Jack,
   Thanks for your input.
   I had the draft document Published in March 2000.
   My question is the different between Nc&Oc, same to Np&Op,Nt&Ot.   IPC7912 shows Nc stand for number of components on assembly(Np is number of placements on assembly), Oc is number of component opportunities(Op is number of placement opportunities), what is different with them.
   
Best Regards,
Liu Changkang 
   
   
    
  ----- Original Message ----- 
  From: Jack Crawford 
  To: [log in to unmask] ; [log in to unmask] 
  Sent: Tuesday, March 06, 2001 1:13 AM
  Subject: Re: [TN] Nc&Oc


  Liu--

  You indicate that you are referring to a draft document and I don't know which draft you are looking at.  IPC-7912 was published as an ANSI approved standard in July 2000, and that is the only version you should be referring to at the present time.  The term references were changed around, where O stands for Opportunities, and the subscript C is for components, P for Placements, and T for Terminations. It's quite a bit easier to understand than several of the early drafts. The biggest struggle the committee had with this document (and with the nearly completed follow-on IPC-9261 for in-process DPMO) is that there are multiple methods to develop DPMO numbers. Working within the boundaries of each of the methods, none of them would be "wrong".  However, industry can't compare the results of one method to another (apples to oranges). By establishing a standard method to calculate DPMO everyone's on the same playing field.  Hope this helps.  jack

  "3.1 Component Opportunities (o c ) The term ''component''
  is defined as each device or piece of hardware that
  may be assembled onto a printed wire board (PWB). Solder,
  glue dots, and other similar assembly materials are not
  to be included in this DPMO calculation. The PWB is considered
  to be one component.

  The total opportunity count for each ''component'' is one.
  An electronic component with multiple leads still counts as
  one. Processes, such as cleaning operations, do not add
  component opportunity counts. The finished PWA is not an
  opportunity."

  You have a board with a bunch of components to mount onto it. Count the components and then add "one more" for the board.  So that's the total of component opportunites. Here are the definitions for opportunities for placement

  3.3 Placement Opportunity (o p ) The term ''placement''
  refers to the presence and/or positioning of any component
  on a PWB. The PWB does not have a placement opportunity.
  Therefore, the number of placements will be one less 
  than the number of components. This is because the printed
  wiring board has no ''placement'' of its own.

  The total opportunity count for each placed component is
  one.

  3.5 Termination Opportunity (o t ) The term ''termination''
  is defined as any hole, land or other surface (such as
  component to component attachment) to which a component
  is electrically terminated. This includes any terminal
  onto which a wire is electrically attached.

  The total opportunity count for each ''termination'' is one;
  e.g., a 208 lead component will have 208 opportunities. A
  wire typically will have one termination at each end for a
  total of two opportunities. Note that if a single component
  termination connects to multiple lands on the PWB (i.e.,
  split pad or shield) then only count the terminations of the
  component, not the number of lands. In this case you
  would count only one termination opportunity. In the event
  of a defect to either one or both of the split pads, the maximum
  count of defects would be one.

  >>> lckang <[log in to unmask]> 03/05/01 01:18AM >>>

         When I calculate DPM and Assembly Yield according to IPC-7912(draft document), It shows Nc is number of components on assembly, and Oc is Number of component pooprtunities, what different between them(Np&Op,Nt&Ot)?
         Any other help about Yield&DPM Calculation is appreciated.

  Best Regards,
  Liu Changkang