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

5.4  Procedure IV, "Lot conformance FR inspection".  Where Procedure IV is specified (see 4.6), sample units shall be selected at random from each inspection lot.  As far as practicable each style, value, class, and grade allowed in lot formation, which are in the lot formed, shall be represented in the sample in approximately the same proportion as in the inspection lot.  Permissible combinations of styles and values represented in the inspection lot shall be the same combinations permitted for Procedures I and II.

a.        The minimum number of sample units to be subjected to lot conformance FR inspection shall be as specified in 5.4.1.  Manufacturers may test larger sample sizes than indicated herein; the number of failures permitted shall be 1.

b.        When the volume of production or the frequency of orders, are such that, temporarily, compliance with this procedure presents an economic problem, the qualifying activity shall be contacted.  At the discretion of the qualifying activity, alternate sample sizes and maintenance periods shall be provided.

5.4.1  Sampling plans.  Unless otherwise specified in the ER specification, the lot conformance FR sampling plans shall be as specified in table VII.

TABLE VII.  Lot conformance FR plan.

5.4.2  Extended FR tests.  A minimum number of sample units as specified shall be selected at random from each inspection lot and subjected to the specified extended FR test.  The manufacturer may increase this sample size from lot to lot, if desired, in order to develop the necessary number of unit hours of data required for Procedure III or IV; however, each sample shall be tested for the full length of time specified for Procedure I (see 5.1.5d).  The units selected for the extended FR test may be either:

a.    From those subjected to lot conformance FR inspection tests of Procedure IV, or

b.        From remaining units in the inspection lot.  In any case, the units to be subjected to the extended FR test shall be predetermined before any FR tests are initiated.  The extended FR test may be either initiated periodically with units accumulated from each inspection lot, or may be initiated on a lot by lot basis.  The units selected for extended FR tests from each lot, shall be representative of the styles and values included in the lot to the maximum extent possible.

5.4.3  Action in case of failure:

a.        Where the lot conformance FR inspection enables early shipment of an inspection lot, a failure, to the lot conformance failure definition, should result only in disallowing early shipment of the inspection lot.  If continuation of the full qualification period does not result in an unacceptable number of failures, to the failure definition, the inspection lot may then be shipped.

b.        Those units in a rejected lot which were predestinated for extended FR testing shall either remain or be placed on test for the full length of time.

c.        Failures in excess of those permitted during extended FR testing shall be reported to the qualifying activity, and to all known recipients of the parts from the inspection lots represented by the sample.

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