Formation and evolution mechanism of metal whiskers in extreme aerospace environments:A review

Zekun WANG, Shiming WANG

a School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200444, China

b Department of Mechanical Engineering, Auburn University, Auburn AL 36849, USA

c College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China

d Shanghai Engineering Research Center of Marine Renewable Energy, Shanghai 201306, China

KEYWORDS

Abstract The spontaneous growth and evolution mechanism of metal whiskers have long been scientific problems.With the development of the integration of electronic and electrical productions,short circuits and system failures are raised by metal whiskers continuously.In the meantime, the related theories and mechanisms of whiskering problem are still vague,leading to a deficiency in the studies of environmental factors influencing the whisker phenomenon.Besides,the extreme environments such as aerospace, have been proven the accelerators to the formation of metal whiskers,resulting in a severe threaten to equipment and devices working in such environments including satellite and military equipment.To establish a comprehensive understanding to the whiskering process associated with their applicable control strategies, this study analyzes the growth phenomenon, influencing factors, formation process and evolution mechanism of metal whiskers in extreme service environments,puts forward the corresponding controlling strategies,offers a reference for the establishment of Chinese extreme aerospace strategic environment, and improves the reliability of aerospace systems.

1.Introduction

Metal whiskers are single crystal metal filaments growing spontaneously from metal surfaces of electrical connectors,electronic components and adjacent conductors.1Most whiskers observed are cylindrical in shape, with average diameters(equal to the grain size where it erupts from) varying around 1 μm.Under certain environmental conditions, a whisker can grow to several millimeters.The material of whiskers is much similar to the metals or alloys where they grow from,whiskers usually possess high conductivity.2For example, the whiskers that grow from pure tin(Sn)coating layer have the conductivity about 67 W/(m?K).Whiskers have been observed causing numerous system failures through short circuits mostly, and even fire and explosion accidents on various equipment and devices, and have been reported leading to loss of tens of billions of dollars3–5(Table 1).Multiple systems serving at nonreplaceable and non-repairable positions or equipment which requires extremely stable are also under the threaten of metal whiskers, such as on-orbit satellites (Galaxy IV and HS-601),F15 fighter radar, rocket engines, missiles, atomic reactors,weapons, etc.

Even since the first observation of cadmium whiskers around eighty years ago, the formation mechanism of whiskers, the evolution law, identifying the key influencing factors and control strategies, has long been strong motivation in cutting-edge basic scientific problems in many disciplines such as international materials, mechanics, machinery, microelectronics, etc.6

In recent years, as various components and systems have been developing rapidly towards high frequency, high speed,miniaturization, compact integration and multi-function, the threat of whiskers has been becoming one of the major difficulties and popular points in the research of international theory and engineering circles in the Chinese aerospace industry.7It is also a challenging problem to be solved in the establishment of air, space and sea integrated strategic security system in China.8

In this paper, we briefly introduce the hazards of metal whiskers to crafts and devices working in extreme aerospace environments.We detailedly collect and discuss the major whisker growing mechanisms, list most of the influencing factors and the roles that they play, and analyze the suggestions and conclusions that researchers made to alleviate whiskering problems in aerospace environments.Finally, associated with the latest technology including 5G network and 3D real-time visible simulation and monitoring system, we theoretically derive possible future research areas to deal with whiskering problem and maximize the reliability and life cycle of devices and creatures serving in extreme aerospace environments.

2.Spontaneous growth of metal whiskers and influencing factors

The first record of metal whiskers occurred in the early 1940s,reported by Bell Labs Cobb of the United States, which put forward the conception and problems of metal whiskers to public.9They claimed that cadmium whiskers were the major reason of short circuit in military equipment, and proposed to replace cadmium with tin and zinc to eliminate whiskers.Later, multiple researches reported that whiskers could grow not only on cadmium plated parts, but also on other metals(tin, aluminum, zinc, etc.).10–12Further research settled on the characteristics of metals and alloy which are easy to incubate whiskers, including low melting point, high ductility and high conductivity.They also summarized the common materials which are under the hazardous threat of whiskering problem, including tin, zinc, cadmium, lithium, bismuth, calcium,tellurium, indium, etc.13,14Besides, later researchers expanded this material group with high melting point metals and their alloys, such as aluminum, copper, nickel, cobalt, iron, silver,gold,etc.15,16In addition,non-metallic materials such as semiconductors,ceramics, silicon, graphite,metal oxides, carbides,halides and polymers also produced whiskers.17–20

