Failure of electronic components-Metallisation failures-pcb

- Feb 06, 2017-

Failure of electronic components-

Metallisation failures-pcb

Metallisation failures

Micro-photograph of a failed TO3 power transistor due to short circuit

Metallisation failures are more common and serious causes of FET transistor degradation than material processes; amorphous materials have no grain boundaries, hindering interdiffusion and corrosion. Examples of such failures include:

  • Electromigration moving atoms out of active regions, causing dislocations and point defects acting as nonradiative recombination centers producing heat. This may occur with aluminium gates in MESFETs with RF signals, causing erratic drain current; electromigration in this case is called gate sinking. This issue does not occur with gold gates. With structures having aluminium over a refractory metal barrier, electromigration primarily affects aluminium but not the refractory metal, causing the structure's resistance to erratically increase. Displaced aluminium may cause shorts to neighbouring structures; 0.5-4% of copper in the aluminium increases electromigration resistance, the copper accumulating on the alloy grain boundaries and increasing the energy needed to dislodge atoms from them. Other than that, indium tin oxide and silver are subject to electromigration, causing leakage current and (in LEDs) nonradiative recombination along chip edges. In all cases, electromigration can cause changes in dimensions and parameters of the transistor gates and semiconductor junctions.

  • Mechanical stresses, high currents, and corrosive environments forming of whiskers and short circuits. These effects can occur both within packaging and on circuit boards.

  • Formation of silicon nodules. Aluminium interconnects may be silicon-doped to saturation during deposition to prevent alloy spikes. During thermal cycling, the silicon atoms may migrate and clump together forming nodules that act as voids, increasing local resistance and lowering device lifetime.

  • Ohmic contact degradation between metallisation and semiconductor layers. With gallium arsenide, a layer of gold-germanium alloy (sometimes with nickel) is used to achieve low contact resistance; an ohmic contact is formed by diffusion of germanium, forming a thin, highly n-doped region under the metal facilitating the connection, leaving gold deposited over it. Gallium atoms may migrate through this layer and get scavenged by the gold above, creating a defect-rich gallium-depleted zone under the contact; gold and oxygen then migrate oppositely, resulting in increased resistance of the ohmic contact and depletion of effective doping level. Formation of intermetallic compounds also plays a role in this failure mode.

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