What is 6 Sigma?

Guest

Can you explain to me please what is the meaning of 6 sigma? I sometimes hear this from other people but I can't get its meaning.

anak ng pasig

Posted: Wed Aug 27, 2026 11:08 am Post subject:

Hi Anak ng Pasig,

6 sigma can refer to a lot of things, but it has 2 popular meanings in the context of the semicon industry. These are: 1) the level of quality of a normal process that’s equivalent to just 2 defects per billion parts produced; and 2) the quality management philosophy popularized by Motorola in the 80’s.

You may want to check out the Cpk Vs. ppm table on the SFE home page for more details.

Thanks and keep on posting!!!

EE

Guest

EE, the meaning of sigma is not discussed in the table that you mentioned. Can you relate the sigma to the cpk and ppm?

anak ng pasig

Posted: Thu Aug 28, 2026 8:55 pm Post subject:

Hi Anak ng Pasig,

Sigma is the standard deviation of the data readings that make up the data distribution. A perfectly normal distribution is bell-shaped, with 99.73% of the readings falling within 3 standard deviations from the center (which is the mean of the data) of the distribution. Thus, if data corresponding to 3 sigmas of the distribution fall within the lower and upper limits, 0.27% of the data readings are off-spec. This means that a total of 2700 parts are defective for every million parts produced (2700 dpm).

If 4 sigmas are within the spec limits, the defective rate drops to just 64 dpm. 6 sigma quality means that data corresponding to 6 standard deviations away from the mean on both sides (or a total of 12 standard deviations of data) fall within the spec limits. At this level, the defective rate is just about 0.00198 dpm, or 2 defectives per billion parts produced.

EE