Numerous theories and experiments are approaching one same consensus that the force within the coating material is the fundamental driving factor to whiskering, no matter compressive or tensile.The whisker growth process is illustrated in Fig.1.The inner structure of metal or alloy coating is shown in Fig.1(a) as a cut-section, with numerous grains stacking to form the coating layer, which results in an ununiformed stress dissipation strategy.With the effect of continuous in-layer forces, crystal structures are shaped between the neighboring grains as a method of force releasement(Fig.1(b)).Under various effects dominated by matter exchange, deformation and diffusion, the coating material can get through the grainboundaries(for whiskers,they are most oxidations)and serves as the feedstock of whiskers, as shown in Fig.1(c).Finally, a whisker,consisting of the coating material and covered by oxidations, is observed ‘‘grown” from the layer surface.

Table 1 Accident name and cause of aerospace equipment caused by metal whisker.

Fig.1 Schematic diagrams illustrating whisker growth process.

Associated with the growing mechanism, multiple factors have been carefully tested and classified to several types which influence the whiskering problem in different processes(Table 2).We can easily reach the conclusion that the whiskering phenomenon closed bonded to the whole coating aging process.All the stages of coating layer, including manufacturing, pre-coating, coating, oxidizing and aging, have been concretely confirmed affecting the whiskering process.

Table 2 Classified factors affecting whiskering process.

The major method where most of these factors influence the whiskering process is generating in-layer forces.We collect and organize the process from external conditions to final whisker growth through multiple in-layer process in Fig.2.Fig.2 describes the whiskering problem based on the force accumulation theory, and every influencing factors and external disturbances will stimulate whisker growth through the in-layer stress.Despite the traditional process including crystal dislocation and material diffusion, the Dynamic Recrystallization(DRX)mechanism,as one of the most popular processes illustrating the in-layer phenomenon, is also included.The DRX process is initiated with the strain exceeding one critical threshold, and can provide continuous stress to the coating layer which finally results in the layer deformation, and whiskering formation.For future works, the energy absorption and dissipation should also be considered,as well as the gravity energy and growth velocity changes of whiskers during growth.

Among all the factors that we mentioned in Fig.1, some can be easily accepted as the source of the stress and strain,while the relationship between others and whisker growth is ambiguous.Here,we choose the factor of temperature change as an example to illustrate the correlation between it and whiskers (Fig.3).Fig.3(a) gives a brief view of an ordinary coated item, with the coating layer deposited onto the substrate.Fig.3(b) illustrates a cut-section view of the item.The contact surface of substrate and coating is not smooth as we see with naked eyes,but with numerous convex and concave distributions.Specially, traditional deposition will also introduce hollows between them, which is not illustrated in Fig.3.Those uneven contacting surface can be roughly classified into low-adhesive and high-adhesive areas,marked as blue and red in Fig.3.In a stable environment, such classification does not contribute much to whisker growth but just influences local diffusivity.However,this conclusion is not applied when the item is moved into a different environment.As the temperature increases,both the substrate and coating begin to expand laterally and horizontally.In most cases, there exists a difference between the two Coefficients of Thermal Expansion(CTE), leading to a different expansion dimension laterally.During this process,those high-adhesive areas will be relatively fixed, exerting lateral in-layer forces pointing from highadhesive to low-adhesive ones (Fig.3(c)).Similar situations also happen with variations of humidity, potential, current,etc.

Fig.2 Relationship among multiple processes occurring in coating layer and several major methods of external conditions affecting final whiskering process.

Fig.3 Schematic diagrams illustrating how temperature will affect whisker growth.

Although numerous conclusions and suggestions on whisker alleviation and elimination were proposed, the growth and evolution mechanism of metal whiskers represented by tin whiskers was still vague and interdisciplinary, and none of those conclusions can effectively cease the whisker formation.Ever since the first observation of whiskers in the 1940s, engineers had carried out exploratory research on the formation mechanism of whiskers, associated with the influencing factors, reliability threat and control strategies.21,22The researchers from the US whisker laboratory of National Aeronautics and Space Administration (NASA), National Nuclear Safety Administration (NNSA), European Space Agency (ESA)and many other scholars classify factors affecting whisker growth into internal factors and external environmental factors: (A) Most internal factors are related to characteristics of coating and substrate materials, including thickness, coating area, grid structure, coefficient of thermal expansion,atomic diffusivity,etc.;23(B)External environmental factors contain mechanical stress, temperature, humidity,oxygen content,salinity,vibration,noise,radiation,electromigration,etc.6Among all these conditions,the periodic cycles of temperature and pressure would increase the whisker growth rate progress with a tremendous value.Practical researches suggested controlling these factors to inhibit whisker formation.24

