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

5.3.3 Failure to maintain FR level qualification (see 4.3). Failure to maintain a qualified FR level in accordance with 5.3 shall result in loss of qualification at that FR level. Loss of qualification to a FR shall take place at any time during the qualification maintenance period when the number of failures recorded exceeds the number of failure permitted. The extent of the product affected by the loss of failure rate will be determined by the qualifying activity and may include all product represented by the unit hours on test during the maintenance period in question. If, however, the number of failures recorded exceeds the number of failures permitted (i.e., "c number") by one, the qualifying activity may permit the manufacturer to modify the test plan in an effort to maintain the original failure rate level. This modification to the test plan shall only be allowed with qualifying activity approval following evaluation of the nature and cause of the failure. FR data shall be reviewed by the qualifying activity to justify reestablishment of qualification at a higher FR level. Reestablishment under these circumstances shall be to FRSP-60 or FRSP-90 (see

5.1.5a). All data generated over the production period (corresponding to the qualification maintenance period in FRSP-10) shall be reviewed in reestablishing qualification. FR data shall include the data which caused disqualification. To extend qualification from the reestablished FR level to the lowest level held, the manufacturer shall be required to meet the same requirements specified for Procedure II (see 5.2). Removal of life test samples that have failed during performance of the life test shall be in accordance with the following:

a. When the failure of a sample is within the "C" number predicted for the maintenance period, the manufacturer is permitted to remove the failed sample unit from the life test. Prior notification and approval by the qualifying activity is not required as long as the "C" number is not exceeded. Life test data associated with the failure as well as the failure itself shall be counted toward meeting the number of unit hours and selected "C" number for the maintenance period. Manufacturers are not permitted to discard any sample(s), life test data, or failures without notification and approval by the qualifying activity.

b. When the failure of a sample results in exceeding the specified "C" number for the maintenance period, the manufacturer is permitted to remove the sample(s) from the life test at the manufacturer's risk and shall notify the qualifying activity immediately. Manufacturers are not permitted to discard any sample(s), life test data, or failures without prior notification and approval by the qualifying activity.

c. When life test failures are attributed to an unusual occurrence (test equipment malfunction or failure) the manufacturer is permitted to remove the sample(s) from the life test at the manufacturer's risk and shall notify the qualifying activity immediately. Manufacturers are not permitted to discard any sample(s), life test data, or failures without prior notification and approval by the qualifying activity.

5.3.4 Sublotting failure rate maintenance procedure, failure rates R and S only (optional). The manufacturer may select the sublotting procedure at his option with qualifying activity approval. When selecting the sublotting option, the manufacturer shall submit a sublot procedure plan to the qualifying activity prior to the initiation of the maintenance period. The sublot procedure is used to demonstrate/validate failure rate levels for each sublot in addition to the overall failure rate (see 5.3). the procedure plan shall include:

a. The overall "C" number and unit hours selected for the overall plan.

b. The definition and description of individual sublots (styles, combination of styles, values, and value ranges). c. The "C" number sublot and unit hours for the individual sublot (see 5.3a).

NOTE: The individual sublot unit hours shall meet or exceed the unit hours required by the individual "C" number for the failure rate level (see table IV).

d. The sum of individual sublot unit hours shall meet or exceed the unit hours required by the overall "C" number for the failure rate level (see table IV).

e. The combination of sublots and ranges selected shall meet the requirements of 5.3.1c, except as required to meet the minimum unit-hours requirement.

f. All changes to the previously accepted plan shall be submitted for review and approval of the qualifying activity. The submission and approval shall occur prior to subsequent initiation of a maintenance period.