Marti,
AQL and dpm are apples and oranges. Here's why:
Under an AQL system, you pull a random sample based on the sampling plan,
and accept / reject based on the number of defectives found. A defective
(defective unit) may contain many defects. It is well documented that AQL
plans are biased in favor of the producer. In your case, a 2.5AQL plan
means, that on average, a lot that contains 2.5% defective units will be
accepted almost all the time. Furthermore, each defective unit may contain
many defects. Also, plans with a much higher % defective have a pretty good
chance of slipping by. If we are talking about anything other than minor
defects, this plan seems very liberal.
Looking at the DPM spec, the complexity of the board is a major factor. If
you have a simple board with a low component count, most of your boards may
be "good" with a 500 dpmo. If you have complex boards with a high component
count, you will find you can get below the 500 dpmo number, and still find a
lot of the boards are defective. Another consideration with DPMO is where
is it measured? ...at first pass test / inspect...or as delivered after you
inspect / test and rework?
In summary, AQL is "on average" and looks at defective units. DPMO looks at
defects related to opportunities, so the number of defects and the
complexity of the board are factors.
You are correct in that there is not much of a relationship between the two
measurements.
----- Original Message -----
From: "Marti Tully" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, September 18, 2000 8:06 PM
Subject: [TN] FW: Sampling Plans
> > > Subject: Sampling Plans
> > >
> > > I have a customer who insists that the AQL sampling plans in IPC 6012
are
> > not sufficient to guarantee his desired dppm and he is right. His
product
> > is class 2 and we, in general, follow a 2.5 AQL in final inspection for
> > external defects. This customer wants a dppm of 500. I know of no
cost
> > effective sampling plan that can guarantee this. Only maintaining
rigorous
> > process control can meet this requirement.
> > >
> > > I, however, promised to survey the industry for their thoughts ...
here I
> > am ... comments?
> > >
> > >
> > > Marti Tully, Quality Manager
> > > Sanmina Phoenix
> > > Office 602-824-8497
> > > Cell 602-549-4535
> > >
>
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