3.Analysis on metal whiskering phenomenon in extreme aerospace environment

3.1.Failures caused by metal whiskers in aerospace electronic equipment

Metal whiskers can grow spontaneously at room temperature,but the growth rate is quite slow,at around 0.03–0.90 mm/year generally.Besides, the obvious growth of whiskers follows an overlapping latency in most cases, which is another obstacle for whisker pre-prevention.Therefore, it is difficult to accurately predict when and exactly where whiskers erupt and grow, which makes the pre-prevention for whisker formation much difficult.25Further, in the extreme environments, such as undersea and in space, where multiple factors are coupled,the growth rate of whiskers would increase sharply.26As the metal whiskers possess ultra-conductivity associated with excellent high temperature resistance, they will lead to transient short circuit or permanent short circuit of the circuit,reduce the reliability of aviation and aerospace electronic equipment and lead to catastrophic accidents.Particularly,when faults occur in such occasions, there exist little chances for us to repair them.27

Table 1 lists several recorded accidents of aerospace equipment with their major reasons.Most of them are caused by metal whiskers, leading to severe catastrophe.In 1986, when inspecting faulty circuits of 12-year radar systems, the US Air Force found whiskers up to 2.5 mm growing from the tin coating of the hybrid circuit.28In 1990,several commercial spacecraft failure cases that occurred in the US were caused by tin whiskers.Since 1998, accidents on abnormal operation of satellite electronic equipment and collapse of computer center caused by metal whiskers have occurred for several times in satellites developed and operating in orbit.One of the most famous failures attributing to tin whiskers was the Galaxy IV satellite after entering orbit in 1998, and the telecommunications satellite was accidentally lost due to a short circuit,shutting down nearly 90%of pagers in the US for several days.Later,it was found that the conformal coating was mistakenly sputtered to the circuit board during manufacture process,leading to the fact that the tin whiskers erupted from the pure tin coating and spread through the uncoated area, and finally bridging neighboring circuits and resulting in the failure of the main control computer.29,30Brusse et al.studied the whisker problem of HS-601 satellite, as well as several other HS-601 satellites damaged in orbit due to the growth of tin whiskers in the relay, which caused loss of hundreds of millions dollars to the possessors and their insurance companies.31The continuous short circuit caused by tin whiskers led to the failure of commercial satellites, which prompted Goddard Space Flight Center(GSFC)of the NASA to issue a consultation document(NA-044 and NA-044a), reminding the phenomenon of tin whiskers and the inherent risks associated with the use of pure tin-plated components.32

Several researchers summarized aerospace failure modes caused by whiskers, mainly including:33–36

(1) Transient short circuit

When the current exceeds a certain limit(generally 30 mA),the whisker will be temporarily fused,resulting in an intermittent short-circuit pulse, which is generally difficult to be located.When more than 50 mA fusing current is generated,whisker will cause a short circuit between terminals, terminal to connector and shells to other components.

(2) Permanent short circuit

When a whisker grows over a certain length and diameter,the current of low voltage and high impedance circuit is not enough to fuse the tin whisker, resulting in a permanent short circuit.

(3) Vacuum arc

In vacuum environment (or low air pressure), the solid metal whisker between the tin whisker and the adjacent conductor is evaporated into a highly conductive metal plasma,which can form arc with a hundred of amperes current.With the decrease of air pressure, the metal vapor arc induced by whisker maintains with limited power, resulting in a highly conductive path while causing damage.Richardson’s experiment has proved that at an atmospheric pressure of about 150 Torr(1 Torr=133 Pa),tin whiskers will trigger a continuous metal vapor arc with a voltage of about 13 V(or higher)and a current of 15 A(or higher).The maintenance of current arc depends on the tin on the coating surface.Such arc can last for a long time until one of the following conditions is reached:(A) The tin on the coating surface is consumed; (B) The current is terminated;(C)Interrupted by circuit protection devices(such as fuses and circuit breakers); (D) Other arc extinguishing processes occur.It is reported that the metal vapor arc in the vacuum occurred on at least three commercial satellites,causing the fuse to burn out, resulting in the failure of precision parts of the spacecraft and even failure to work.

(4) Pollution

Loose whisker debris may fall off from the substrate due to external impulses.They further release solid pollutants which may interfere with the circuit surface or bridge the exposed conductor, move around the equipment, and even reach the area not affected by short circuit or arc,so as to generate false signals at the incorrect position and lead to the wrong action of the equipment (Fig.4).

