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68-476: Control of Galaxy IV was lost on May 19, 1998 when the satellite's primary control processor failed. The backup control processor had suffered a previously undetected anomaly, and PanAmSat was not able to regain control of the spacecraft. Galaxy IV was declared a loss on May 20, 1998. Failure of the primary control processor was attributed to tin whisker growth, a phenomenon in which tendrils grow from solder, causing an electrical short circuit. Engineers believe that

136-428: A hillock failure or whisker failure ( short circuit ). Both of these situations can lead to a malfunction of the circuit. In a homogeneous crystalline structure, because of the uniform lattice structure of the metal ions, there is hardly any momentum transfer between the conduction electrons and the metal ions. However, this symmetry does not exist at the grain boundaries and material interfaces, and so here momentum

204-510: A 2003 NASA internal memorandum. The effects of metal whiskering were chronicled on History Channel 's program Engineering Disasters 19. Several approaches are used to reduce or eliminate whisker growth, with ongoing research in the area. Conformal compound coatings stop the whiskers from penetrating a barrier, reaching a nearby termination and forming a short. Termination finishes of nickel, gold or palladium have been shown to eliminate whisker formation in controlled trials. Galaxy IV

272-520: A conductance of G = v e 2 N / L E {\displaystyle G=ve^{2}\!N/LE} . In nano scale bridges the conductance falls in discrete steps of multiples of the quantum conductance G = 2 e 2 / h {\displaystyle G=2\,e^{2}\!/h} . Electromigrated Nanogaps have shown great promise as electrodes in use in molecular scale electronics. Researchers have used feedback controlled electromigration to investigate

340-483: A hole developed in the conformal wax coating over the solder, allowing whiskers to develop. The satellite manufacturer, Hughes, has replaced pure tin plating with nickel to alleviate the problem in newer designs, adding 100 to 200 pounds (50 to 90 kg) per payload. The loss of this satellite was very disruptive to telecommunications in the United States . 80% of pager service in the U.S. went down; service

408-453: A major focus of research efforts. Due to difficulty of testing under real conditions, Black's equation is used to predict the life span of integrated circuits. To use Black's equation , the component is put through high temperature operating life (HTOL) testing. The component's expected life span under real conditions is extrapolated from data gathered during the testing. Although electromigration damage ultimately results in failure of

476-673: A major industry effort to correct this problem. The first observation of electromigration in thin films was made by I. Blech. Research in this field was pioneered by a number of investigators throughout the fledgling semiconductor industry. One of the most important engineering studies was performed by Jim Black of Motorola , after whom Black's equation is named. At the time, the metal interconnects in ICs were still about 10 micrometres wide. Currently interconnects are only hundreds to tens of nanometers in width, making research in electromigration increasingly important. Electromigration decreases

544-459: 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. The possible increase in the use of pure tin in electronics due to the RoHS directive drove JEDEC and IPC to release

612-546: A rate of up to a millimeter per year with a diameter of a few micrometers. Whiskers can form on the underside of zinc electroplated floor tiles on raised floors. 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 do not have to be airborne to damage equipment, as they are typically already growing directly in

680-550: A stretched limit for electromigration. Here, a mechanical stress buildup causes an atom back flow process which reduces or even compensates the effective material flow towards the anode. The Blech length must be considered when designing test structures to evaluate electromigration. This minimum length is typically some tens of microns for chip traces, and interconnections shorter than this are sometimes referred to as 'electromigration immortal'. Particular attention must be paid to vias and contact holes. The current carrying capacity of

748-546: A tin whisker acceptance testing standard and mitigation practices guideline intended to help manufacturers reduce the risk of tin whiskers in lead-free products. 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 plants and paper mills . Whiskers over 20 μm in length were observed on gold-plated surfaces and noted in

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816-425: A via is much less than a metallic wire of same length. Hence multiple vias are often used, whereby the geometry of the via array is very significant: multiple vias must be organized such that the resulting current is distributed as evenly as possible through all the vias. Attention must also be paid to bends in interconnects. In particular, 90-degree corner bends must be avoided, since the current density in such bends

