TABLE OF CONTENTS Tim Williams
The reliability of electronic equipment can to some extent be quantified, and a separate discipline of reliability engineering has grown up to address it. This chapter will serve as an introduction to the subject for those circuit designers who are not fortunate enough to have a reliability engineering department at their disposal.
Reliability, itself, has a strictly defined meaning. This can be stated as "the probability that a system will operate without failure for a specified period, subject to specified environmental conditions." Thus, it can be quoted as a single number, such as 90%, but this is subject to three qualifications:
Agreement as to what constitutes a "failure." Many systems may "fail" without becoming totally useless in the process.
A specified operating lifetime. No equipment will operate forever; reliability must refer to the reasonably foreseen operating life of the equipment, or to some other agreed period. The age of the equipment, which may well affect failure rate, is not a factor in the reliability specification.
Agreement upon environmental conditions. Temperature, moisture, corrosive atmospheres, dust, vibration, shock, supply and electromagnetic disturbances all have an effect on equipment operation and reliability is meaningless if these are not quoted.
If you offer or purchase equipment whose reliability is quoted for one set of conditions and it is used under another set, you will not be able to extrapolate the reliability figure to the new conditions unless you know the behavior of those parameters which affect it.