To alleviate with the threat of whiskers to satellites, Abtew and Selvaduray,37Morgan38of NASA stated that the satellite failures had sounded an alarm to users, which arose concerns about the pure tin coating problem of the European Space Agency.The tin whiskering problem of aerospace, national defense and medical equipment have attracted numerous attentions.Leidecker and Brusse summarized the history of system failure caused by whiskers and the failure problems of satellite and other equipment and electromagnetic relay parts caused by thermal cycling whiskers and electromigration.39

Though multiple factors have been confirmed contributing to the whisker growth process, the quantitative relationship between these factors and whisker growth is still in vague.To overcome this theoretical gap, researchers need to start with the mechanism analysis of environmental factors on whisker growth, derivate quantitative theories, and then find the strategy to inhibit whisker growth.

3.2.Analysis on whisker factors in extreme environment

During the process of aircraft from take-off to landing, electronic components bear different environmental condition with multiple factors varying extreme other than in the lab:an acceleration of more than 0.5g, a large temperature range from less than –60 °C at high altitude to more than 1000 °C during landing, a low pressure maintaining at 5% of the ground pressure,40etc.Such special environmental conditions have been proven will accelerate the growth rate of whiskers in avionics systems and components.41,42

To understand the meteorology, geology and life activities of space and outer planets, researchers designed and launched various satellites, manned spacecraft, space shuttles and other detectors.The environmental data of these equipment change greatly between actions in the space, from launch to orbit change, orbit determination, landing, operation and recovery.Electronic equipment in these areas usually serves in an extreme harsh service environment, such as heavy load and weight loss caused by the high acceleration, circulating pressure and temperature with large variations, supercritical high vacuum, strong noise, strong magnetic field, strong radiation of almost all frequencies, heavy corrosion, etc.

Orbits of earth orbit missions are generally classified as the Geosynchronous Orbit (GEO), and the Low Earth Orbit(LEO)which we concern frequently.The LEO spacecrafts with the altitude less than 1852 km are designed for satellites of communication, military, and scientific applications.Generally, the orbits of satellites are approximately circular, but some are elliptical.One type of LEO satellites is the solar synchronous orbit.The orbital plane of the spacecraft maintains an almost fixed angle with the sun,which allows the spacecraft to cross the equator at the same earth time in each orbit.

During the launch process, the electronic system and highvoltage solar cell array power supply system of the satellite withstand positive and negative acceleration of ±8g and high temperature flow.Meanwhile, electronic components suffer large noise and vibration from rocket launchers.When the satellites separate from the rocket in space, a high-amplitude vibration occurs to the whole structure of the satellite.Such dynamic impact makes the satellite structure bear highfrequency and amplitude waves with concentrated frequencies,which propagate in the whole structure.Therefore, the effects of shock vibration, thermal stress and high-frequency vibration on whisker growth should be considered in the design stage of electronic components used in space.35

Fig.4 Examples of short circuit caused by tin whiskers of a transistor and detailed inside view in satellites.

When in operation in orbit,the satellites work in a different environment.The organic matter on spacecraft surface will react and peel with O3from the top of the atmosphere associated with an oxygen atom.The plasma flux and energy in synchronous altitude region and low altitude polar region increase sharply.In space, the temperature circulates cycling between–30 °C and 125 °C periodically.The satellites release voltage pulses of up to 20000 V.Besides, the temperature of low orbit spacecrafts and other objects are majorly maintained by the acquisition or loss of radiant energy as the energy dissipation method.There are three major sources where the LEO spacecrafts receive radiation from: solar radiation from the sun,solar energy reflected from the earth’s surface (albedo) and long wave radiation emitted by the earth’s atmosphere.In most of the year, LEO spacecrafts are exposed to solar radiation,resulting in a quite slight thermal cycle for the period of a year.While the time for LEO satellites to operate surround the earth is about 90 min, they rotate more than 6000 cycles around the earth in one year; especially, at the frequency of 2000–3000 cycles per year,the thermal cycle stimulates whisker growth most,which could cause vacuum plasma arc discharge,leading to a disastrous impact to the electrical system.Such accidents of on-orbit satellites occur from time to time.43

As mentioned above, radiation is the major method for heat transfer in the vacuum environment, while the solar system is full of thermal radiation.In geostationary orbit, the majority radiation is transferred through outer electrons and solar protons, making the temperature exceed 2000 °C in certain range on the orbits.The radiation source is affected by the solar activity.The solar activity cycle can be classified into two stages:the solar minimum and the solar maximum.As a result,related electronic devices have to withstand extreme temperature cycles.It is still to be determined which period stimulates or alleviates the whisker growth.