884-545: A waveguide for electrons. The nanocontact essentially acts like a one-dimensional wire with a conductance of G = 2 e 2 / h {\displaystyle G=2\,e^{2}\!/h} . The current in a wire is the velocity of the electrons multiplied by the charge and number per unit length, I = v e N / L   {\displaystyle \,I=veN/L\ } or   G = v e N / L V {\displaystyle \ G=veN/LV} . This gives

952-460: Is also intrinsically less susceptible to electromigration. However, electromigration (EM) continues to be an ever-present challenge to device fabrication, and therefore the EM research for copper interconnects is ongoing (though a relatively new field). In modern consumer electronic devices, ICs rarely fail due to electromigration effects. This is because proper semiconductor design practices incorporate

1020-436: Is constrained by increasing frequencies, the more marked decrease in cross-sectional areas (compared to current reduction) will give rise to increased current densities in ICs going forward. In advanced semiconductor manufacturing processes, copper has replaced aluminium as the interconnect material of choice. Despite its greater fragility in the fabrication process, copper is preferred for its superior conductivity. It

1088-451: Is gaining importance. Results of TCAD studies in combination with reliability tests lead to modification of design rules improving the interconnect resistance to electromigration. The electromigration degradation of the on-chip power grid network/interconnect depends on the IR drop noise of the power grid interconnect. The electromigration-aware lifetime of the power grid interconnects as well as

1156-496: Is mainly due to the higher electromigration activation energy levels of copper, caused by its superior electrical and thermal conductivity as well as its higher melting point. Further improvements can be achieved by alloying copper with about 1% palladium which inhibits diffusion of copper atoms along grain boundaries in the same way as the addition of copper to aluminium interconnect. A wider wire results in smaller current density and, hence, less likelihood of electromigration. Also,

1224-517: Is significantly higher than that in oblique angles (e.g., 135 degrees). The typical current density at which electromigration occurs in Cu or Al interconnects is 10 to 10 A/cm . For solder joints (SnPb or SnAgCu lead-free) used in IC chips, however, electromigration occurs at much lower current densities, e.g. 10 A/cm . It causes a net atom transport along the direction of electron flow. The atoms accumulate at

1292-457: Is susceptible to electromigration. Research shows that adding 2-4% of copper to aluminium increases resistance to electromigration about 50 times. The effect is attributed to the grain boundary segregation of copper, which greatly inhibits the diffusion of aluminium atoms across grain boundaries. Pure copper wires can withstand approximately five times more current density than aluminum wires while maintaining similar reliability requirements. This

1360-523: Is the Boltzmann constant , T {\displaystyle T} is the temperature in kelvins , and n {\displaystyle n} a scaling factor (usually set to 2 according to Black). The temperature of the conductor appears in the exponent, i.e. it strongly affects the MTTF of the interconnect. For an interconnect of a given construction to remain reliable as the temperature rises,

1428-401: Is the electric charge of the ions, Z e {\displaystyle Z_{e}} and Z p {\displaystyle Z_{p}} the valences corresponding to the electrostatic and wind force respectively, Z ∗ {\displaystyle Z^{*}} the so-called effective valence of the material, j {\displaystyle j}

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1496-536: Is the atomic volume and N 0 {\displaystyle N_{0}} is initial atomic concentration , H = ( σ 11 + σ 22 + σ 33 ) / 3 {\displaystyle H=(\sigma _{11}+\sigma _{22}+\sigma _{33})/3} is the hydrostatic stress and σ 11 , σ 22 , σ 33 {\displaystyle \sigma _{11},\sigma _{22},\sigma _{33}} are

1564-469: Is the conventional mass balance (continuity) equation where N ( x → , t ) {\displaystyle N({\vec {x}},t)} is the atom concentration at the point with a coordinates x → = ( x , y , z ) {\displaystyle {\vec {x}}=(x,y,z)} at the moment of time t {\displaystyle t} , and J {\displaystyle J}

1632-463: Is the effective activation energy of the thermal diffusion of metal atoms. The vacancy concentration represents availability of empty lattice sites, which might be occupied by a migrating atom. At the end of the 1960s J. R. Black developed an empirical model to estimate the MTTF (mean time to failure) of a wire, taking electromigration into consideration. Since then, the formula has gained popularity in

1700-448: Is the local current density, k {\displaystyle k} is the Boltzmann constant , T {\displaystyle T} is the absolute temperature . D ( x → , t ) {\displaystyle D({\vec {x}},t)} is the time and position dependent atom diffusivity. here Ω = 1 / N 0 {\displaystyle \Omega =1/N_{0}}

