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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