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An electronic equipment is wholly made up of
electronic components or a combination of electronic and electrochemical parts
and several equipment constitute a system. Therefore, a discussion on
component failure presumes reference to failure of equipment or system. It
therefore follows that the failure of a component in an equipment may lead to
the failure of the entire equipment. As well, the failure of an equipment
within a system may in turn lead to the failure of the system.
Before we proceed into discussing failure and
it causes, I will first explain reliability
and quality of an equipment
RELIABILITY
Reliability is the
characteristic of an item expressed as the probability that it will perform a
required function under stated conditions for a stated period of time. Other
literature sources may define reliability by slightly different statements;
regardless of the approach, the three operative phrases ‘PERFORM A REQUIRED FUNCTION’, ‘UNDER STATED CONDITION’, ‘FOR A STATED
PERIOD OF TIME’ are always emphasized. Based on mathematical reasons,
reliability lies between 0 and 1.
For
example, when
Reliability = 1.00; means equipment will work as expected.
Reliability = 0.90; means 90% likely to work as expected.
Reliability = 0.00; means absolutely that equipment will not work as expected.
Thus the reliability of a public address system amplifier may be given as 0.90
over a ten hour (10hrs) period, with an ambient temperature of 250c.
Here, the required function of the amplifier is to amplify electrical signal under the stated temperature condition,
for a specific period of 10 hours.
It may be observed that all possible
environmental conditions to which the amplifier may be exposed to have not been
stated. Besides, reliability of 0.90 may cease to be valid if the
operating period is increased. Furthermore, the reliability may also be
affected by mechanical conditions such as vibration and electrical condition
such as variation in the main voltage. It should therefore, be noted that the
general definition of reliability given in this paper means that, each
reliability figure quoted for an item or equipment relates only to the
operating period concerned, and the required function to perform and its
working conditions.
QUALITY
The Oxford
Dictionary defines Quality as the standard of something when it is compared to
other things like it; (i.e. how good or bad something is.)
Quality is the conformity to specification or fitness for the purpose, and it
can be sub-divided roughly into two:
PHYSICAL FEATURES: Provides information
as to whether an equipment had dimension within acceptable limits, and has a
satisfactory appearance.
PERFORMANCE: Provides information as to whether an item or
equipment works correctly. This is an attribute which may not be ease to
observe in figures.
Now we can see the relationship between Reliability and
Quality, though they are synonymous. It may be argued that quality is a static feature and reliability
is a dynamic feature. For example, an electronic equipment may be of
high quality, but its reliability is measured in use. Quality control can be
seen as a major subset of reliability engineering. It encompasses such as testing and selection of materials used in manufacturing process. It
also has to do with the design and control of processing assembly parts and
equipment. Finally, it also includes testing and inspection. In contrast,
reliability tends to focus on design and it is concerned with the probability
of failure at some instant in time and therefore, has its own mathematics which
is different from that in use by quality personnel.
FAILURE
Failure is
caused by the stress that acts on the components within a circuit board of an
electronic system and fault may be classified into environmental stresses and operating stresses.
ENVIRONMENTAL STRESS: This is due to the effect of factors which are external
to the equipment such as the weather, (atmospheric
temperature, atmospheric pressure, humidity, wind). any equipment which
operate outdoors cannot escape the full effects of the weather in its locality.
OPERATING STRESS: This stress can be further subdivided in to Frequency change,
Voltage surge and current surge.
FAILURE IN
ELECTRONIC COMPONENTS
Due to environmental and operating stresses and at times manufacturing defects,
electronic components usually experience failures, such as in:
Fixed resistors: The failure in
fixed resistor is due either to increase in resistance value or open-circuitry
which is common in carbon composition resistor, and this is due to any of the
following;
• Operative stress
• Movement of carbon under the influence of
heat, voltage or moisture
• Absorption of moisture, resulting in swelling
or forcing of carbon particles to separate.
Capacitors: Failure of capacitors may be characterized by
open-circuiting, short-circuiting, intermittent contact and fluctuation in
capacitance value, which may be as a result of mechanical/thermal shock, as in
paper foil capacitors, electrolytic capacitor and silver not adhering to mica
as in mica capacitor. It can also occur due to fracture in dielectric resulting
from shock or vibration caused by high humidity.
Relays: Failures in relays are due to corrosion of fine
wire and consequent open-circuiting and metal fatigue in the armature spring.
Dirty contacts caused by action of chemicals produced sparking in an enclosed
space may also lead to failure.
Semiconductors: Failures in semiconductor devices are mainly those
of an open or short circuit at a junction. For example, bipolar transistor may
fail due to open or short circuiting between base and emitter, collector and
base or emitter and collector. Apart from environmental stresses, electrical
interference is another cause of failure in semiconductor.
