder metal alloy used to join together metal pieces with higher melting points

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid". In fact, solder must first be melted in order to adhere to and connect the pieces together after cooling, which requires that an alloy suitable for use as solder have a lower melting point than the pieces being joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Ball grid array

A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.

Tin pest Tin pest is an autocatalytic, allotropic transformation of tin, deterioration of tin objects at low temperatures. Tin disease, tin blight tin leprosy, lèpre détain

Tin pest is an autocatalytic, allotropic transformation of the element tin, which causes deterioration of tin objects at low temperatures. Tin pest has also been called tin disease, tin blight or tin leprosy.

Flux (metallurgy) type of chemicals used in metallurgy

In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

The Restriction of Hazardous Substances Directive 2002/95/EC, (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Union.

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

Electronic throttle control automobile technology

Electronic throttle control (ETC) is an automobile technology which electronically "connects" the accelerator pedal to the throttle, replacing a mechanical linkage. A typical ETC system consists of three major components: (i) an accelerator pedal module, (ii) a throttle valve that can be opened and closed by an electric motor, and (iii) a powertrain or engine control module. The ECM is a type of electronic control unit (ECU), which is an embedded system that employs software to determine the required throttle position by calculations from data measured by other sensors, including the accelerator pedal position sensors, engine speed sensor, vehicle speed sensor, and cruise control switches. The electric motor is then used to open the throttle valve to the desired angle via a closed-loop control algorithm within the ECM.

Gold plating

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

Galaxy IV was a model HS-601 satellite built by Hughes Space and Communications Company (HSC). The satellite, which carried a payload of both C band and Ku band transponders, was launched on June 24, 1993 and operated by PanAmSat Corporation. It was in geostationary orbit at 99°W.

Tinning covering object with layer of tin

Tinning is the process of thinly coating sheets of wrought iron or steel with tin, and the resulting product is known as tinplate. The term is also widely used for the different process of coating a metal with solder before soldering.

Dip soldering

Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board, creating a reliable mechanical and electrical connection.

Mechanical plating, also known as peen plating, mechanical deposition, or impact plating, is a plating process that imparts the coating by cold welding fine metal particles to a workpiece. Mechanical galvanization is the same process, but applies to coatings that are thicker than 0.001 in (0.025 mm). It is commonly used to overcome hydrogen embrittlement problems. Commonly plated workpieces include nails, screws, nuts, washers, stampings, springs, clips, and sintered iron components.

A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting the package to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.

Soldering process of joining metal pieces with heated filler metal

Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained lead, but environmental and health concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

2009–11 Toyota vehicle recalls

The 2009-11 Toyota vehicle recalls involved three separate but related recalls of automobiles by Toyota Motor Corporation, which occurred at the end of 2009 and start of 2010. Toyota initiated the recalls, the first two with the assistance of the U.S. National Highway Traffic Safety Administration (NHTSA), after reports that several vehicles experienced unintended acceleration. The first recall, on November 2, 2009, was to correct a possible incursion of an incorrect or out-of-place front driver's side floor mat into the foot pedal well, which can cause pedal entrapment. The second recall, on January 21, 2010, was begun after some crashes were shown not to have been caused by floor mat incursion. This latter defect was identified as a possible mechanical sticking of the accelerator pedal causing unintended acceleration, referred to as Sticking Accelerator Pedal by Toyota. The original action was initiated by Toyota in their Defect Information Report, dated October 5, 2009, amended January 27, 2010. Following the floor mat and accelerator pedal recalls, Toyota also issued a separate recall for hybrid anti-lock brake software in February 2010.

Sudden unintended acceleration (SUA) is the unintended, unexpected, uncontrolled acceleration of a vehicle, often accompanied by an apparent loss of braking effectiveness. Such problems may be caused by driver error, mechanical or electrical problems, or some combination of these factors.

Failure of electronic components Ways electronic elements fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.

References

  1. Lyudmyla Panashchenko. "Whisker Resistant Metal Coatings" (PDF). NEPP NASA. Retrieved 23 October 2013.
  2. Craig Hillman; Gregg Kittlesen & Randy Schueller. "A New (Better) Approach to Tin Whisker Mitigation" (PDF). DFR Solutions. Retrieved 23 October 2013.
  3. Sun, Yong; Hoffman, Elizabeth N.; Lam, Poh-Sang; Li, Xiaodong (2011). "Evaluation of local strain evolution from metallic whisker formation". Scripta Materialia. 65 (5): 388–391. doi:10.1016/j.scriptamat.2011.05.007.
  4. George T. Galyon. "A History of Tin Whisker Theory: 1946 to 2004" (PDF). iNEMI. Retrieved 21 December 2012.
  5. "Whisker Effect". INELCO. Retrieved 5 January 2011.
  6. "Zinc whisker induced failures in electronic systems". ERA Technology. Archived from the original on 16 January 2013. Retrieved 21 December 2012.
  7. "Event Notification Report for July 12, 1999". U.S. Nuclear Regulatory Commission. Retrieved 21 December 2012.
  8. "ITG Subject: Tin Whiskers – Problem, Causes, and Solutions". Food and Drug Administration. 1986-03-14. Archived from the original on October 18, 2007. Retrieved 21 December 2012.
  9. Jay Brusse; Henning Leidecker; Lyudmyla Panashchenko (December 5, 2007). "Metal Whiskers: Failure Modes and Mitigation Strategies" (PDF). NASA. Retrieved 21 December 2012.
  10. "JEDEC and IPC Release Tin Whisker Acceptance Testing Standard and Mitigation Practices Guideline". JEDEC.org. 4 May 2006. Retrieved 5 January 2011.
  11. Alexander Teverovsky (April 2003). "Introducing a New Member to the Family: Gold Whiskers" (PDF). NASA. Retrieved 21 December 2012.
  12. 1 2 John Burke (September 2010). "Tin Whiskers Eliminated".
  13. Keun-Soo Kim, Suk-Sik Kim, Seong-Jun Kim, Katusaki Suganuma, ISIR, Osaka University, Masanobu Tsujimoto, Isamu Yanad, C. Uyemura & Co., Ltd., Prevention of Sn whisker formation by surface treatment of Sn plating Part II, TMS Annual Meeting, 2008
  14. Bruce Felps. "'Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon". Archived from the original on March 3, 2009. Retrieved 25 October 2013.
  15. "Reactor Shutdown: Dominion Learns Big Lesson From A Tiny 'tin Whisker'" (PDF).
  16. "Treatise" (PDF). nepp.nasa.gov.
  17. Bunkley, Nick (27 March 2018). "Toyota Issues a 2nd Recall Over Accelerators". NYTimes.com.
  18. "NHTSA-NASA Study of Unintended Acceleration in Toyota Vehicles". NHTSA. Retrieved 14 November 2014.
  19. "NHTSA rejects 'tin whiskers' theory for Toyota's unintended acceleration incidents". Automotive News. Retrieved 14 November 2014.
  20. "'Tin Whiskers' and Other Discredited Unintended Acceleration Theories". Toyota. 24 January 2012. Retrieved 29 September 2019.