Besides, in the near space operations, spacecrafts periodically enter and exit the sunshine area and shadow area.When entering the sunshine area from the shadow area, the solar radiation heat flow increases sharply, causing the vibration of the spacecraft (thermal vibration).When operating into the dark side, the temperature decreases by 300 °C compared with the sunshine area.Such vibration and thermal cycles accelerate whisker growth process, and bring threat to spacecrafts attitude response and data signal acquisition.44

Further, the temperature range varies widely on different planets.During the day and night of the moon, the temperature of the surface varies from –160 °C to 200 °C, resulting in a temperature step of 360 °C, while the working environment temperature of lunar rovers is designed between–150 °C and 600 °C.Carbon dioxide in the Martian atmosphere takes a proportion of approaching 96vol%, making the atmosphere thin and dry, and resulting in a poor thermal insulation performance.The temperature difference between day and night is as large as 120 °C∶20 °C in the day and suddenly drop to–100°C during night at places near the equator.The temperature at the two poles is much lower, with a minimum of –139 °C.45The atmosphere of Venus is distinguished from the former two, where Sulfur takes most of its atmosphere.The average global temperature is as high as 480 °C due to the greenhouse effect, day and night.Spacecrafts such as the Orion are designed to take humans to Mars and deep space, where the temperature can be close to 2000 °C.46

In deep space exploration,due to the long-distance flight of spacecraft,the advantages of high specific energy and low selfdischarge of lithium-ion battery are particularly prominent.Lithium battery possesses the lowest standard electrochemical redox potential but relatively high theoretical specific capacity,which make it the most adaptable cathode material of rechargeable battery.Sub-detectors of Galileo launched into Jupiter’s atmosphere in 1989 were powered by lithium sulfur batteries.Lithium-ion batteries are now widely utilized as energy provider in deep space exploration programs in recent years, such as the smart January ball probe launched by the ESA,the Mars Express,the‘‘Spirit”and‘‘Opportunity”Mars probes launched by NASA, etc.NASA cooperated with the University of Miami to develop a new type of solid-state lithium battery for microsatellites use, such as ‘‘CubeSats”.Chinese lunar probe program II also designs to use lithiumion batteries as the energy storage and power supply for landers and lunar rovers.However, in the practical application,batteries are often affected by the vibration, noise, impact and operating temperature of the active section, during which the lithium whisker grows spontaneously.Whiskers continuously consume the electrolyte, deplete the active lithium and eventually lead to the short circuit of the battery.47

In fact,accidents caused by lithium batteries in the aviation field can be traced back to as early as in 2013.On January 7,2013,a fire case on an empty Boeing 787 Dreamliner of Japan Airlines happened at Boston airport, where the auxiliary power unit of lithium-ion battery was suspected of causing the accident.Similar incident occurred to the lithium-ion battery in one Chevrolet voltage collision test.Engineers of Boeing, a multinational aerospace and defense company of the United States started an investigation on the battery problem,and revealed that the metal whiskers led to system short circuit.48Based on these accidents,scholars from the Preparatory School of Notre Dame University studied the metal whiskers of nickel cadmium batteries.49

4.Formation and evolution mechanism of whiskers in extreme space environment

4.1.Formation of whiskers in extreme space environment

Multiple factors have been confirmed able to influence the whiskering problem.Among them,the major factors affecting the whisker formation are temperature,50,51thermal stress,35thermal shock,52pressure,53etc.Under the comprehensive effect of high temperature,thermal shock,large-variation thermal cycle associated with other ones,the growth of whiskers is accelerated sharply.54–56

In extreme environments such as space, multiple factors build the environments universal but complex for all the spacecrafts including vacuum, weightlessness, noise, magnetic storm, electric and magnetic field, radiation, electron irradiation, etc.57,58Therefore, the destructive damage to electronic equipment by space radiation effect is the primary consideration during the design and operation of spacecraft.59,60The research results in this field have illustrated the effect of these factors on whiskering process,such as strong magnetic field on the behavior of whiskers from Bi,Ga and Sb doped tin-plated films,61the high-frequency vibration of whiskers caused by microwave radiation,62γ and X-rays on the accelerated growth of whisker,63,64and high-frequency radiation on the formation and evolution of tin whiskers.26,65Killefer et al.studied γ ray,and the experimental results show that the electrostatic field induced by γ ray affects the dynamics process of whisker formation, where the growth of tin whiskers is accelerated under non-destructive γ ray by 50%.66

