Nowadays, electronic systems have quite large circuits having many components though many of them are of similar type. A lot of new components are also being developed and studies are going on for recognition of their failure mechanisms. Typical electronic circuit has components like resistors, capacitors, inductors, microcircuits (ICs), connectors, switches and others. We will have a brief overview for some of the components and discuss their failure mechanisms.
5.2.1
Resistors
Resistors are the basic components of any electronic circuit. A failure occurred with resistor usually make it an open circuit or a very high resistance is developed across the ends. A failure in resistance can occur due to high current stress or high voltage stress that results in excessive heating of the components and as the heating con- tinues increasing its temperature, there may be a time the temperature gets over the melting point of the material used. This causes the resistor to be fused and hence an open circuit is developed. Fabrication defects and electrostatic discharge also some time causes failure to the components. This may usually make the component noisy. Sometimes instead of an open circuit the parameters of the component also get changed due to above mentioned causes.
5.2.2
Capacitors
Capacitors are used in almost every electronic circuit as a storing component. The failures in capacitor usually make it open circuit except in some cases it gets short circuit after failure as in the case of electrolyte capacitor. If no voltage is applied over a long period, they get short-circuited. High voltage stress is usually the dominating aspect for the failure of capacitors. Some special purpose capacitor, as
in the case of electrolyte capacitors it has got polarity so, reversing the polarity may damage the component. Generally, a diode or other protecting circuit is used to save valuable component and to avoid the failures as well.
5.2.3
Inductors
Inductors are also part of many electronic systems. Inductors are very prone to the failure due to excessive current stress. High current may cause the heating of the conductor, which as a result may damage the insulation and causes a failure. In addition, when there is a large variation in current through the inductor the designer should choose proper insulation and conducting material. A diode is used across the ends to protect an inductor which provides a path for the current toflow when the inductor suddenly get out of the circuit.
5.2.4
Relays
Relays are very important parts of any electrical or electronic circuits from the safety point of view. They not only save the system from any damage due to some faulty situation in the system itself or from the outer side but they also take care of the parts, which are not really going to be affected by these faults. Thus, the failure in relay itself may sometime cause vary dangerous situation. Electro-mechanical type relay which are being used for a long time have metal contact which is used to trip the circuit when high current or voltage or any other parameter get out of the tolerance of the circuit. The main failure cause for this type of relay is due to the large heat, which generate excessive power and results in the contact failure. Apart from this as it is a mechanical element as well mechanical failure may also occur if proper handling is not done. Now a days, pure solid-state relays are used which do not have any mechanical contact in there. These types of relays are also sensitive to non-resistive loads, surge currents that may create high junction temperatures that degrade the component.
5.2.5
Semiconductor Devices
Semiconductor devices are always part of a larger, more complex piece of elec- tronic equipment. Semiconductor devices like diode, transistor, MOSFET, solar cells etc. have P-N junctions. For avoiding failure of these components, manu- facturers provide failure characteristics for the components, which are determined with the application of stress bias voltages. A semiconductor device generally fails when excessive reverse voltage is applied across the P-N junction, which results in
the breakdown of the P-N junction. That is why the peak inverse voltage (PIV) is usually specified for these devices. Like other electronic devices, these components are also very sensitive to the ambient temperature. Heating may produce excessive charge carriers, which results in widening of the depletion region in P-N junction. One of the important failure mechanisms of semiconductor devices is related with the dislocation in silicon such as diffusion, precipitation or photo effects. Experiments have shown that dislocations are usually unavoidable for practical devices. Failure generally caused through doping non-uniformities caused by dif- fusion enhancement and precipitation of metals at dislocations, which may destroy the P-N junction.
5.2.6
Microcircuits (ICs)
Integrated circuits (ICs) are one of the important elements in any electronic circuit. Fabrication defects in silicon chip are major failure causes for these components. Apart from this, ICs are very prone to failure due to electrostatic discharge (ESD), electro-migration and antenna effect. Electrostatic discharge causes the metal to melt and bond wires to fuse, usually causes an open circuit. To avoid failure due to ESD some assembly protection are used like RIF/EMI design zoning in which sensitive parts are shielded by less sensitive parts and Faraday shielding. Electro-migration causes large current density in the conductors and results in slow wear out. Impact of electrons causes gradual shifting of aluminum atoms (con- ducting material) from their normal lattice sites, which also give rise to voids between grains. It also increases the resistance of the conductor. Some alloys are being used instead of aluminum as the conducting material in order to reduce this phenomenon. Proper packaging of the silicon chip is also an important issue for the failure due to excessive heating. Most of the times, failure in integrated circuits results in modification of their actual functions. Specially, at high temperature ICs start giving incorrect outputs. Therefore, proper temperature should be maintained for sophisticated electronic circuits.
There are a lot other electronic components, which are also being developed to provide more flexibility in designs. Some of these components are Field Programmable Gate Arrays (FPGAs), Micro-Electro-Mechanical Systems (MEMS), embedded systems and others. The failure mechanisms for these systems and components are being studied and will be discussed later in details.
Table 5.1 summarizes the different failure modes of some of the electronic components discussed above and their failure causes with an estimation of the corresponding probability of the failure modes. Comprehensive list is given in Electronic Reliability Design Handbook [1].