How to decide AQL value? Sampling Plans

H

Helenliu

#1
I am in one semi-conductor industry company. I meet a real problem about how to decide AQL value.The defect average ratio was found by 100% pertent Visual inspection about 20 ppm. We want to change the inspection ratio from 100% to sampling check, but I don't know which AQL level is effective and meaningful.
Any friends:thanx: can help?
 
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#2
Re: How to decide AQL value?

I am in one semi-conductor industry company. I meet a real problem about how to decide AQL value.The defect average ratio was found by 100% pertent Visual inspection about 20 ppm. We want to change the inspection ratio from 100% to sampling check, but I don't know which AQL level is effective and meaningful.
Any friends:thanx: can help?
OK, so your process average is 20 ppm or 0.002%, excellent yield that. If you maintain your process under the existing control you can expect to achieve Average outgoing quality (AOQ) of 0.002% defectives.
From the preferred AQL for single sampling plan under tightened inspection level the best AQL is 0.01 by which the Average outgoing quality limit (AOQL) would be 0.0184.
So perhaps if you follow this plan you could expect a worst quality of 184 ppm going out.
Tightened inspection, Single sampling plan, General inspection level III, AQL 0.01.
You could try this and check your results if it sounds acceptable.
Note here that your process consistancy is of utmost importance, since this ready plan can statistically pass 20 ppm to 184 pm defectives.
These are preferred AQL and if you wish to opt for still better AQL then the standard table does not work .
(Ref: ISO 2859-1:1999)
 

Miner

Forum Moderator
Leader
Admin
#3
My experience with processes at that level of ppm has been that sampling inspection is ineffective. You basically have two choices: 100% inspection or no inspection.

I would recommend that you assess the risk of passing a nonconforming product and base your decision between 0% and 100% inspection on that risk. I also recommend that you consider some type of process control to ensure that your process stays at 20 ppm and does not deteriorate.
 
Last edited:

Bev D

Heretical Statistician
Leader
Super Moderator
#4
Miner is absolutely correct: that defect rate is going to require a very large sample size. For example if you wanted a plan that detected a 20ppm defect level 95% of the time (an RQL plan) your sample size would be 150,000 (149,786 to be exact). If you want to accept a 20ppm rate 95% of the time (an AQL plan) and you have a common c=0 plan, then your sample size would be 2,550.

Your management probably doesn't understand statistical sampling though right?
If not we can guide you through what the sample size would have to be depending on the level of protection you would require.

A few questions:

PROCESS: I assume that you are doing 100% human visual inspection of all die on all wafers? Is this post wafer/probe test and pre assembly? If not can you tell us where in the process the inspection is occurring?

VARIATION: When you do see defects are they randomly distributed across all wafers, only on some wafers and not on others or are they clustered in a specific location on a few wafers (such as might happen given some fixture location related cause). Or is there any other common pattern to the defects? Are there are any specific patterns when you have an abnormally high defect rate (when a ‘special’ cause occurs).

PROTECTION: Do you want to allow the current average defect rate to escape or do you want to detect that rate? What is the rate that you are comfortable shipping to your Customers? What rate would you be uncomfortable shipping to your Customers?

Once we know the answers to these questions we can up with a suitable AQL/RQL and sample size. In all likelihood the sample size will be very high and then you can let your management make an informed decision about sample inspection or 100% inspection
 
H

Helenliu

#5
Thanks for all the answers.
First let me give more details about the process.
Actually it¡¯s refering to inspection after wire bonding process, and the upper mentioned defect is "no wire"(which is "missing bonding", ppm level is no more than 10ppm based on former 100% visual inspection). Management don't want to see such failure as if such failure could be detected then it could be reworked to reduce chip loss.
Due to manpower restriction, sampling is requested to replce 100% VI.
From above answers, I understand that if follow AQL=0.01 it's not able to detect such failure, thanks you all.
So there's no need to perform sampling inspection for a single defect of "no wire". But I want understand more on this question.
Now let's consider all the detects of wirebonding. The target is 140ppm, and now the actual average ppm level is about 80ppm(based on a self inspection conducted by wire bonding operator, sample size is about 8.3% of lot size(n/N=8.3%, lot size is about 10k-25k).
The question is:
1. If accept a ppm of 80ppm, AQL=? Sample size=? How to calculate?
2. Is the sample size of about 8.3% lot size enough to detect the defects of about 80ppm?
 
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