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

5.3.4.1 Failure to meet overall "C" number. The following criteria, only applies where the overall "C" number has been exceeded: Where the combined lot submission exceeds the selected "C" = number, the sublotting plan is reviewed to determine if the submission exceeds the select "C" = number, the sublotting plan is reviewed to determine if the sublot numbers were exceeded. If failures have occurred in only one style above and beyond the stated limit in the sublotting table, those styles that did not exceed the sublot "C"= number can be released with the qualifying activity's approval. Where a "C" number has been exceeded for more than one sublot, the manufacturer has failed to maintain FR and shall notify the qualifying activity and proceed as stated in 5.4.4.

5.3.4.2 Overall "C" number not exceeded. Where the overall "C" number is not exceeded, evaluation of the individual sublot is not required, even though the individual sublot "C" number is exceeded.

5.3.4.3 Example of sublot calculation. The following example provides guidance for use of the sublot maintenance option (see table VI):

a. Five styles are combined for "S" failure rate level under period A.

b. A C = 2 failure rate maintenance plan is established for a 12 month period. c. This requires 110 million unit hours for the 12 month period.

TABLE VI. Example of sublot calculation plan.

		Failures occurring
Unit hours	C = #	A	B	C	D	E
Sublot 1 > or = 10.5 million	0	1	0	2	0	1
Sublot 2 > or = 53.2 million	1	1	2	1	0	0
Sublot 3 > or = 10.5 million	0	1	0	0	0	1
Sublot 4 > or = 10.5 million	0	0	1	0	0	0
Sublot 5 > or = 10.5 million	0	0	0	0	3	0
Total > or = 110.0 million	C = 2

A. All sublots affected by reduction of failure rate. B. All sublots affected by reduction of failure rate.

C. Only sublot "1" affected by reduction of failure rate. D. Only sublot "5" affected by reduction of failure rate.

E. Overall "C" number not exceeded; all lots acceptable.

5.3.5 Details to be specified. The following details are to be specified in the ER specification:

a. Applicable qualification maintenance period (see 5.3.2). For those ER specifications where Procedure IV is specified, the maintenance period letter shall correspond to the lot sampling plan letter of Procedure IV, e.g., period "A", when plan "A" is specified.

b. Data combinations permitted (see 5.3d).