In the vacuum environment, in addition to the growth rate of whiskers which can be measured directly,the whisker mechanism is more complex due to the coupling of many extreme factors.67Without air as insulator, the electronic system in spacecrafts produces plasma arc between circuits.Studies have illustrated that in vacuum,the average diameter of tin whisker is 3 μm with low strength (tensile modulus of 8–85 GPa and tensile strength limit of 8 MPa) but high current conductivity is up to 32 mA.Besides, such whiskers will not even be disturbed by broadband spectrum vibration or mechanical shock up to 200g.35,68,69

It has been confirmed that high temperature accelerates the growth of whiskers, but under vacuum associated with low temperature, whiskers will also be produced due to the phase transformation of the coating.70Scholars have studied the nonlinear dynamics of tin whisker micro resonators under vacuum and extremely high temperature, which largely damage the reliability of electronic devices on the lunar rover on duty due to low temperature phase transformation triggering tin whiskers to grow.Besides,in the vacuum and anhydrous space environment, radiation, extreme temperature and sharp thermal alternation all accelerate the chemical corrosion of metals,which is also an important reason for the accelerated growth of whiskers.71,72

4.2.Whisker evolution mechanism

Theoretical and experimental studies show that the whisker evolution mechanism does not vary between in aerospace environment and in general environment.However,due to the coupling of various factors in extreme environment, the performance of whisker growth is affected: the reproducibility of whiskers is poor, the growth rate and unpredictability are large,and the whisker shapes are multiple.All these characteristics make the research on whisker growth mechanism more complicated.

Over the past 80 years, international researchers have put forward multiple mechanisms concerning the whisker formation process, including compressive stress mechanism, recrystallization mechanism, dislocation mechanism (including Frank read dislocation theory, two-stage dislocation of dislocation ring climbing and sliding), oxide film rupture mechanism, mass diffusion mechanism, creep mechanism, atom migration theory caused by strain gradient, process migration mechanism, ferromagnetic resonance mechanism, interfacial flow and grain boundary diffusion mechanism, etc.73–75In recent years, the DRX mechanism discussed by Vianco et al.of Sandia National Laboratory has now been widely accepted in the industry.76,77

With these mechanisms established, further research and verification are carried out through qualitative analysis, quantitative analysis, statistical analysis, approximate estimation,and physical model test, and produced new research results,such as spontaneous driving force and mechanism,78random growth mechanism,74double stress zone model,79stress driven constitutive model and numerical simulation,80main driving force mechanism,81,82etc.Osenbach et al.comprehensively discussed the concept, observation method, growth mode,driving force, modeling and control strategy of whiskers in his work ‘‘Sn-whiskers: Truths and myths”.83Several classic papers of Joe such as ‘‘Theory of tin whisker growth: The end game” have greatly promoted the theoretical research of metal whiskers.84The research group of Professor Sun Zhengming of Southeast University, associated with theoretical calculations and experiments, studied the spontaneous growth mechanism of tin whiskers in Ti2SnC from the perspective of atomic motion.85Through the regulation of Ti2SnC matrix composition, it is clear that the existence of free tin atoms is necessary for the spontaneous growth of tin whiskers in Ti2-SnC.The characterizations of the microstructure near whisker roots show that the tin whisker root is not connected with the free tin beneath the oxide Ti2SnC layer, implying that the method of tin atom supplying the whisker growth is similar to diffusion through the tin atomic layer in Ti2SnC.Ref.85 clarifies the atomic diffusion mechanism and the morphology formation mechanism of tin whiskers in the growth process of tin whiskers from the atomic scale, which also provides a theoretical basis for understanding the mechanism behind the growth of metal whiskers in numerous other materials.

