5LTPD PlansThe quality level at 10% probability of acceptance (consumer’s risk) is called the LTPD.This rejectable quality level (RQL) is highest percent defective (poorest quality) tolerable in a small percentage of product.Borderline of distinction between a satisfactory lot and an unsatisfactory one.LTPD plans are used for many product qualification plans to assure consumer protection.
6Common Sampling Problem in Industry There are constraints on sample size based on limited time, money, or other resources.There is often the need to adjust sample size and corresponding acceptance number while holding LTPD constant.
8Limitations of Tables LTPD values restricted to only those listed. There are finite ranges of sample sizes and acceptance numbers.
9Example CaseReliability qualification plan for integrated circuits calls for stressing a sample of 300 units for 1000 hours. Pass requirement is no more than three failures.Early samples are precious, costing approximately $10,000 each and are needed for other evaluations.How can the engineer reduce the sample size and allowed failures while holding the LTPD constant?
10Approaches by Engineer First, the LTPD value must be determined.Then, LTPD tables may be consulted to see if n = 300 and c = 3 are tabulated.Approximation may be necessary:Checking LTPD table, we see n = 333 and c = 3 for LTPD = 2%.For c = 1, LTPD = 2%, we need n = 195.
11Graphical Techniques* *Applied Reliability, 2nd ed., P. Tobias and D. Trindade
12Graphical Results For n = 300, c = 3, LTPD = 2.2%. For LTPD = 2.2%, c = 1, n ~ 180.There is a limitation in these graphs toonly c = 0, 1, 2, or 3.