1768-459: Is the major electromigration process in aluminum wires, whereas surface diffusion is dominant in copper interconnects. In an ideal conductor, where atoms are arranged in a perfect lattice structure, the electrons moving through it would experience no collisions and electromigration would not occur. In real conductors, defects in the lattice structure and the random thermal vibration of the atoms about their positions causes electrons to collide with

1836-679: Is the total atomic flux at this location. The total atomic flux J {\displaystyle J} is a combination of the fluxes caused by the different atom migration forces. The major forces are induced by the electric current , and by the gradients of temperature, mechanical stress and concentration. J → = J → c + J → T + J → σ + J → N {\displaystyle {\vec {J}}={\vec {J}}_{c}+{\vec {J}}_{T}+{\vec {J}}_{\sigma }+{\vec {J}}_{N}} . To define

1904-574: Is transferred much more vigorously. Since the metal ions in these regions are bonded more weakly than in a regular crystal lattice, once the electron wind has reached a certain strength, atoms become separated from the grain boundaries and are transported in the direction of the current. This direction is also influenced by the grain boundary itself, because atoms tend to move along grain boundaries. Diffusion processes caused by electromigration can be divided into grain boundary diffusion, bulk diffusion and surface diffusion. In general, grain boundary diffusion

1972-508: The National Highway Traffic Safety Administration (NHTSA) and a large group of other NASA researchers that found no electronic defects. In 2012, NHTSA maintained: "We do not believe that tin whiskers are a plausible explanation for these incidents...[the likely cause was] pedal misapplication ." Toyota also maintains that tin whiskers were not the cause of any stuck accelerator issues: "In

2040-423: 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. Germanium , zinc , cadmium , and even lead whiskers have been documented. Many techniques are used to mitigate

2108-606: The "high-tech trash" problem, leading to a re-focusing on the issue of whisker formation in lead-free solders . 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 . The mechanism behind metal whisker growth is not well understood , but seems to be encouraged by compressive mechanical stresses including: Metal whiskers differ from metallic dendrites in several respects: dendrites are fern -shaped and grow across

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2176-449: The affected IC, the first symptoms are intermittent glitches, and are quite challenging to diagnose. As some interconnects fail before others, the circuit exhibits seemingly random errors, which may be indistinguishable from other failure mechanisms (such as electrostatic discharge damage). In a laboratory setting, electromigration failure is readily imaged with an electron microscope, as interconnect erosion leaves telltale visual markers on

2244-413: The anode, while voids are generated at the cathode and back stress is induced during electromigration. The typical failure of a solder joint due to electromigration will occur at the cathode side. Due to the current crowding effect, voids form first at the corners of the solder joint. Then the voids extend and join to cause a failure. Electromigration also influences formation of intermetallic compounds , as

2312-417: The atoms and scatter , which is the source of electrical resistance (at least in metals; see electrical conduction ). Normally, the amount of momentum imparted by the relatively low- mass electrons is not enough to permanently displace the atoms. However, in high-power situations (such as with the increasing current draw and decreasing wire sizes in modern VLSI microprocessors ), if many electrons bombard

2380-499: The atoms with enough force to become significant, this will accelerate the process of electromigration by causing the atoms of the conductor to vibrate further from their ideal lattice positions, increasing the amount of electron scattering . High current density increases the number of electrons scattering against the atoms of the conductor, and hence the rate at which those atoms are displaced. In integrated circuits, electromigration does not occur in semiconductors directly, but in

2448-487: The chip decreases if the chip suffers from a high value of the IR drop noise. Recent work demonstrates MTTF prediction using a machine learning model. The work uses a neural network-based supervised learning approach with current density, interconnect length, interconnect temperature as input features to the model. Electromigrated nanogaps are gaps formed in metallic bridges formed by the process of electromigration. A nanosized contact formed by electromigration acts like

2516-463: The company's reputation. Electromigration can be a cause of degradation in some power semiconductor devices such as low voltage power MOSFETs , in which the lateral current through the source contact metallisation (often aluminium) can reach the critical current densities during overload conditions. The degradation of the aluminium layer causes an increase in on-state resistance, and can eventually lead to complete failure. The material properties of

