Failure of electronic components
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.
Failures most commonly occur near the beginning and near the ending of the lifetime of the parts, resulting in the bathtub curve graph of failure rates. Burn-in procedures are used to detect early failures. In semiconductor devices, parasitic structures, irrelevant for normal operation, become important in the context of failures; they can be both a source and protection against failure.
Applications such as aerospace systems, life support systems, telecommunications, railway signals, and computers use great numbers of individual electronic components. Analysis of the statistical properties of failures can give guidance in designs to establish a given level of reliability. For example, the power-handling ability of a resistor may be greatly derated when applied in high-altitude aircraft to obtain adequate service life.
A sudden fail-open fault can cause multiple secondary failures if it is fast and the circuit contains an inductance; this causes large voltage spikes, which may exceed 500 volts. A broken metallisation on a chip may thus cause secondary overvoltage damage. Thermal runaway can cause sudden failures including melting, fire or explosions.
Packaging failures
The majority of electronic parts failures are packaging-related. Packaging, as the barrier between electronic parts and the environment, is very susceptible to environmental factors. Thermal expansion produces mechanical stresses that may cause material fatigue, especially when the thermal expansion coefficients of the materials are different. Humidity and aggressive chemicals can cause corrosion of the packaging materials and leads, potentially breaking them and damaging the inside parts, leading to electrical failure. Exceeding the allowed environmental temperature range can cause overstressing of wire bonds, thus tearing the connections loose, cracking the semiconductor dies, or causing packaging cracks. Humidity may also cause cracking, as may mechanical damage or shock.During encapsulation, bonding wires can be severed, shorted, or touch the chip die, usually at the edge. Dies can crack due to mechanical overstress or thermal shock; defects introduced during processing, like scribing, can develop into fractures. Lead frames may contain excessive material or burrs, causing shorts. Ionic contaminants like alkali metals and halogens can migrate from the packaging materials to the semiconductor dies, causing corrosion or parameter deterioration. Glass-metal seals commonly fail by forming radial cracks that originate at the pin-glass interface and permeate outwards; other causes include a weak oxide layer on the interface and poor formation of a glass meniscus around the pin.
Various gases may be present in the package cavity, either as impurities trapped during manufacturing, due to outgassing of the materials used, or chemical reactions, as is when the packaging material gets overheated. To detect this, helium is often in the inert atmosphere inside the packaging as a tracer gas to detect leaks during testing. Carbon dioxide and hydrogen may form from organic materials, moisture is outgassed by polymers and amine-cured epoxies outgas ammonia. Formation of cracks and intermetallic growth in die attachments may lead to the formation of voids and delamination, impairing heat transfer from the chip die to the substrate and heatsink and causing a thermal failure. As some semiconductors like silicon and gallium arsenide are infrared-transparent, infrared microscopy can check the integrity of die bonding and under-die structures.
Red phosphorus, used as a char-promoting flame retardant, facilitates silver migration when present in packaging. It is normally coated with aluminium hydroxide; if the coating is incomplete, the phosphorus particles oxidize to the highly hygroscopic phosphorus pentoxide, which reacts with moisture to form phosphoric acid. This is a corrosive electrolyte that in the presence of electric fields facilitates dissolution and migration of silver, short-circuiting adjacent packaging pins, lead frame leads, tie bars, chip mount structures, and chip pads. The silver bridge may be interrupted by thermal expansion of the package; thus, disappearance of the shorting when the chip is heated and its reappearance after cooling is an indication of this problem. Delamination and thermal expansion may move the chip die relative to the packaging, deforming and possibly shorting or cracking the bonding wires.
Contact failures
Electrical contacts exhibit contact resistance, the magnitude of which is governed by surface structure and the composition of surface layers. Ideally contact resistance should be low and stable, however weak contact pressure, mechanical vibration, and corrosion can alter contact resistance significantly, leading to resistive heating and circuit failure.Soldered joints can fail in many ways including electromigration and the formation of brittle intermetallic layers. Some failures show only at extreme joint temperatures, hindering troubleshooting. Thermal expansion mismatch between the printed circuit board material and component packaging strains the part-to-board bonds. Thermal cycling may lead to fatigue cracking of the solder joints. Loose particles, like weld flash and tin whiskers, can form in the device cavity and migrate inside the packaging, causing often intermittent and shock-sensitive shorts. Corrosion may cause a buildup of oxides and other nonconductive products on the contact surfaces. When closed, these then show unacceptably high resistance; the oxides may also migrate and cause shorts.