However, most existing theories and mechanisms lack rigorous theoretical derivation and associated convincing experimental verification, which makes them fragmented.The recrystallization mechanism is not rigorous enough due to the fact that it has not been observed that the grains of the coating material have recrystallized at room temperature and there are obvious differences in the size and shape of the coating layer during the whole whisker growth process.The theory of atomic migration induced by strain gradient also failed to prove its the essence of whiskers theoretically.The mechanism of universality has not yet been formed.In particular, the single mechanism of whisker growth in the extreme environment of space has not been confirmed.86

The author’s team believes that the whisker growth mechanism is composed of material atoms of both coating and substrate.Though there are multiple internal and external factors affecting this process, the essence of their generation and evolution is the dynamic exchange of energy inside and outside the system and between atoms.The process can be divided into three stages: recovery, recrystallization and grain expansion.Therefore, with the support of the National Science Foundation (NSF), NASA, the National Nuclear Safety Administration, Texas Instruments and Tyco Electronics, based on the calculations of system energy and atomic micro diffusion dynamics,the atomic motion of whisker growth and evolution process can now be uniformly described, and the relationship among kinetic energy, length, density and growth rate of whisker in extreme environment is established in combination with experimental and theoretical calculation (Fig.587), and the geometric equations, growth and evolution equations of different scale structures, which is expected to open up a new way to uncover the myth of whisker mechanism.

Fig.5 Statistics of whisker density on step height samples with multiple structures.87

5.Whisker control strategy

To alleviate the threat of metal whiskers,scholars put forward a series of measurements from the aspects of management policy, risk assessment, mitigation and control strategy targeting to decrease or interrupt certain processes or steps of whisker growth in the extreme environment.

The first paper on tin whiskers concerning the reliability of electrical connectors was published by Diehl of Burndy connector company in 1993.88Dugenske et al.of the National Electronic Manufacturing (NEM) program put forward the integrated development roadmap of electronic production plants for the whiskering phenomenon in extreme environment.89From the perspective of management, Henshall et al.proposed method of bridging the gap in the supply chain to maximize the reliability of electronic components of high tin under that situation.90Wilson conducted research on the lead-free RoHS strategy in the procurement of military electronic products.91Sampson discussed the formulation of NASA’s lead-free policy for electronic products.92Chudnovsky put forward the conclusion as ‘‘fighting the military’s tin whisker threat that there is no leading strategy for products”.93

In terms of risk assessment, the advanced major research involves the following: comprehensive analysis of whiskering problem from pure tin coating on Hughes satellite,94evaluation method of battery for safe air transportation,95whisker risk assessment of space system,96research on conformal coating value and risk assessment of Sandia satellite project,97and new methods of metal plasma vapor arc potential and risk assessment.34Kadesch and Brusse studied the tin whisker inorganic coating evaluation project (twice) entrusted by the U.S.Department of Defense in 2001, which was reported potentially alleviating the impact of new lead-free alloys or processing on high-performance electronic systems and improving aerospace and defense supply and manufacturing systems.98

Aimed at the whisker suppression and control strategies in the aerospace field,scholars such as Brusse and Leidecker from the Green Belt Perot Systems Company in Maryland and Leidecker of NASA Goddard Space Flight Center analyzed multiple cases caused by tin whiskers on the commercial satellites of the center since 1998, as well as the electronic components of US space equipment since 1990, including electromagnetic relays, transistors, and hybrid microcircuit packaging, where the short circuits took the majority.99Other NASA scholars tried to use conformal coating to control tin whiskers in space environment and inhibit whiskers caused by satellite thermal stress.45There are also various studies on the common heatresistant materials such as Al2O3and SAC305 (tin-slivercopper or Sn-Ag-Cu) for the preparation of aerospace parts(Fig.6100).41,100–102Nishimi investigated the system failure raised by tin whiskers observed on the avionics box and hardware of the space shuttle control system,and put forward mitigation and elimination suggestions.103Kostic illustrated the latest research progress on the reliability of aerospace’s lead-free electronic products and proposed appliable corresponding whisker control strategy.104Based on the previous research, the standards for reducing and suppressing whiskers in aerospace and high-performance electronic systems have been gradually formed.105In 2012,to deal with the tin whiskering problem, the American National Standards Institute(ANSI) issued the standard for mitigating the effects of tin whiskers in aerospace and high-performance electronic systems(GEIA-STD-0005–2), and the process management for avionics—Aerospace and defence electronic systems containing lead-free solder—Part 2: Mitigation of deleterious effects of tin (IEC TS 62647–2-2012).