2584-484: The components of principal stress. Assuming a vacancy mechanism for atom diffusion we can express D {\displaystyle D} as a function of the hydrostatic stress D = D 0 exp ⁡ ( Ω H − E A k T ) {\displaystyle D=D_{0}\exp \left({\tfrac {\Omega H-E_{A}}{kT}}\right)} where E A {\displaystyle E_{A}}

2652-406: The conducting material, preventing the flow of electricity. In narrow interconnect conductors, such as those linking transistors and other components in integrated circuits, this is known as a void or internal failure ( open circuit ). Electromigration can also cause the atoms of a conductor to pile up and drift toward other nearby conductors, creating an unintended electrical connection known as

2720-406: The current density within the conductor must be reduced. However, as interconnect technology advances at the nanometer scale, the validity of Black's equation becomes increasingly questionable. Historically, aluminium has been used as conductor in integrated circuits, due to its good adherence to substrate, good conductivity, and ability to form ohmic contacts with silicon. However, pure aluminium

2788-411: The current density, and ρ {\displaystyle \rho } the resistivity of the material . Electromigration occurs when some of the momentum of a moving electron is transferred to a nearby activated ion. This causes the ion to move from its original position. Over time this force knocks a significant number of atoms far from their original positions. A break or gap can develop in

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2856-403: The durability of the interconnects. There are also important differences with time dependent current: direct current or different alternating current waveforms cause different effects. Two forces affect ionized atoms in a conductor: 1) The direct electrostatic force F e , as a result of the electric field E {\displaystyle E} , which has the same direction as

2924-512: The effects of electromigration into the IC's layout. Nearly all IC design houses use automated EDA tools to check and correct electromigration problems at the transistor layout-level. When operated within the manufacturer's specified temperature and voltage range, a properly designed IC device is more likely to fail from other (environmental) causes, such as cumulative damage from gamma-ray bombardment. Nevertheless, there have been documented cases of product failures due to electromigration. In

2992-424: The electric field, and 2) The force from the exchange of momentum with other charge carriers F p , toward the flow of charge carriers, is in the opposite direction of the electric field. In metallic conductors F p is caused by a so-called "electron wind" or " ion wind ". The resulting force F res on an activated ion in the electrical field can be written as where q {\displaystyle q}

3060-565: The environment where they can produce short circuits, i.e., the electronic equipment itself. 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. Pacemakers have been recalled due to tin whiskers. Research has also identified

3128-474: The first place." Electromigration 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,

3196-404: The fluxes mentioned above: Here e {\displaystyle e} is the electron charge, e Z {\displaystyle eZ} is the effective charge of the migrating atom, ρ {\displaystyle \rho } the resistivity of the conductor where atom migration takes place, j → {\displaystyle {\vec {j}}}

3264-410: The late 1980s, one line of Western Digital 's desktop drives suffered widespread, predictable failure 12–18 months after field usage. Using forensic analysis of the returned bad units, engineers identified improper design-rules in a third-party supplier's IC controller. By replacing the bad component with that of a different supplier, WD was able to correct the flaw, but not before significant damage to

3332-470: The logic board responsible for monitoring the steam pressure lines in the power plant. In September 2011, three NASA investigators claimed that they identified tin whiskers on the accelerator position sensors of sampled Toyota Camry models that could contribute to the "stuck accelerator" crashes affecting certain Toyota models during 2005–2010. This contradicted an earlier 10-month joint investigation by

3400-405: The metal grain size has influence; the smaller grains, the more grain boundaries and the higher likelihood of electromigration effects. However, if you reduce wire width to below the average grain size of the wire material, grain boundaries become "crosswise", more or less perpendicular to the length of the wire. The resulting structure resembles the joints in a stalk of bamboo. With such a structure,

3468-507: The metal interconnects deposited onto them (see semiconductor device fabrication ). Electromigration is exacerbated by high current densities and the Joule heating of the conductor (see electrical resistance ), and can lead to eventual failure of electrical components. Localized increase of current density is known as current crowding . A governing equation which describes the atom concentration evolution throughout some interconnect segment,

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3536-418: The metal interconnects have a strong influence on the life span. The characteristics are predominantly the composition of the metal alloy and the dimensions of the conductor. The shape of the conductor, the crystallographic orientation of the grains in the metal, procedures for the layer deposition, heat treatment or annealing , characteristics of the passivation and the interface to other materials also affect

