Passive element failures-Potentiometers and trimmers-pcb

- Feb 06, 2017-

Passive element failures-Potentiometers

and trimmers-pcb

Passive element failures

Potentiometers and trimmers

Potentiometers and trimmers are three-terminal electromechanical parts, containing a resistive path with an adjustable wiper contact. Along with the failure modes for normal resistors, mechanical wear on the wiper and the resistive layer, corrosion, surface contamination, and mechanical deformations may lead to intermittent path-wiper resistance changes, which are a problem with audio amplifiers. Many types are not perfectly sealed, with contaminants and moisture entering the part; an especially common contaminant is the solder flux. Mechanical deformations (like an impaired wiper-path contact) can occur by housing warpage during soldering or mechanical stress during mounting. Excess stress on leads can cause substrate cracking and open failure when the crack penetrates the resistive path.

Electrolytic capacitors

In addition to the problems listed above, electrolytic capacitors suffer from these failures:

  • Aluminium versions having their electrolyte dry out for a gradual leakage, equivalent series resistance and loss of capacitance. Power dissipation by high ripple currents and internal resistances cause an increase of the capacitor's internal temperature beyond specifications, accelerating the deterioration rate; such capacitors usually fail short.

  • Electrolyte contamination (like from moisture) corroding the electrodes, leading to capacitance loss and shorts.

  • Electrolytes evolving a gas, increasing pressure inside the capacitor housing and sometimes causing an explosion; an example is the capacitor plague.

  • Tantalum versions being electrically overstressed, permanently degrading the dielectric and sometimes causing open or short failure. Sites that have failed this way are usually visible as a discolored dielectric or as a locally melted anode.

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