Fig.6 Failure on SAC305 solder joint in thermal cycle environment.100

For solar cells and lithium batteries in aerospace, the researchers from California State University cooperated with San Diego State University, the US Army Research Laboratory and General Motors R&D Center to study the formation mechanism of lithium metal battery whiskers.106Besides,there are multiple cases of improving and controlling whiskers through the artificial intelligence methods, such as Monte Carlo analysis of tin whisker interaction,107whisker nonlinear viscosity modeling,108,1093D microstructure whisker evaluation based on finite element analysis and other intelligent control methods.37,110,111

The whisker research group of the author is working in the CAVE3 laboratory,one of the international top research institutions on extreme environment reliability established by the NSF and the Department of National Defense relying on Auburn University.This lab aims to solve the whiskering problem between the microstructure surface of aerospace components and the interface of microsystem in extreme environments.Based on the principle that tin atom diffusion is the major reason of whiskers, we have reached the following two conclusions:

(1) The energy of the system needs to be reduced.

(2) Metal and alloy atoms must move at the whisker grain boundary, which also proves that whiskers are mainly produced at the grain boundaries rather than inside grains.

To reveal the volume threshold of whiskers, our lab proposed the strategy of inhibiting whisker growth by intelligently meshing the unit volume of base material and limiting the movement of atoms, which can effectively reduce the whisker growth by more than 90%.Researchers have formed a series of reliability intelligent control strategy system.87

Currently, there are multiple adaptable suggestions which can be applied to alleviate or even eliminate the growth of whiskers, as follows:

(1) Replace the material of substrate and coating to ones with low whisker sensitivity.Certain metals and alloys have been confirmed can grow very few and shorter whiskers compared to others, such as lead and leadcontained alloys, tin-silver alloys, etc.However, when alleviating whiskering problem, such materials usually cannot play its role well.For example, lead-contained alloys have long been applied as the coating material before manufacturers turned to pure tin, not only because pure tin possesses high conductivity, ductility lower melting point and especially, lower price, but also because lead-contained alloys are strongly harmful to human’s body.Furthermore, they would contaminate the environment for several hundreds of years during operating and after abolished.Ever since the leadcontained alloys were regulated, the alloys of tin-slivercopper (Sn-Ag-Cu, SAC) have been popular and might have the potentials to replace pure tin as coating material, for their excellent resistance, excellent resistance,compatibility with most flux type,and low melting point for Pb-free alloys (217–218 °C).

(2) Maintain the working environment stable and under low temperature.In multiple theories and studies, the eruption of whiskers starts from a weak place on the coating layer surface where the in-layer force and deformation are easy to release,and in the meantime the unpredicted stimulus from the outer environment usually initiates the whiskering process.Especially, in extreme space environment, the thermal cycling condition provided by spacecrafts operating in orbits can accelerate the whisker growth rate a lot.As a result, a stable working environment with less stimulus can effectively reduce the productivity of whiskers.However, the operation of devices will inevitably bring mechanical vibrations with certain frequencies, and researchers concerning this problem provided a new perspective that with stable vibration to stimulate whiskers to grow in large numbers but reduce in whisker length, which can also decrease the inner forces accumulated and the probability of short circuits due to the low whisker length.

(3) Improve the manufacturing process and protection methods.The places where whiskers grow most are the coating layers, interfaces and outer surfaces of devices,which provide conductivity, protection, or isolation to devices and equipment.Due to the characteristics of these parts and layers, whiskers are easy to grow with relatively high rate.So, we can improve and optimize the manufacturing processes to reduce the whiskering probability.For example, adapting compact devices with less interfaces could eliminate the area where whiskers might grow.Otherwise, optimizing the circuits and enlarging the safety distance between neighboring circuits on devices would also extend the service life of them.

6.Conclusions

The formation mechanism and suppression methods of spontaneous growth of metal whiskers in aerospace extreme environment on the reliability of electronic systems are studied, which can also be used for reference to improve the reliability of other extreme environment electronic components and systems.Due to the complex mechanism of metal whiskers, it is necessary to apply multi-disciplinary theories and knowledge such as mathematics, physical chemistry, material science, and cybernetics.From the perspective of system science and life cycle of production, further improvement can be conducted through the following methods:

(1) Conduct in-depth research on the integrated whisker reliability of equipment for the usage in extreme environments within each manufacturing and maintaining processes.

(2) Break through the logical and self-consistent basic scientific and theoretical reliability of systems.

(3) Establish the scientific knowledge system of whisker regulation and principal design.

(4) Develop the physical model of service equipment in typical extreme environment.

(5) Establish the digital model through physical law reconstruction and feature extraction.

(6) Realize the scene simulation and knowledge system expression of typical extreme environment.

(7) Realize the digital twin three-dimensional visual simulation based on 5G network.

(8) Realize the in-situ characterization, growth regulation,reliability prediction and inhibition strategy verification of whiskers in extreme coupling environments such as aerospace, and establish its standard system.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No.41976194).