3604-487: The metal layers of the IC. With increasing miniaturization, the probability of failure due to electromigration increases in VLSI and ULSI circuits because both the power density and the current density increase. Specifically, line widths will continue to decrease over time, as will wire cross-sectional areas. Currents are also reduced due to lower supply voltages and shrinking gate capacitances. However, as current reduction

3672-406: The metal or the presence of an electromagnetic field. 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. The European Restriction of Hazardous Substances Directive (RoHS), which took effect on July 1, 2006, restricted

3740-524: The migration rates are a function of atomic mass. The complete mathematical model describing electromigration consists of several partial differential equations (PDEs) which need to be solved for three-dimensional geometrical domains representing segments of an interconnect structure. Such a mathematical model forms the basis for simulation of electromigration in modern technology computer aided design (TCAD) tools. Use of TCAD tools for detailed investigations of electromigration induced interconnect degradation

3808-456: The practical significance of this effect increases. The phenomenon of electromigration has been known for over 100 years, having been discovered by the French scientist Gerardin. The topic first became of practical interest during the late 1960s when packaged ICs first appeared. The earliest commercially available ICs failed in a mere three weeks of use from runaway electromigration, which led to

3876-591: The problem, including changes to the annealing process (heating and cooling), the addition of elements like copper and nickel, and the inclusion of conformal coatings . Traditionally, lead has been added to slow down whisker growth in tin-based solders. 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

3944-401: The reliability of integrated circuits (ICs). It can cause the eventual loss of connections or failure of a circuit. Since reliability is critically important for space travel , military purposes , anti-lock braking systems , medical equipment like Automated External Defibrillators and is even important for personal computers or home entertainment systems, the reliability of chips (ICs) is

4012-547: The resistance to electromigration increases, despite an increase in current density. This apparent contradiction is caused by the perpendicular position of the grain boundaries; the boundary diffusion factor is excluded, and material transport is correspondingly reduced. However, the maximum wire width possible for a bamboo structure is usually too narrow for signal lines of large-magnitude currents in analog circuits or for power supply lines. In these circumstances, slotted wires are often used, whereby rectangular holes are carved in

4080-567: The risk of whisker growth. The trade-off has been an increase in weight, adding 50 to 100 kilograms (110 to 220 lb) per payload. 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

4148-412: The semiconductor industry: Here A {\displaystyle A} is a constant based on the cross-sectional area of the interconnect, J {\displaystyle J} is the current density, E a {\displaystyle E_{\text{a}}} is the activation energy (e.g. 0.7 eV for grain boundary diffusion in aluminum), k {\displaystyle k}

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4216-435: The surface of the metal, while metal whiskers are hair-like and project normal 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

4284-488: 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. Airborne zinc whiskers have been responsible for increased system failure rates in computer server rooms . Zinc whiskers grow from galvanized (electroplated) metal surfaces at

4352-412: The wires. Here, the widths of the individual metal structures in between the slots lie within the area of a bamboo structure, while the resulting total width of all the metal structures meets power requirements. There is a lower limit for the length of the interconnect that will allow higher current carrying capability. It is known as "Blech length". Any wire that has a length below this limit will have

4420-490: 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

4488-492: 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 misapplied, allowing whiskers formed in the pure tin plating to find their way through a missing coating area, causing a failure of the main control computer. The manufacturer, Hughes, has moved to nickel plating, rather than tin, to reduce

4556-655: Was briefly occupied by Galaxy 4R , and is now occupied by the Galaxy VI satellite. Galaxy VI's relocation required an emergency order from the FCC, taking one week drift time to change orbit. Galaxy IV remains in space. This article about one or more communications satellites is a stub . You can help Misplaced Pages by expanding it . Whisker (metallurgy) Metal whiskering is a phenomenon that occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in

4624-434: Was not restored until the following day. Many fast-pay gas pumps were not able to verify credit card transactions. Wire news services, like Reuters , were also affected. The TV network CBS had to use alternate means of transmitting its programs. All 600 NPR -affiliated stations (as well as numerous other broadcasters) lost access to current network shows and newscasts. After the incident, Galaxy IV's assigned orbital slot

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