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October 2000

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 23 Oct 2000 18:17:52 +0300
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Daan

I agree that it is very rough. The rationale is that the last 15 mins is
probably leaching of some sort, but that the first 5 minutes is almost
100% from what you want to measure, so it is a very crude way of
separating fast-dissolving from very-slow-dissolving contaminants. In
the instruments I used to make, the near-asymptote (<1% of the total
absolute change in 1 minute) was detected between 3 and 15 minutes and
the instrument itself cut the measurement. If it had not cut by 15
minutes for this reason, it did so by time. The contamination/time curve
was then analysed mathematically, which allowed the separate curves for
each contaminant type to be determined and extrapolated to infinite
time. This is obviously precise, provided that there was little or no
"noise" on the curve and we usually got 95-99.9 percentile curve fits,
in practice, and the print-out would give the final values for each
contaminant, with a text appreciation of the type and possible causes.
However, this kind of instrument was a far cry from the type that this
thread originator mentioned (e.g. it could detect uncleaned flux under
components).

Brian

"d. terstegge" wrote:
>
> Hi Brian,
>
> This assumes that the type of contamination, and thus the speed of dissolving, is always the same. From experience I know that the speed at which the asymptote is approached varies a lot from product to product, depending on design, bare board materials, components.
> But maybe that's one of the reasons why you already called it a very rough guideline.
> For practical reasons we've set the standard measuring time for our Alpha Ionograph to 15 minutes.
>
> Kind regards,
>
> Daan Terstegge
> Unclassified mail
> Personal Website: http://www.smtinfo.net
>
> >>> Brian Ellis <[log in to unmask]> 10/23 2:55 pm >>>
> Albert
>
> How long is a piece of string? It depends on how quickly the
> contaminants dissolve. I suggest you do two or three tests for an hour.
> You will observe that the resistivity drops fairly rapidly and then very
> slowly. Draw a straight line approximating the last 15 minutes backwards
> to zero time. Draw another straight line approximating, say, the first
> five minutes. Note the value and the time where the two lines intersect.
> If the value is > 3/4 of the final value, then double the time to
> intersection. If >1/2, then quadruple the time. If >1/4 then octuple the
> time, if <1/4 then 16 times the time. This is only a **very** rough
> guideline until you acquire sufficient experience to judge when you
> approach asymptote.
>
> Brian
>
> > ¿àÁJ·½( sylai ) Q80_5381 wrote:
> >
> > Hi TechNetters,
> >     I need some help regarding Cleanliness. Now I have an automatical
> > test machine whose name is omega 600. I usually test 10 minutes
> > for cleanliness of PCB. Can anyone give me the best advice about the
> > test time of omega 600 machine which is used in PCB and tell me the
> > cause ?
> > Thanks,
> >
> > Best Regards,
> >
> > >Albert Lai
> > > TEL : 886-3-323-1111 ext 5381
> > > FAX : 886-3-323-6611
> > > E-MAIL : [log in to unmask]
>
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