CLASSIFICATION
OF CAUSES OF COMPONENT FAILURE
Failures
are classified in the following ways;
Misuse Failures: Failure attributable to the application of stresses
beyond the stated capacities of the equipment. For example, using of a 230v AC
mains to an equipment specified for use with 110v AC mains only.
Inherent Weakness Failure: Failures attributable to weakness
inherent in the equipment itself when subjected to stress within the stated
capacities of the equipment.
BY TIMING
OF FAILURE
Sudden Failure: failures that could not be anticipated by prior
examination.
Gradual Failure: failures that could be anticipated by prior
examination.
BY DERGREE OF FAILURE
Partial Failure: failure resulting from deviations in characteristic(s)
beyond specified limits but not such as to cause complete lack of the required
function.
BY A
COMBINATION OF FAILURES
Catastrophic failure: failure which is both complete and sudden. Examples
of such are (i). Blowing of fuse; (ii). Open circuit in wire wound resistors or
short circuit failure in capacitors.
Degradation of Failure: failure which is both gradual and
partial. Example is a change in the value of the resistance of a resistor due
to over-operational stress.
FAILURE
RATE
One indicator of the reliability an item or equipment is the rate at which the
item or equipment fails. The number of
failures occurring per unit time is called failure rate.
But it is hardly used in any computation, strictly failure rate is
defined by mathematical relation,
Where
λ(t) = failure rate
Ns = number of surviving items after life test.
ΔNf = number of
failed items during the time interval.
Example: 10 items
have failed out of 1000 put on test during a period of 5000 hours. Calculate
the failure rate.
Solution:
And the percentage failure rate,
= 0.002 percent/hour or 0.2 percent/103
The bath-tub curve shows three distinct phases in the life of an equipment.
The first phase shows the early failure period i.e. the time when very weak
components fail, which is the infant
mortality.
The second phase is useful life period i.e. the period that may last for many
years or thousands of continuous hours. The third phase is the wear-out period
i.e. the period of few years or few thousands of continuous working hours.
EARLY FAILURE PERIOD
The most common causes of this failure are;
Manufacturing Fault: which are not detected before the dispatch of the
produced items to market or customers. E.g. weld, joints, connections, coating
flaws, etc.
Design Fault: are caused by wrong or inaccurate design. E.g. a
design for production without prototype.
Misuse Fault: are due to incompetent operation of equipment mostly
by the user or an operator of equipment. E.g. operating equipment in a wrongful
manner.
CONSTANT FAILURE PERIOD
Once the early failure period is over, the parts of an equipment or a system
usually will settle down for what may be relatively long period, during which
failure rate is approximately constant. It is the period in which equipment is
mostly usefully employed and any failure that occurs is usually assumed to be
stress related. During this period the failure that occur are purely chance
fault.
WEAROUT FAILURE PERIOD
When an equipment comes to the end of
its useful life period its failure rate may increase because in addition to
chance failures, parts start to deteriorate and wear out. This may be caused by
corrosion, oxidation, breakdown of insulation, friction wear, shrinkage,
fatigue etc. The possible way to extend the useful life period of equipment is
employing the planned preventive maintenance.
CONCLUSION
Conclusively, I want to say that when a fault is observed on any equipment and
the cause(s) is /are known, then and only we can boldly talk of effective
repair of such equipment. Don’t forget that when a system is given maintenance
attention, it will last longer and serve better. This is a way of extending the
life span of equipment.
REFERENCES
Electronic Materials Handbook: Packaging By
Merrill L. Minges, ASM International. Handbook Committee, 1989, p. 970 ISBN 0-87170-285-1
Istfa 2008: International Symposium for Testing and
Failure Analysis ASM International, 2008, p. 61 ISBN 0-87170-714-4
Shangguan, Dongkai (2005-12-05). "Lead-free solder interconnect reliability".
ISBN 978-0-87170-816-8.
Microelectronic failure analysis: desk reference: 2002
supplement By Thomas W. Lee, ASM International, 2002, p. 161 ISBN 0-87170-769-1
Ragnar Holm (1958). Electric Contacts Handbook (3rd ed.).
Springer-Verlag, Berlin / Göttingen / Heidelberg. pp. 331–342.
"Lab Note #105 Contact Life - Unsuppressed vs.
Suppressed Arcing". Arc Suppression Technologies. August 2011.
Retrieved March 10, 2012.
Microelectronics failure analysis: desk reference
By Electronic Device Failure Analysis Society. Desk Reference Committee, ASM
International, 2004 ISBN 0-87170-804-3 p. 79
Corrosion and reliability of electronic materials and
devices: proceedings of the Fourth International Symposium. The
Electrochemical Society. 1999. p. 251. ISBN 1-56677-252-4.
Chapter 4. Basic Failure Modes and Mechanisms,
S. Kayali
