The importance of accurate and clearly specified product requirements can not be overstated. The reliability specification is usually part of a larger document, called the product specification or product requirements document. This document contains information such as the product description, performance specifications, environmental requirements, electrical interface specifications, physical specifications and reliability specifications.
Because the definition of reliability depends on the various product specifications, including the reliability requirements, it is important that they are well understood and properly stated in the product specification. Nothing should be left to interpretation or to be assumed by the reader, unless it is a legitimate design choice. Any operating condition or performance requirement that will affect the reliability value should be nailed down somewhere in the body of this document. To help in the understanding of how the product’s reliability will or should be measured, definitions of failure and MTBF (Mean-Time-Between-Failure) should appear in the reliability specification section. An example of a properly developed reliability specification for a printer follows:
The MTBF of the Model X-100 printer shall not be less than 2000 hours, based on the following operating conditions:
Power On: 8 hours per day; 168 hours per month
Printing: 25% Duty Cycle; 42 hours per month
Characters Printed: 40 characters per second average; 3.3 million per month
Columns and Lines: 40% average density
MTBF for purposes of this specification shall be defined as:
MTBF = Total Operating Hours for the Period / Total Number of Chargeable Failures
Where the total operating hours is the total power on time accumulated for the period during the specified useful life of a population (minimum of 200) of field installed printers, when operated within the specifications stated in this document. To establish a meaningful MTBF, operating hours per unit must be greater than 1000 hours.
A failure is defined as the inability of a unit to perform its specified function when operated within the defined limits of the product specification, requiring unscheduled maintenance to restore performance. Failures excluded from MTBF calculations include stoppage or substandard performance caused by operator error, environments beyond specified limits, power source failure, failure of associated supplies and equipment supplied by other vendors, or any other failures not caused by the printer.
In this example, a minimum operating time per unit was specified to insure that enough hours have been accumulated to obtain a statistically meaningful MTBF calculation. A minimum population was specified because a specified MTBF is an average value for a large population. Measurements for a small number of units can result in a wide range of values that will not reflect the true reliability level of the product. Some individuals have from time to time requested that manufacturers specify product MTBF requirements to a confidence level, such as 90%. This practice should be avoided because there is no direct relationship between a MTBF specification and confidence levels. The addition of a confidence level statement does not add anything to the specification, and only leads to confusion and a false sense of security by customers. The minimum population requirement is all that is usually required to eliminate small sample size measurement fluctuations. Sample size in this case equals the number of failures and the associated number of operating hours. With a large enough sample, all confidence level values will converge around the true population value. Therefore, it is not necessary to indicate a confidence level.
Another misconception that sometimes appears in reliability specifications is the stating of MTBF as a guaranteed life expectancy, or confusing a ‘guaranteed’ MTBF for an extended warranty. Based on a random distribution of failures, 2/3 of the units of a population of units are expected to fail prior to the MTBF value being reached and 1/3 after for an overall average value equal to the MTBF. Therefore, a specified MTBF does not guarantee a cost free failure repair period equal to the MTBF value. A warranty period is a period of time in which the vendor agrees to pay the cost of repairs. The product MTBF certainly helps determine the number of such repairs and should be taken into consideration when setting the warranty period. However, the two should not be confused.
Senior Reliability Consultant
Ops A La Carte