Peebos
In essence, the purpose of measuring resistivity in your cleaning process is
to maintain quality in your manufacturing process.
Electronic circuit assemblies that have ionic contaminants (salts such as
tin,lead, copper and finger etc..) on their surfaces may become somewhat
unreliable particularly when exposed to condensation or high humidity. Why?
Because water dissolves salt and salt is conductive, so your electronic
circuit may behave in a somewhat unintended and unpredictable way.
If the resistivity meter on your cleaner is reading low values, say less
than 0.1 megohm, then you may have dirty assemblies; whereas if the reading
is greater than 1 megohm, then you will be likely to have cleaner and
therefore perhaps, more reliable assemblies.
The current ANSI / IPC- J-STD-001 manufacturing standard, uses resistivity
measurements to help control your process. It refers to a maximum
recommended limit of 1.5 microgrammes per square centimetre (10.06
microgrammes / square inch) sodium chloride equivalence on your assembly.
This measurement comes about from taking a reading of test solution
resistivity before and after the test and then extrapolating this result and
by calculation express it as a measure of salt (ionic) contamination.
To get a much more thorough explanation of this, get a copy of the above
spec and IPC-TM-650.
There are some questions that you should consider, e.g.:
Suppose some of my assemblies have much more flux residues than others, will
they all be cleaned to the same level? You really need to "cleanliness test"
individual assemblies because you have only measured the rinse water
resistivity and maybe the rogue assembly got through the process before the
cleaner had got hold of those nasty residues.
So if I clean to 10.06 megohms / inch2 I am OK? Not necessarily. By
definition, the spec is saying it is acceptable to leave UP TO 10.06
microgrammes of salt on every square inch but if you are into fine line -
fine pitch, sophisticated packages, COB, BGA etc., and working in a hostile
environment, then this level will almost certainly be too high.
Hope this helps and if you want more, let me know
Best regards
Graham Naisbitt
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Concoat Ltd
Alasan House, Albany Park
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Application Engineers in Chemical Compounds to the Electronics Industry
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