Printed circuit board failures
s are vulnerable to environmental influences; for example, the traces are corrosion-prone and may be improperly etched leaving partial shorts or may crack under mechanical loads, while the vias may be insufficiently plated through. Residues of solder flux may facilitate corrosion; those of other materials on PCBs can cause electrical leaks. Polar covalent compounds like antistatic agents can attract moisture, forming a thin layer of conductive moisture between the traces or may dissipate high-frequency energy, causing parasitic dielectric losses; ionic compounds like chlorides tend to facilitate corrosion. Alkali metal ions may migrate through plastic packaging and influence the functioning of semiconductors. Chlorinated hydrocarbon residues may hydrolyze and release corrosive chlorides; these are problems that occur after years.Above the glass transition temperature of PCBs, the resin matrix softens and becomes susceptible to contaminant diffusion. For example, polyglycols from the solder flux can enter the board and increase its humidity intake, with corresponding deterioration of dielectric and corrosion properties. Multi-layer substrates using ceramics suffer from many of the same problems.
Conductive anodic filaments may grow within PCBs along the fibers of the composite material. Metal is introduced to a vulnerable surface typically from plating the vias, then migrates in the presence of ions, moisture, and electrical potential; drilling damage and poor glass-resin bonding promotes such failures. The formation of CAFs usually begins with poor glass-resin bonding; a layer of adsorbed moisture then provides a channel through which ions and corrosion products migrate. In the presence of chloride ions, the precipitated material is atacamite; its semiconductive properties lead to increased current leakage, deteriorated dielectric strength, and short circuits between traces. Absorbed glycols from flux residues aggravate the problem. The difference in thermal expansion of the fibers and the matrix weakens the bond when the board is soldered; the lead-free solders, which require higher soldering temperatures, increase the occurrence of CAFs. Besides this, CAFs depend on absorbed humidity; below a certain threshold, they do not occur. Delamination may occur to separate the board layers, cracking the vias and conductors to introduce pathways for corrosive contaminants and migration of conductive material.
Relay and switch failures
Every time the contacts of a conventional electromechanical relay, switch or contactor are opened or closed, there is a certain amount of contact wear. An electric arc may occur between the contact points both during the transition from closed to open or from open to closed. The arc caused during the contact break is akin to arc welding, as the break arc is typically more energetic and more destructive. The heat and current of the electrical arc across the contacts creates specific cone & crater formations from metal migration. In addition to the physical contact damage, there appears also a coating of carbon and other matter. This degradation limits the overall operating life of a relay or contactor to a range of perhaps 100,000 operations, a level representing 1% or less than the mechanical life expectancy of the same device without arcing.Semiconductor failures
Many failures produce hot electrons. These are observable under an optical microscope, as they generate near-infrared photons detectable by a CCD camera. Latchups can be observed this way. If visible, the location of failure may present clues to the nature of the stress. Liquid crystal coatings can be used for localization of faults: cholesteric liquid crystals are thermochromic and are used for visualisation of locations of heat production on the chips. Nematic liquid crystals respond to voltage and are used for visualising current leaks through oxide defects and of charge states on the chip surface.Examples of semiconductor failures relating to semiconductor crystals include:
- Nucleation and growth of dislocations. This requires an existing defect in the crystal, as is done by radiation, and is accelerated by heat, high current density and emitted light. With LEDs, gallium arsenide and aluminium gallium arsenide are more susceptible to this than gallium arsenide phosphide and indium phosphide; gallium nitride and indium gallium nitride are insensitive to this defect.
- Accumulation of charge carriers trapped in the gate oxide of MOSFETs. This introduces permanent gate biasing, influencing the transistor's threshold voltage; it may be caused by hot carrier injection, ionizing radiation or normal use. With EEPROM cells, this is the major factor limiting the number of erase-write cycles.
- Migration of charge carriers from floating gates. This limits the lifetime of stored data in EEPROM and flash memory.
- Improper passivation. Corrosion is a significant source of delayed failures; semiconductors, metallic interconnects, and passivation glasses are all susceptible. The surface of semiconductors subjected to moisture has an oxide layer; the liberated hydrogen reacts with deeper layers of the material, yielding volatile hydrides.
- Laser marking of plastic-encapsulated packages may damage the chip if glass spheres in the packaging line up and direct the laser to the chip.