Cpk for Test Parameters

Guest

Can someone explain to me what's Cpk in the context of test parameters? Is this the same as Cpk in normal processes? Is the computation for Cpk in test the same?

Confucius

Joined: 20 Jun 2026 Posts: 20

Confucius,

I think Cpk is still the same when used for test. The USL and LSL will be your test param's max and min limits (suggest you use the prod limits, not QA). The mean and sigma will be based on your actual test data.

Test Engg 99

Joined: 20 Jun 2026 Posts: 20

I forgot to mention that you can only use this for params with normal distributions.

Test Engg 99

Posted: Tue Mar 22, 2026 3:12 pm Post subject:

CPKs are a good indication of where your mean is located with respect to the upper and lower spec limits. the higher the value of the cpk, the farther the mean is from the spec limit of interest.

CPK_hi = (usl-mean)/3sigma
CPK_lo = (lsl-mean)/3sigma

Posted: Thu Apr 21, 2026 3:57 pm Post subject:

I just have a correction. CPK_lo should be computed as shown below.
CPK_lo = (mean-lsl)/3sigma

In addition to the explaination above, cpk's are computed using the three factors:
a. Limits
b. mean
c. The standard deviation(sigma)

Assuming that the limits are known, the cpk's would then be :
1. directly related on how far the mean is from the limits(USL or LSL)
2. and indirectly related to the standard deviation(sigma)
= this means that the cpk would be reduce if readings on a certain parameter is erratic(resulting to a large Standard Deviation)

Asmoth

Guest

Con-fuse-use,

wag ka nang maghirap pang umintindi sa mga notations sa expalanations ng mga to.

just go to www.google.com and research about Cpk and statistics.

or go to a good, I mean a GOOD engineering library. I suggest you go to UP library, its the only good engineering library.

masyado ka kasing outdated. upgrade yourself.

Ka Totoy

Posted: Mon Jun 13, 2026 10:16 am Post subject:

Cpk is a measure of how capable your process is. So if you are talking about the Cpk as applied to a test parameter, then the question is what process are you considering? It is either you are measuring the capability of your test process ( tester,test program, etc ) or the capability of the fabrication of the die itself. In a product engineer's point of view, I would think the Cpk which is of more importance will be the capability of the wafer fab process as this translates to yield,cost,reliability,etc. This does not mean that your test setup should not be given importance as well.