Rick,
     
        We recently went through this same exercise.  We count 
     opportunities the same way you do although I have read papers and 
     articles which recommend multiplying the components by 2 as Vicki 
     mentioned.  We keep track of electrically defective components but do 
     not count them in our DPMO.  Our rationale was that we just wanted to 
     measure our own processes and not factor in vendor defects.  
     
        We count defects the same way Vicki mentioned.  A short between 
     three legs as you mentioned would be two defects not one.
     
        It would be helpful if there was a conventional wisdom in order to 
     compare ourselves against some standard but if you are consistent in 
     the way you count opportunities and defects then you will still know 
     when you are improving, getting worse or out of control relative to 
     your initial baseline data.
     
     
     - David Bruni      [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: PPM Defect Counting
Author:  <[log in to unmask]> at cc_gate
Date:    7/9/97 6:02 PM


Rick --
     
Great question! I wish the industry as a whole would standardize on this! I 
would be very interested in hearing what other replies you get back.
     
We count defects basically the same way you do,  but with some variations 
depending on the defects were tracking: 
     
Solder defect rate:
     
        # of defective solder joints / total # of solder joints
     
Placement defect rate or defective component defect rate:
        # of defective placements or defective components / total # of 
components
     
Total process-related defect rate:
     
        (# of defective solder joints + # of placement defects) / (total # of 
solder joints + total # of components)
     
Over-all defect rate:
        (# of defective solder joints + # of placement defects + # of defective 
components) / (total # of solder joints + 2*(# of components))
     
 Since the "over-all" defect rate includes electrically defective components, 
each component has an additional opportunity to be defective.   
     
# of defective solder joints = the number of pins affected. If one whole side of
     
a 60-pin QFP is shorted together, that's 15 defective solder joints.
     
 The exception is that if just two pins are shorted together, I suspect this 
just gets counted as 1 defect most of the time.
     
If the QFP is misclocked, that's one defective placement.
     
If the QFP has an internal defect, that's one defective component.
     
     
-- Vickie Chapman
     
     
     
     
On Wed, 9 Jul 1997 10:33:41 -0700 RICK VERNON wrote:
     
> From: RICK VERNON <[log in to unmask]> 
> Date: Wed, 9 Jul 1997 10:33:41 -0700
> Subject: PPM Defect Counting
> To: [log in to unmask]
> 
> We count our defect opportunities as follows: one per component plus
> one per solder joint. A resistor would be three defect opportunities, a 144 
> pin QFP would be 145 defect opportunities.
> 
> We count actual defects as follows: Tombstone part is one defect.
> Insufficient solder on one pad is one defect. Solder short between three 
> legs of an SOIC is one defect. Skewed SOIC is one defect.
> 
> I have concerns that we are presenting an artificially low PPM rate by 
> counting defects this way. Others have concern that if we count defects 
> the same way we create opportunities, the PPM numbers will be inflated. 
> 
> I'm looking for info on how others in the industry count actual defects that 
> affect more than one opportunity.
> 
> Thanks.  Rick Vernon, QA Manager   [log in to unmask] 
> 
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