Ken
Doug's answer is fine, but the allowable reading for a given application
cannot be specified, in my opinion. It depends on a host of
circumstances. If you use a "no-clean" flux, for example, the reading -
as an absolute value - is totally meaningless. The oft-quoted value for
cleaned circuits of 1.56 µg/cm2 eq. NaCl is also relatively meaningless
unless you take it into context of the original MIL-P-28809 spec and the
work that Hobson et al. did on it in the late 60s, early 70s plus the
subsequent flawed equivalency factors work leading up to the A revision.
In any case, in those days, the average PCBs were through-hole types
with an average component density of 4-6 eq. DIL14/dm2, track widths and
spacings of 0.5 mm. No-one can convince me that the figure which was
arbitrarily considered as the statistically invalid pass/fail threshold
on a sampling basis can be applied to today's PCBs, quite apart from the
fact that the flux/cleaning process specified in 28809 is no longer
used.
The value of ionic contamination testing is as a means of cheap and
simple process control, not as a measure of reliability. If you have a
process that you know is good, producing reliable circuits, and you
measure the contamination on a sample of, say, a dozen or a score
circuits of each type you manufacture and you calculate the mean and
standard deviation of each set, you establish your norm. If subsequent
measurements on the same PCB assembly show the mean has drifted
significantly **in either direction** or that the SD has widened, then
you know that your overall process has gone off the rails. This does not
necessarily mean that your circuits are less reliable. If all subsequent
measurements fall randomly within your gaussian curve established with
the norm, with little skew, you know your process hasn't changed and you
can sleep comfortably, on both ears, at night.
IMHO, FWIW
Brian
Ken Patel wrote:
>
> Assembly Expert,
> Is the Omegameter reading taken for Ionic contamination measurement is
> considered Dynamic or Static test? What is the maximum allowable limit for
> class 2? Our product is for telecommunications application.
>
> re,
> ken patel
>
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