Dave,

   See the example bellow :

  Imagine one process with a ppm level equal a 100. Thats mean 1 error in
10000.

  One board with 100 components, producing 100 boards you have 10000
components and 1 failure
  One board with 1000 components, producing 10 boards you have 10000
components and 1 failure

  The yield to the first board is 99% ( 1 fail board in 100 )
  The yield to the second board is 90% ( 1fail board in 10 )

  PPM measurement tells you that the "quality " level is exactly the same.
  Firts Pass Yeild tells you that the " quality " of the first board is
better.

  You choose what is more suitable for your company, but try explain the
difference to the managers ( this is the hard part !!! )
   I prefer PPM level because is more easy to process control and you can
compare different products ...

  I hope this help ...

Jorge Dourado de Santana
Maintenance / Process Engr
Microtec - Brazil





> -----Original Message-----
> From: Ahne Oosterhof [SMTP:[log in to unmask]]
> Sent: 24 de Agosto de 1999 13:29
> To:   [log in to unmask]
> Subject:      Re: [TN] Defect Count
>
> The best method of reporting defects is to calculate the ratio of "errors
> per possible errors", hopefully expressed in parts per million (ppm). So
> when you are placing parts, it is "placement errors per total number of
> placements". Or if you are printing solder paste it can be the "number of
> print errors per number of apertures".
> When you start using "number of boards with any error per total number of
> boards"  it becomes impossible to compare results. Very simple boards with
> a
> low defect ratio does not mean you are performing well, and neither does a
> higher defect ratio for very complex board indicate that you are out of
> control. The "First Pass Yield" number that some people use (number of
> correct boards per total number of boards) has the same problem: you
> cannot
> compare a line that makes very simple boards to a line that makes very
> complex boards.
> But if you use ppm for all your work, any board can be compared to any
> other
> board and the data can even be combined for a complete plant.
>
> Have fun,
>         Ahne Oosterhof
>         A-Laser, Inc
>         (503)641-9428
>
> PS: But I have no idea how you do this for a circuit design engineer or
> even
> for a board layout person.
>
> -----Original Message-----
> From:   TechNet [mailto:[log in to unmask]] On Behalf Of Dave Roesler
> Sent:   Monday, August 23, 1999 18:50 PM
> To:     [log in to unmask]
> Subject:        [TN] Defect Count
>
> Technetter's,
>
> When reporting defects, do I count all defects individually or do I count
> the number of bad circuits.  For example, if I have a circuit which
> contains 3 defects, do I count this as 3 (for the 3 defects) or 1 (for the
> single, bad circuit).
>
> Thanks in advance for the clarification.
>
> Dave Roesler
> TRI-C Design, Inc.
>
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