This document discusses failure analysis of electronic circuits. It defines failure and describes the traditional bathtub curve which indicates three stages of component life: infant mortality period, random failure period, and wear out stage. The document outlines objectives, failure symptoms, causes including design deficiencies and production deficiencies. It also defines failure types such as early failure, random failure, and wear out failure. Finally, it discusses useful terms for failure analysis such as failure rate, mean time to failure, mean time between failures, and mean time to repair.

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Practical Troubleshooting of Electronic
Circuits for Engineers and Technicians
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Objectives
• Understand the technical meaning of failure
• Know the symptoms of failure
• Study the causes of failure
• Study Early Failure, Random Failure and Wear out Failure
• Know what is Failure Rate, Mean Time to Failure, Mean Time between Failure
and Mean Time to Repair
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Failure Symptoms
• Failure
A deviation in the properties of a component or item
from prescribed condition is considered as a fault
• Failure Curve
• Traditional bathtub curve indicates component life in three stages

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Failure Symptoms
• Reliability Curve (generalized)
First stage Failure rate begins high and decreases rapidly with time.
This stage is known as Infant Mortality Period and has a
Decreasing Failure Rate (DFR)
Second
stage
State failure rate period, which is usually long also
known as random failure period or useful life of the
equipment
Third stage Beyond useful life period, there is a gradual increase in
failure rate. A period of ageing and wear out with
increasing failure rate

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Equipment Failure
Partial failures Resulting from deviation in
characteristics or parameters
beyond the specified limits
Complete failure If characteristics deviate
beyond specified limits causing
complete breakdown
Catastrophic failure A sudden and complete change
in an equipment's performance
Degradation failure Failures occurring gradually
and in a partial manner
• Failed item:
Completely inoperable
Operable but no longer able to perform its
intended function satisfactorily
Serious deterioration made it unreliable

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Failure Causes
• Classification
• Design Deficiency
a) Little engineering efforts / changes in design
b) Improper choice of components
c) Upgrading of a part
d) Inadequate information on failure analysis of
components
e) Insufficient design criteria
f) Insufficient prototype testing for meeting both
reliability and performance specification
g) Excessive heat development inside
equipment and lack of cooling
h) Poor mechanical layout of components,
assemblies and panels

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Failure Causes
•Production
Deficiency
Improper / substandard purchase of material
Unsuitable storage method and long storage period of
components
Absence of training programs to employees
Negligence in performing environmental tests like
temperature cycling, operation of equipment at elevated
temperature for specified period
Lack of proper working environment, ill-ventilated, poor
illumination and dusty assembly shops
Lack of inspection of goods, sampling tests and inspection
Insufficient testing and inspection of finished product

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Assembly Errors
• Careless Storage and Transport
Excessive vibrations and mechanical
shocks during transportation
Improper packaging
Undue long storage of equipment
• Undetected errors in assembly by manufacturer / purchaser
• Improper matching of various signals used or generated by the
components on the terminal board at the time of assembly
• Improper prescribed order of the components collected on the terminal
board

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Improper Working Conditions
• Presence of abnormal severe conditions of speed,
temperature etc.
• Using equipment without following manufacturer's
instructions, warnings and cautions
• Inadequate maintenance policy and after sale service
• Usage of equipment beyond its lifetime
• Aging of the equipment
• Operator error
• Lack of preventive measures

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Failure Types
• Early Failure
• Random Failure
• Wear out Failure

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Failure Types
a) Early Failure
b) Random Failure
c) Wear out Failure
Poor quality control techniques during production
Causes:
Also known as Infant Failures, can be
eliminated by de-bugging or burn-in process
Weak and substandard components

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Failure Types
c) Wear out Failure
Predominant during actual operation of system
Occur at random, irregularly and unexpectedly
Caused by sudden stress accumulations
beyond specification of component
Phase during which only chance failure occur
is called useful life of the system
Reliability techniques help to reduce occurrence of
random failures
b) Random Failure
a) Early Failure

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Failure Types
b) Random Failurec) Wear out Failure
a) Early Failure
Caused due to aging or wearing out of components
Improper maintenance
Need of preventive maintenance
Period between two maintenances should be shorter
than the mean wear out life of the system

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Some Useful Terms in Failure
• Failure Rate
Failure Rate = f / T
Where:
f = number of failures during the test interval
T = total test time
• Smaller the value of the failure rate, the higher is the reliability of the system
• Mean Time to Failure (MTTF)
For one transistor used in the system,
MTTF = 1 / 5 * 10 -5
= 20,000 hours

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Some Useful Terms in Failure
• Mean Time to Failure (MTTF)
• Calculated on the basis of the results of life testing of components
MTTF = Total test time / number of components
• Mean Time between Failures (MTBF)
MTBF = 1 / Failure rate
= T / f
Where:
f = number of failures during the test interval
T= total test time

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Some Useful Terms in Failure
• Mean Time to Repair (MTTR)
Average time required to bring a system from a failed
state to an operational state.
Defined as the total corrective maintenance time divided
by the total number of corrective maintenance actions
during a given period of time.

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Summary
• A deviation in the properties of the component or item from prescribed
condition is considered as a fault. A state of fault is denoted by the term
failure
• Component life is divided into three stages. Infant Mortality Period,
Random Failure Period and Wear out stage
Production deficiency, Processing deficiency,
Assembly errors, inadequate storage and
transport conditions
Failure Rate, Mean Time to Failure, Mean Time
between Failure and Mean Time to Repair
• Important mathematical terms in failure
• Some of the major causes of failure:

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