Should I use Xbar-R chart?


Blueberry Nut
Hello all,

I work with a semiconductors company. My division manufactures wafers, and from each wafer we'll get thousands of devices. We don't run many wafers, about one wafer every week or so (one wafer/wk makes a LOT of product), so it takes us a long time to gather lot by lot info.

If we sample, say, 9 devices and test them, could we plot an X-bar/R chart of the test results? My concern is that the 9 samples would not be consecutive in time, but taken "at the same time" because they were all created together. About the only thing we could randomize would be the location the samples come from on the wafer itself.

Am I on the right track? Or am I worrying about something irrelevant and I should go ahead with my original chart? All the other industries I've worked with had running production lines so I never considered this question before.

I should probably mention that this question was originated by a new customer for a new part. They want us to sample 9 random test sites, plot them on an SPC chart, CALCULATE Cpk on the 9 and make sure its >1, before shipment (shows how much they know about Cpk's! Scary...At least they seem to know what CPk=1 means) Almost sounds like they want us to plot each inidividual point on the 'SPC Chart' (eeck!!) :frust:

Any wiser heads care to comment/ help me out on this one?
Elsmar Forum Sponsor

Laura M

Interesting question. You did a good job describing your process, but I do not understand wafer manufacturing very well, so here's my theoretical opinion.

In order to answer it, you need some understanding of the variation of the process. Your within subgroup variation in the 9 pieces should ensure "common cause variation" is captured. (The Cpk calculation doesn't make sense to me.)
Randomizing location makes sense to me. What % of production does 9 pieces represent? How many do you get from one wafer?

Samples for X'bar R charts must be "rational subgroups." Does the R-bar estimate of std dev. represent the process variation? Can you compare one week's wafer to the next? Have you established if the process is in control?

Rick Goodson


I believe your basic question deals with the sampling technique. Since your are randomly selecting from the wafer you are not using the commonly used instant-time method (all the devices sampled being made one after another). You are using the period-of-time method (similar to random sampling from a lot of material). With period-of-time method the within subgroup variation is maximimzed and the between subgroup variation is minimized. The period-of-time method is less sensitive to changes in the average.

Regardless of the method used for sampling, in statistical process control there is an assumption of homogeneous lots. That is, the pieces in each lot (read wafer) are as alike as possible; same machine, operator, robot, material, etc. I would be more concerned about the week or two time lag between samples and changes to the process. IMHO too many things will change in a week or two for the charts to be of value in process control.
Thread starter Similar threads Forum Replies Date
S Should I use Grand Mean or Target Value as the CL for Xbar-R, MA, CUSUM? Statistical Analysis Tools, Techniques and SPC 6
S What should I use for Variable Sample with Size 3, 4, and 5 - xbar-R or xbar-S? Statistical Analysis Tools, Techniques and SPC 4
M Should DoC be updated every time Technical Documentation is revised? EU Medical Device Regulations 2
F How often should Gage R&R's be updated? Is there a recommended time-frame? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
A Which certificate should I start with from ASQ, CQIA or CQE? (or should I take something else) Professional Certifications and Degrees 4
E Please help! ! I AM FRASTRATED SO MUCH! Should inactive ingredient be included in the unit formula if it is removed during the manufacturing process? US Food and Drug Administration (FDA) 0
I IEC 60812 or ISO 14971 for PFMEA? What should we use? ISO 14971 - Medical Device Risk Management 3
qualprod Do sum of results of quality objectives should met a high level goal? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 10
F Should I perform several Gage R&R for every caliper to cover all specs range of use? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
E Opening meeting for Third-Party Audit--Who should say what? General Auditing Discussions 22
8 MSA--Should I Use Parts or Gage Pins? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
B Should I buy IEC 62304:2006, IEC 62304:2006/AMD 1:2015 or both? EU Medical Device Regulations 1
W "Accurate to ±2%" means "64% of measurements should be within 2% of the actual value"? US Medical Device Regulations 4
H Should we stop inspecting a part if its never been rejected? Reliability Analysis - Predictions, Testing and Standards 6
D What distributor requirement should we (manufacturer) require from the Distributor? EU Medical Device Regulations 0
M Should there be another column in the Optimization section (step 6) of AIAG-VDA DFMEA form? FMEA and Control Plans 1
C By when should harmonized standards be complied with? EU Medical Device Regulations 5
W Strategy for determining which components from a system should be "ME EQUIPMENT" -- home healthcare environment IEC 60601 - Medical Electrical Equipment Safety Standards Series 6
B Documented information - Should be controlled? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 10
Q Is engineering a process and should it have its own process turtle? IATF 16949 - Automotive Quality Systems Standard 7
P Which MDCG Document should I use for PMS Plan and PSUR? EU Medical Device Regulations 2
M Who should quality representative report to? ISO 13485:2016 - Medical Device Quality Management Systems 5
H When should the first PSUR be issued? EU Medical Device Regulations 5
I If i do not want to be an initial importer should i register with FDA? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 0
O Should a Covid vaccine and testing policy be included as part of ISO9001 or AS9100 risk management? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
B UKRP to what level should you audit Class I Technical Documentation? UK Medical Device Regulations 0
C When should you quit programming? Job Openings, Consulting and Employment Opportunities 9
C Should resolution be included in uncertainty budget for digital caliper or micrometer calibration? Measurement Uncertainty (MU) 5
Ed Panek External Standards List - Should this document include previously revised standards? ISO 13485:2016 - Medical Device Quality Management Systems 4
T How should I approach REACH, CM, etc. as a job shop? RoHS, REACH, ELV, IMDS and Restricted Substances 18
A Should we assign the PRRC before the date of application of MDR (26 May 2021)? EU Medical Device Regulations 0
J UDI-DI how should we interpret Device version or model to determine if a new UDI-DI is needed? EU Medical Device Regulations 0
Sidney Vianna Interesting Discussion Should ISO 9004 be changed from a guidance document to a requirements standard? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 13
A Should I take an online course for a career in Occupational Health and Safety? Career and Occupation Discussions 2
J Should a Class 1 medical device with an option to measure body weight be considered Class 1m? EU Medical Device Regulations 0
K Should APQP/PPAP has its own section in a QM? Quality Management System (QMS) Manuals 1
S What should i choose for "testing procedure" characteristics? (N95) General Information Resources 0
P Should eIFU link per ISO 15223-1:2016 be added to labels out of scope of Reg 207/2012? EU Medical Device Regulations 1
S Which Sampling Plan(s) Should I Use? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 13
A Document release vs its related training. Which should come first? ISO 13485:2016 - Medical Device Quality Management Systems 18
S Which department should prepare the control plan? could you show me a standard regarding to this matter. FMEA and Control Plans 17
J Help settle a disagreement: Should external providers of preventive maintenance be on your ASL? AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 5
N Master Samples - What should we be keeping? IATF 16949 - Automotive Quality Systems Standard 9
G Supplier delivered recent PPAP, should he deliver yearly layout inspection? IATF 16949 - Automotive Quality Systems Standard 7
John Broomfield Should ISO 9004 Become a Requirements Standard? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 70
A Capability Study - in the beginning of your career what should you have known about the tool Quality Tools, Improvement and Analysis 11
J Should Loading and Unloading be Included in Cycle Times? Lean in Manufacturing and Service Industries 14
E Manufacturers should develop a testing device for covid19 Service Industry Specific Topics 0
T 510(k) submission - Which name should I use in the submission? Other US Medical Device Regulations 3
N ISO 19011:2018 - 5.4.2 "...audit program should engage in appropriate continual development..." Training - Internal, External, Online and Distance Learning 4

Similar threads

Top Bottom