Gage R&R with ATE (Automatic Test Equipment)

J

JStain - 2008

#11
Mintab Anova Model

Hey KEN K:

I'm using MINITAB 14.

When I tried to run ANOVA GLM it wants to have my MODEL. :yuk:

Also RANDOM FACTORS, OK no clue here. :frust:

Any Ideas?
 
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J

jstevensss

#12
[please excuse any redundant questions. I composed this prior to finding this thread]


Hi All,

First off, this forum is a fantastic resource for information... I’ve only been perusing the threads for a day or so but I’ve learned a lot of new things already. By way of background, I’m new to my company and have essentially made a career change from PC-based instrument design to test platform development and analysis. That said, I’m sailing uncharted waters and am in need of some advice regarding a starting point.

One of the first buzz words I heard when I arrived at my new job was "Gage R&R." Right now there’s no identifiable control or understood capability of the ATE that is used to test our semiconductor products. Being told to go "do a GR&R" is fine but what does that really mean and what will it buy me? Is it really the analysis that I need?

The problem as I see it:

I’m concerned that there may be a misunderstanding of what GR&R will provide. The product specifications are starting to push the specified accuracy of the ATE in some areas. In particular, voltage and frequency measurement. On paper, the specified accuracy is generally at least 10x better than the parameters of interest so I believe we’re following the 10% rule.

I’m concerned that people here want a GR&R to see how far we can push the ATE beyond specification. I believe this is incorrect and not the point of a GR&R. GR&R will only tell us how our measurement system performs from device-to-device, operator to operator. It does not tell us anything about absolute accuracy performance. I believe that’s better suited for an MSA.

Am I correct is saying that a well-constructed MSA will include a GR&R but GR&R itself will not tell you anything about pushing a tester beyond its specification? Even if there is capability beyond the specs verifying and guaranteeing that performance becomes an issues.

Some general questions:

1. Since a piece of ATE is comprised of a number of measuring instruments, I would think that GRR /MSA would be performed on each instrument or at least those considered critical. (That may be obvious but I had to ask). You don’t qualify a tester with GR&R; you qualify the components of that overall tester that are critical to the parameters being measured.

2. It seems to me, although more involved, that the correct approach may be to perform and MSA on the instruments in question. That way we’ll be forced to understand stability, bias and linearity before attempting the GR&R.

3. From all I’ve read, the reproducibility part of the GR&R involves operators. Since the tests are performed with automatic part handlers under program control I’m wondering where the reproducibility part fits in? Yes, a human sets everything up but then basically watches the automatic test of 10K devices. I suppose the handler itself could have some influence on the measurement but I’m having a tough time conceptualizing where this fits n the overall R&R.

4. Is anyone out there in the semiconductor test business that has performed this sort of exercise? I’d be most interested in understanding your experience and findings with regard to this application. I searched the web far and wide for GR&R/MSA specifically as it applies to ATE and I’m surprised at the lack of available information.

Thank you for being patent with my long-winded introduction to this forum. And yes, I have the Fluke “Calibration: Philosophy in Practice” and the AIAG “MSA” books on order. Both are due this week.

Best regards to all, :thanx:

Jim
 

Ron Rompen

Trusted Information Resource
#13
I'll try and keep my answer short. You ask some VERY good questions, which (IMNSHO) also apply to CNC-controlled measuring devices such as CMM's and Vision Systems.

1) Yes, I agree.

2) Absolutely. Before doing GR&R, you need to establish linearity, bias and stability of the measuring device in question.

3) What you are looking at is machine repeatability. But are you confident that the operator cannot influence the measurement? What if the incorrect part is loaded, or the orientation is incorrect? Will this still allow a measurement to be taken, or will the ATE 'fault out'

4)Not me. I'm a metal stamper....but I am sure that there are Covers here who either are in that field, or have been in the past.
 

Caster

An Early Cover
Trusted Information Resource
#14
When you get the MSA book

jstevensss said:
[please excuse any redundant questions. I composed this prior to finding this thread]

3. From all I’ve read, the reproducibility part of the GR&R involves operators. Since the tests are performed with automatic part handlers under program control I’m wondering where the reproducibility part fits in? Yes, a human sets everything up but then basically watches the automatic test of 10K devices. I suppose the handler itself could have some influence on the measurement but I’m having a tough time conceptualizing where this fits n the overall R&R.

Thank you for being patent with my long-winded introduction to this forum. And yes, I have the Fluke “Calibration: Philosophy in Practice” and the AIAG “MSA” books on order. Both are due this week.

Jim
When you get the MSA book, check out pages 53,73,97 and 197 Appendix D for answers about ATE. Your suspicions are correct. There is no human error.

The MSA is actually a pretty good document, now that I took the time to sit down and read it.

For automated test systems you can either ignore Reproducibility (Operator) and do a GR study or if you have multiple test stations, set them up as Operators and do a GRR. (Pit them against each other)

We plan to do this with our two CMM machines - Repeatability is within machine, Reproducibility is between machines (I hope I said that right)

Don't get me started on within part variations! I'm still bedazzled by that!

Good luck, let us know how it turns out.
 
A

Arvind

#15
Understanding intent is important for selecting # of parts and # of repeats

I am amazed by different people throwing numbers for # of parts versus # of repeats for Gage R & R calculations without considering the usefulness or effect of these.
Here are some guidelines.

1) In gage R & R, we are essentially comparing variation of measurement system with variation within the parts. Therefore, sample size will depend on how much is variation with parts. If your samples selected show lower variation than actual, then gage R & R will be poor.

2) If you increase repeats, gage R & R will improve by 1/(SQRT(n)). This is from central limit theorm. Therefore, repeats selected for R & R must represent reality. Else R & R using high repeats will give a false security of good R & R.
Arvind
 
J

jstevensss

#16
More thoughts regarding GRR for ATE...

Thanks much for all the responses up to this point. They've been very helpful.

Arvind, you do make a good point regarding the number of devices in a GR study. I've seen something like 5-20+ pcs depending on the document. I would think that the idea is to have enough devices that are representative of the overall process variation. I would assume that I would need samples that represent nominal, somewhere between nominal and the spec limit, some very close to spec and some outside of spec. A couple of devices in each one of these bins would suggest something like a 10pc sample?

With regard to the number of repeats, it looks like many examples call for two or three. I struggle a bit with the concept of what qualifies as a "repeat" on an automated measurement (say a low voltage measurement). Averaging is a common technique used in electrical measurements. The intent is to minimize any sprurious noise contributions from external sources (not the device) that may be getting into the system. The same often holds for frequency or time measurements. The "result" is actually a collection of many repetitive measurements averaged together. I can't convince myself that averaging to get a particular result adversely affects the outcome of a GRR; Averaging is part of the measurement technique. Any comments on this subject would be much appreciated.

In practice, there is only one attempt at any measurement with ATE. The GRR that I need to run on several pcs of (identically configured) ATE is shaping up as:

1. Identify instrument types - voltage meaurement, current measurement, frequency measurement
2. Identify critical parameters/tests that rely upon these intruments. Keep the 10:1 rule in mind when doing so.
3. Perform a repeatability study on:

Instrument A, Tester 1
Instrument A, Tester 2
Instrument A, Tester 3

4. Perform a reproducibility study of Instrument A across testers 1,2,3 (thanks Caster)

5. Repeat steps 3 and 4 for the other instruments.

6. Qualify a particular tester based on having acceptable GR results for all selected instruments. Qualify a group of testers based on acceptable GR&R result for selected intruments across machines.

Conceptually, does anyone out there agree with this sort of strategy? This dialog has been very helpful and I appreciate all of the responses.

Jim
 
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