With all due respect, I believe I was the first to describe the
equivalence factors as a load of bulls..t, when they were first
determined by NAFI in 1975 and published in MIL-P-28809A in 1978. This
was confirmed by the European Space Organisation in about 1980, who
published a matrix of the results obtained from three instruments (the
then Kenco OmegaMeter, the Alpha Ionograph and the Protonique
Contaminometer) and four or five different flux types. The "equivalence
factors" were different in each cell of the matrix and were not even in
the same order for either instruments or flux type. This was later
confirmed by the UK Ministry of Defence in their Standard published in
1986. They defined the permissible contamination according to a test
method based on the Contaminometer principle. There was no question of
equivalence factors which were, in fact, a measure of the efficiency of
each type of instrument to extract the "wanted" ionic contamination and
the "unwanted" ions (e.g., extracted from the constructional materials
of the instrument, carbon dioxide from the air and irrelevant ions out
of the board materials etc.). Both ESO and the MoD chose the
Contaminometer as their reference method because it was the only
instrument to have an efficient residue removal and software
compensation for the "unwanted" ions, based on mathematical modelling,
giving an answer closer to the real introduced contamination on their
test boards, under most conditions.

In any case, the so-called equivalence factors were determined on
instruments of the mid-1970s, which bear no resemblence to instruments
in use today, even if they bear the same name. It would suffice to
change, e.g., the pump rate to change the equivalence factor.

The notion of continuing testing until there is no rise over 5 minutes
is a good way of counting all the "unwanted" ions as part of your
contamination, giving a meaningless result, as they are not a function
of surface contaminants on the board. My research showed that the best
method was to continue testing for either 15 minutes or a rise of less
than 1% in 1 minute, whichever was the shorter, and then to
mathematically extrapolate the compensated results to infinite time.
This was a game of mathematical fun because, in most cases, you had
several sources of contamination, each providing a different-shaped
exponential rise curve, so the measured curve had to be split into its
components and each component extrapolated and then the lot added
together. In the CM-4 (PC model), we split the curve into two, but the
later software for the CM-5 took advantage of the 486/487 PCs and their
extra speed and split into 3. Today, if I were to do this, with a
Pentium 3.06 GHz, I could probably do it into 10 components and still
get the results in a small fraction of the time! The number-crunching
(curve analysis and extrapolation) used to take up to five minutes after
the test itself ended - and that was before Windows slowed down PCs by a
factor of five!

In fact, there has been more bulls..t talked about ionic contamination
testing over the years than almost any other subject in our industry.
This was because the original method, published by Hobson et al., based
on testing in 1969, had 28 identified inaccuracies but became the method
described in the original 28809 spec. Subsequent revisions always tried
to bring it back to the original figures. When the various instruments
were developed, they became more accurate as time went on, reaching
their peak about 1990, but they were always referred back to the 1969
method. I used to say it was like trying to calibrate a micrometer with
a wooden foot-rule.

Please excuse this historical diversion and the trumpet-blowing for the
Contaminometer. I allow myself this luxury, because it is all very much
past history and I now have absolutely no vested interest in the
Contaminometer, which I partially divested to Multicore 12 years ago and
totally divested 2 years ago to Henkel/Loctite who, in turn, divested to
Concoat.

Brian

Charlie Pitarys wrote:
> I need to meet a Bellcore requirement on an assembly of
> 6.45ugrams/sq.in.  (Based on the pwb surface area)
>
> The assembly has 633.5 sq.in. surface area.
>
> The customer has supplied me with documentation from;
> Physical Design and Manufacture of Telecommunications Products PWB
> Assembly Requirements,  GR-78-CORE, Issue 1, Sept 1997. Section 7.1.3.9
>
> I have an Omegameter 600. Is there an equivalence factor, etc? Or is it
> a 1 to 1 deal?
>
> Thanks in advance.
>
> Charlie
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