Peter
As the original designer of the Contaminometer, but no longer involved
with it, in any way, I guess that I'm as qualified as any to respond to you.
You are partially right, but not entirely. First of all, it must be said
that the 1.56 ug/cm2 eq. NaCl or, better, ug.cm^-2 eq. NaCl(which is the
correct way of expressing the units cf. ISO 2000) dates from an
interpretation of Hobson's original tests, way back in 1969. Please
don't tell me that modern electronics are the same as they were 34 years
ago. This figure is as antiquated as the dinosaur and has no meaning
today. The voltage gradients on our substrates have increased by at
least an order of magnitude, actually to the limit of acceptable with
HDIS, even for low voltage operation. I therefore concur that 0.2 ug/cm2
eq. NaCl would seem reasonable for modern electronics with conductor
spacings of less than 0.2 mm, **provided** that it is measured
reasonably accurately and for hi-rel end-products.
So, how can this be done? First of all, you need to maximise the
sensitivity. The first and obvious way is to use the UK Mod standard
solution of 50% v/v IPA and not the US MIL standard of 75%, which is a
relic of Hobson's work which had an integration time of 1 min, because
it was sprayed on from a wash bottle, and not immersed with longer
integration times. The 50% solution gives twice the sensitivity of the
75% solution.
Secondly, contamination in these equipments is directly and linearly
proportional to the delta conductivity, so is inversely and non-linearly
proportional to the delta resistivity. Unfortunately, for fine
measurements, because resistivity meters have a range, you are working
at the cramped end of the scale. It is therefore more accurate to use
conductivity with its linear scale.
Then the board size enters into play. If you are stuck with a small
board and a large tank, you simply measure on a plurality of boards. For
example, if you have a tank size of, e.g., 250 x 350 mm, you can put in
8 boards 100 x 100 mm at a time, to obtain maximum sensitivity. Of
course, your answer will be an average of the eight, but this is not a
disadvantage.
Then there is the reduction of errors. These can come from many sources.
A bad one comes from CO2 absorption. This can be reduced by presenting a
minimal surface area of solution at the air interface (rectangular,
rather than funnel-shaped, tanks) and further reduced by software
compensation of the absorption, as well as keeping the lid on during the
test, with a minimal air volume over the solution. Another point is to
keep the test down to under, say, 15 minutes, so that what you measure
is the dissolved contamination and not leachates from the substrate or
the instrument constructional materials. For the same reason, you should
use solution at low temperatures (20-25 deg C). At higher temperatures,
the heterogeneous structure of the polymeric surfaces tend to open and
provoke more leaching from the substrate.
I won't enter into the so-called "static" and "dynamic" methods. Our old
CM-5 and MCM-2 models could use either. Each had advantages and
disadvantages but, overall, the "static" method is the better,
especially for low contamination levels. (I put the words between
inverted commas, "", because they are very misleading misnomers.)
If you observe all these points, it is possible to have a reasonable
accuracy at 0.2 ug/cm2 eq. NaCl.
For the anecdote, I used to manufacture the Microcontaminometer MCM-1
and MCM-2, which had a sensitivity that it could measure down to better
than 0.05 ug eq. NaCl absolute. This would have measured better than
0.01 ug/cm2 eq. NaCl on a 100 x 100 mm board (it had an interchangeable
tank for a 4" hybrid substrate). I believe Multicore stopped the MCM-2
in 1993. It was partially replaced by the CM-20, but I don't know what
happened to that. I can only answer for the CM-1 to CM-5 and MCM-1 to
MCM-2 series, which were the ones I designed.
Anyway, what you wish to do is possible, provided you have the right
instrument and know how to use it correctly.
Brian
[log in to unmask] wrote:
> Morning, All
>
> I was interested to see the cleanliness level of <0.2ug NaCl eq/cm^2 that
> is being adopted by many manufacturers, actually being stated in a data
> booklet published by Concoat Systems. Having done a bit of poking around
> into the world of cleanliness testing, I have a question for discussion if
> you're up for it:
>
> Most ionic cleanliness testers are still marketed as measuring cleanliness
> to the level specified in the MIL and IPC specs (1.56ug NaCl eq/cm^2). Fair
> enough. Having looked quite closely into such machines as the Ionograph,
> Concoat's own CM11 Contaminometer, Omegameters and the Zero Ion, which
> seems to be teacher's pet in the IPC books (TR-583, anyway). I find that
> none of the machines reads at all reliably at levels as clean as 0.2ug,
> very especially whan the boards are small. Does anyone know, and is willing
> to discuss, what work is therefore being done to improve ICT machines in
> terms of the following:
> a) using the test solution at much higher starting resistivity (e.g.
>
>>200MOhm-cm +), assuming that at this level of resitivity, small amounts of
>
> contamination will have a greater impact on the drop in resistivity when
> added (?)
> b) how the test solution is presented to the resistivity probe for
> measurement of these slight amounts of contamination. This is of especial
> concern since small amounts of contamination don't affect the resistivity
> of the test solution very much at current values (150MOhm-cm or so), which
> thus remains relatively close to its starting value. If the test solution
> is not homgeneous and the measuring portion of the probe is not in contact
> with all the test solution that passes it, solution of a higher or lower
> resistivity will not be measured and included in the machine's reading.
> c) the capability of the probes to accurately measure these relatively very
> small differences in high resistvity values.
> d) the capability of the filters to clean the test solution without adding
> further contanination back in.
>
> Suppose you have a board that only measures 4cm x 4cm, and it has to be
> 0.2ug NaCl eq/cm^2 clean. This means that the max amount of salt-equivalent
> contamination allowable is only 6.4ug. To measure this board alone is
> impossible - no machine is remotely capable of accuratey measuring a
> contamination level this low - so a batch of these little boards would have
> to be tested all together and the reading taken across the combined board
> area. After being divided by the number of boards, you are left with an
> average reading only. This is maybe OK if you have a lot of boards to test,
> but not so OK if you're repairing only one or two of them and want them
> clean again to original requirement.
>
> What is on the cards for future Ionic Contamination Testing equipment?
>
> Peter
>
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