Visual Inspection - Capability Study on Attribute Data

D

DJ5657

#11
Re: Capability Study on Attribute Data? Visual Inspection

Guys, and experts here, I also need your help. I want to know how to determine the process capability of my machine. My given data are weekly ppm of visual mechanical defects on ICs such as lead defect and package defect.

My idea as step by step procedure to get this are :
1. Get the measurement readings on IC samples that was processed on my machine. Let's say 100 samples. This will give me the value for standard deviation.
2. Determine my limits for parameters on lead and package defect.
3. Substitute the derived values in CPK formula to get the CPK or process capability.

The result will be a value and will tell to me my process capability. This will tell me how far my machine produces defects away from this limits. However, how can I translate the result to the recurrence rate? Or interpret it the chance of having a defect on processed units?

Does my idea posted above is correct? Please help me. I am new in this field. Thank You.
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#12
Re: Capability Study on Attribute Data? Visual Inspection

I want to know how to determine the process capability of my machine. My given data are weekly ppm of visual mechanical defects on ICs such as lead defect and package defect.

My idea as step by step procedure to get this are :
1. Get the measurement readings on IC samples that was processed on my machine. Let's say 100 samples. This will give me the value for standard deviation.
2. Determine my limits for parameters on lead and package defect.
3. Substitute the derived values in CPK formula to get the CPK or process capability.
just for clarity - are you only dealing with counts of defect data? no continuous or ''varables' data on input parameters?
if only using defect data do you also have the count of inspected units (with and without defects)
the best approach to defect data is to use a c chart (if the number of units per period is roughly equal) or a u chart if the number of inspected units has substantial variation.
If this is a standard inspection why use 100 units? why not use ALL of the inspection data?

you can use the upper control limit as the capability of the process. this provides a direct report of the defect rate that your process creates. (without calculating the Cpk.) IF you need a Cpk value for your customer, you can back calculate it using the Z formula.
 
D

DJ5657

#13
Re: Capability Study on Attribute Data? Visual Inspection

Hi, Bev D. Thank you so much for your reply. Yes, I am dealing only with a pass/fail result which was obtained from sampling inspection. Each sampling inspection is per lot and each lot has different lot size. So no same number of lot size from sampling inspection from previos weeks. I guess the U chart you mentioned is more appropriate with my condition, but can you help me how can I obtain from my defect rate data per week in PPM the upper and lower control limit? And how the U chart result will help me to assess the current process capability of my machine?

Thank you so much for helping a newbie like me.

Regards,
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Re: Capability Study on Attribute Data? Visual Inspection

Formulas for the u chart:

each plotted point = defect rate per lot = ui = sum of defects in the lot/sum of units inspected in the lot

center line = u_bar = (sum of all defects)/(sum of all items inspected in all lots)

LowerControl Limit = u_bar-3*sqrt(u_bar/n)
Upper Control Limit = u_bar+3*sqrt(u_bar/n)
each data point will have it's own control limits
n= the number inspected in each individual lot.

u_bar is the average defect rate across all lots and represents your capability.
the individual control limits represent the highest expected defect rate in an individual lot, given its sample size...

you can translate to a Cpk value (But only if required to do so by a Custoemr as it really is meaningless compared to the control chart which shows in direct terms the capability of the process):
Cpk = NORMSINV(1-defect proportion)/3
NORMSINV is an EXCEL function that converts teh defect proportion to a Z-score. Cpk = Z/3...
 
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