Failure of electronic components-Electrical overstress-pcb

- Feb 06, 2017-

Failure of electronic components-Electrical

overstress-pcb

Electrical overstress

Most stress-related semiconductor failures are electrothermal in nature microscopically; locally increased temperatures can lead to immediate failure by melting or vaporising metallisation layers, melting the semiconductor or by changing structures. Diffusion and electromigration tend to be accelerated by high temperatures, shortening the lifetime of the device; damage to junctions not leading to immediate failure may manifest as altered current-voltage characteristics of the junctions. Electrical overstress failures can be classified as thermally-induced, electromigration-related and electric field-related failures; examples of such failures include:

  • Thermal runaway, where clusters in the substrate cause localised loss of thermal conductivity, leading to damage producing more heat; the most common causes are voids caused by incomplete soldering, electromigration effects and Kirkendall voiding. Clustered distribution of current density over the junction or current filaments lead to current crowding localised hot spots, which may evolve to a thermal runaway.

  • Reverse bias. Some semiconductor devices are diode junction-based and are nominally rectifiers; however, the reverse-breakdown mode may be at a very low voltage, with a moderate reverse bias voltage causing immediate degradation and vastly accelerated failure. 5 V is a maximum reverse-bias voltage for typical LEDs, with some types having lower figures.

  • Severely overloaded Zener diodes in reverse bias shorting. A sufficiently high voltage causes avalanche breakdown of the Zener junction; that and a large current being passed through the diode causes extreme localised heating, melting the junction and metallisation and forming a silicon-aluminium alloy that shorts the terminals. This is sometimes intentionally used as a method of hardwiring connections via fuses.

  • Latchups (when the device is subjected to an over- or undervoltage pulse); a parasitic structure acting as a triggered SCR then may cause an overcurrent-based failure. In ICs, latchups are classified as internal (like transmission line reflections and ground bounces) or external (like signals introduced via I/O pins and cosmic rays); external latchups can be triggered by an electrostatic discharge while internal latchups cannot. Latchups can be triggered by charge carriers injected into chip substrate or another latchup; the JEDEC78 standard tests susceptibility to latchups.


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