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July 1997

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Subject:
From:
Ed Cosper <[log in to unmask]>
Date:
Wed, 9 Jul 1997 11:44:41 -0500
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Dear Carrie,

Be careful when you start getting involved in how your customers perform receiving inspection. To answer your question as to is there a statistical way to determine a serious systemic problem? Sure there is. Your customer can simply follow any sampling plan and plot finding to determine process variation. The problem is who determines what constitutes
serious. 

It sounds to me that your customer is aiming for zero defects using the old xx-0-1 plan. The problem with this plan is the fact that in every board shipment, if you look long enough and close enough you can find 1 part that technically wont meet some part of the specification.
This is where the ability of the customer to apply "common sense" comes in. There is also a 
major variation between inspectors and inspection techniques.

My suggestion to you is to ask your customer to use a standard 1.0 AQL normal sampling plan. I have always likes the sample tables called out in the old 105E standard. Probably because I've used them so long and have become familiar with them. However, all sample plans have an inherent degree of the probability of escape. Even the xx-0-1 plans. If you have a record of providing good parts and there are no apparent assembly or function problems, I would suggest that your customer go to a dock to stock program. The savings in inspection costs alone would justify such a move and I doubt any additional assembly costs would arise. After ll, we are all fully aware that the effectiveness of visual inspection on todays designs are limited at best. Even if parts pass a sample, some defects are getting to the line and only function defects usually detected.

Ed Cosper
Director Quality Assurance and Engineering
Graphic Electronics Inc.


----------
From:  [log in to unmask][SMTP:[log in to unmask]]
Sent:  Wednesday, July 09, 1997 10:24 AM
To:  [log in to unmask]
Subject:  RE: PWB OEM Incoming Inspection

     One of my customer's is interested in feedback from other companies 
     regarding incoming inspection as follows:  Any responses would be most 
     appreciated and will be sent to them.  
     
     
     1)  What would a normal telecommunications industry AQL and sampling 
     plan be for a company that has contractual obligations to meet 
     Bellcore's TR-NWT-000078 and that uses IPC-A-600, class 2, as an 
     inspection criteria for printed wiring boards?  This company also 
     wishes to use a reduced labor sampling plan.
     
     2)  Example:  Items being inspected at Incoming Inspection are printed 
     wiring boards. They are inspected to Bellcore TR'78 and IPC-A-600, 
     class 2.  The company inspection methodology is that a single 
     manufacturing process defect found on one board out of the sample 
     group (e.g. qty. 9) will cause the entire lot of boards (e.g. qty. 
     5,000) to be rejected.  Is this proper taking into consideration that 
     there can be hundreds of electrical nets per board and the defect 
     (say, a circuit width reduction of 80%) has occurred to only one of 
     the 9 sample boards' circuit runs?  If rejecting the lot based upon 
     finding this one defect on one sample board is too stringent, what 
     would be more reasonable?  How many defects of a particular kind 
     should be found across the samples before the defect can be considered 
     a seriously systemic process issue?  Is there a statistical way of 
     determining this?
     
     
     Carrie Lawn
     HADCO Corporation

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