Tolerance calculations - Page 1
Forum section: electronics / beginners

author: edba
author_rank: Contributor
timestamp: September 19, 2020, 07:33:44 pm
content: HiDoes anyone know how the min max values on a data sheet are calculated? Presumably the manufacturer takes a sample of the components from production and does some statistics to calculate the min max tolerances and put them on the data sheet?

author: Alti
author_rank: Frequent Contributor
timestamp: September 19, 2020, 08:15:30 pm
content: The min max values are part of the contract, the agreement between customer and the manfufacturer.How do they calculate it, if at all, depends on various factors and for sure a bit of statistics and a lot of marketing is involved in that.I presume you ask about regulations or norms that describe how to provide tolerance of resistance or operating temperature range of some electronic component. Either this information is explicitly provided in the given datasheet (usually as a testing circuit drawing with DUT) or by reference to some regulation, or it is left, many times unspecified, assuming there must be some benign circumstances where this min max holds in some narrow range.Of course min max might come from sampling components but many times it is just stated: "this parameter is not tested in production but based on a design this is going to be somewhere in between 5 and 8".Some other times datasheet does not state min/max but the probability distribution of some lot is given. Of course not necessarily the lot that you buy but some other lot. Then you have to calculate that min and max by yourself and hope your lot is going to be quite similar to the sampled one.It depends.

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2020, 09:14:22 pm
content: Quote from: edba on September 19, 2020, 07:33:44 pmThe min/max values on datasheets are not "calculated" but are tested during production as go/nogo parameters for every unit."Typical" specs are a different story.("Alibaba" parts can be different.)

author: edba
author_rank: Contributor
timestamp: October 06, 2021, 09:40:08 pm
content: I thought most manufacturers estimate the min and max tolerance by first sampling their production process and tweeking it untill the standard deviation is one third of the min max required i.e. min max is +/-3sd of normal production variation?

author: TimFox
author_rank: Super Contributor
timestamp: October 06, 2021, 10:10:38 pm
content: They may tweak the process when setting up, but ATE ("automatic test equipment") is used during production with "go/no-go" settings. Some parameters take a long time to measure, and if they are guaranteed the component will cost extra. I talked with film capacitor and wire-wound resistor manufacturing engineers years ago, and they were proud of the tight performance of their production setups.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2021, 03:12:59 am
content: If the listed minimum and maximum specifications are not tested, then the datasheet should say something like "guaranteed by design". Typical specifications come from the statistical sampling.Testing is a big cost of final parts and is taken seriously. Their specifications better fit within the listed minimums and maximums.