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August 1999

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Subject:
From:
Ahne Oosterhof <[log in to unmask]>
Reply To:
Date:
Tue, 24 Aug 1999 09:28:34 -0700
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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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