MIL-STD-690-SUP Failure Rate Sampling Plans and ProceduresThis standard provides procedures for failure rate FR qualification, sampling plans for establishing and maintaining FR levels at selected confidence levels, and lot conformance inspection procedures associated with FR testing for the purpose of direct reference in appropriate military electronic parts established reliability ER specifications. Figures and tables throughout this standard are based on exponential distribution. Weibull distribution will be acceptable in certain components such as capacitors. Use of Weibull distribution for any component must be approved by the qualifying activity. This standard also provides guidance to specification writers in the use of this standard see appendix A and references material for users of ER parts.

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MIL-STD-690D

4.1.4.1  FR marking upgrading.  Where parts have been produced for part manufacturer's stock to a specific failure rate level and qualification has been subsequently extended to a lower failure rate level, the existing stock may be remarked to the latest qualified failure rate level provided:

a.        The lower failure rate shall have been achieved solely through the accumulation of FR test data with no change in materials, process controls, process limits, except as accepted by the qualifying activity in accordance with the corrective plan of action requirements of MIL-STD-790.

b.        The manufacturer provides a written detail procedure of the remarking process and test data to substantiate that the remarking procedures do not affect the part reliability or performance and the procedure is accepted by the qualifying activity.

c.        Parts shall have passed the ER specification conformance inspection and shall have been in stock for no more than 3 months, except where the manufacturer provides test data substantiating longer storage periods.

d.    If parts are remarked, date and lot code shall not be changed.

4.2  Failure criteria.  Deviation of one or more specified parameters beyond the specified limits shall constitute a failure.  If more than one parameter is to apply to the FR test, failure of more than one parameter on a single sample constitutes one failure in determining conformance to the acceptance criteria.

4.3  FR test records.  Test records shall be maintained for the period required to substantiate the FR level qualification and shall include the data derived from the FR tests.  The format is suggested on figure 1.  Any measurement that indicates failed parts shall be clearly marked and identified as such in the test record (for exemption of data see 4.4).  A sample unit which fails any given time interval shall be recorded as a failure immediately following the previous measurement and for all subsequent measurements.  The manufacturer may remove failed sample units from the test.  If the "C" number predicted for the maintenance period is exceeded or the failure is attributed to an unusual occurrence, the manufacturer shall immediately notify the qualifying activity (see

5.3.3.).  If a failed sample unit is not removed, the test hours accumulated subsequent to its failure shall not be recorded with the cumulative component hours.  All failures occurring during any FR test shall be reported to the qualifying activity at the time of failure.  All FR data recorded shall be variables type data; attributes type data is not acceptable, except as permitted in the applicable ER specification.  Figure 1 type data shall be made available when requested by the Government.  A cumulative type record as shown on figure 2 shall be submitted to the qualifying activity at the end of each applicable maintenance period (see 5.3), or when requested by the qualifying activity.

4.4  Exception of data.  Where FR test is known to be faulty as a result of test equipment failure, the test data obtained shall be entered in the test record along with a complete explanation and submitted to the qualifying activity. The qualifying activity shall determine whether the failure will be used in the computation of the FR level.  There shall be ample technical and statistical evidence that the cause of equipment failure has been removed and will not recur in future production.  No FR sample lots can be removed from test without the approval of the qualifying activity.

4.5  FR qualification procedures.  FR qualification is an integral phase of the total qualification requirements in ER specifications.  The procedures specified herein pertain only to the reliability requirements of these specifications. Qualification at any FR level shall be specified at either a 60 percent or a 90 percent confidence level.  FR qualification procedures are as follows:

a.    Procedure I, "Qualification at the initial FR level" (see 5.1).

b.        Procedure II, "Extension of qualification to lower FR levels" (see 5.2). c.        Procedure III, "Maintenance of FR level qualification" (see 5.3).

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