From a 1999 response:
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,
In addition:
If you are a customer you don't really care how complicated a board is, if
it has one error the product you just bought does not work. Therefore "First
Pass Yield" (good boards per batch) is great to make your customers feel
good, it is a Marketing tool. On the other hand, "DPMO" is the number you
need to show how good your assembly process is, or how much you need to
improve it; it is a Manufacturing tool.
Regards,
Ahne Oosterhof, GM
A-Laser, Inc.
503-641-9428
Beaverton, OR, USA
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair K. Hogg
Sent: Thursday, October 02, 2003 05:17
To: [log in to unmask]
Subject: Re: [TN] DPU and PPM
We use DPMO / PPM at our inspection step, and %yield at test. We're not
following 7912 exactly, but pretty close.
I don't think it is really important how you get the measures, but how
you use them. When looking for improvement opportunities, we look at
essentially defects per board as opposed to PPM. This is because a small
run of a low volume product with a few defects would have a high PPM,
but our high volume boards could have more defects and more
opportunities even though they have a lower PPM.
Blair Hogg
QA Manager
GAI-Tronics
>>> [log in to unmask] 10/01/03 05:22PM >>>
Hi everyone,
The upper management team here uses a DPU metric for our board shop,
for
what reason I am trying to comprehend. Basically, one solder joint
defect
on a board equates to the whole board counting as a defect. The
opportunities are irrelevant - every board gets one opportunity. When
the
"PPM" numbers are published, which is what they call them, they are
outrageously high. My questions are:
Does anyone use Defects per Unit at the board level?
If so, what kind of numbers are you seeing.?
I am pursuing a change to use real DPMO metrics, but so far I've been
unsuccessful. I even presented IPC-7912, but it was ignored. Thanks
in
advance for the help.
Howard Watson
SMT Manufacturing Engineer
AMETEK/Dixson
Grand Junction, CO
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