IMHO, to say 1.56 ug/cm2 eq NaCl or any other figure is useless without
qualification. Would it be reasonable to use this figure on both a
through-hole circuit with conductor spacings of 0.5 mm and no
significant traps under components and a high density interconnect
structure with shadowing components and gaps under them equal to the
copper thickness? Of course not, it is ridiculous. The figure quoted was
derived from conditions in the first case, in the 1970s, before SMDs
were current. IMHO, the figure, for equal reliability, must be
proportional to the maximum voltage gradient. In the first case, we are
talking of e.g. DILs working at 5 V, so we have a voltage gradient of 10
V/mm. I agree my example is perhaps extreme, so let's say 50 V/mm, for
the sake of a more practical argument. Your HDIS may be using
semiconductors working at 3.0 V with minimum track/pad spacings of 25
µm, ie a voltage gradient of 120 V/mm (roughly the maximum advisable
with FR-4, before dissociation starts), so if 50 V/mm and 1.56 µg/cm2
are OK, then you would want 1.56 * 50/120 = 0.65 µg/cm2, assuming equal
accessibility of cleaning fluids under the components. As this is not
the case, I suggest we have to weight the figure to compensate. I
propose a factor of 3 (from experience, we know that a tight SMA is 3
times more difficult to clean to an identical level than a 1970s style
assembly). It would therefore seem that 0.2 to 0.25 µg/cm2 eq. NaCl
would seem the most judicious figure to get an identical level of
reliability, all other things being equal. Empirically, this argument
would extend to ~0.5 µg/cm2 eq. NaCl for a tightish non-HDIS SMA.
That having been said, these arguments apply only to cases before
conformal coating is applied (MIL-P-28809n is specific on this point) to
avoid vesication and to apply the same arguments to circuits without
coating is totally unreasonable, because the conditions of operation are
totally different. Specifications have never considered this and are
therefore useless. IMHO, the only thing to do is to determine your
figure empirically. Unfortunately, this cannot be done overnight and
requires great knowledge of how your products are going to be used and
under what climatic conditions. You can try accelerated tests but they
are difficult, even impossible, to correlate with real-life conditions,
but they may give you a starting point. If products coming back for
subsequent repair show any signs of environmental damage, then you have
to tighten the figure. If they come back in a pristine condition or
don't come back at all, then you may be able to relax your figure slightly.
Again, IMHO, no one here can advise you, without knowing a lot more
about your products, the required reliability over a length of time and
the conditions of assembly, cleaning and use with a specific figure. It
may be that your 20 µg/cm2 is OK for you (although I very much doubt it,
as this figure is outside my knowledge of acceptable figures - and I am
one of the pioneers of ionic contamination testing, having worked for
over three decades on this and related subjects).
Please do not assume any figure is correct for you, without verification.
Brian
Angela Gregor wrote:
> First I wanted to thank you all for the gasket information you gave.
> Evidently this forum is very much respected because I didn't get any
> arguments from anyone here. Here's another one. At our company we are
> currently using Ionograph 500m version 3.02 to test our assembled boards
> after wash. Our pass/fail limit is 20 micro grams of sodium per square
> centimeters. I'm not sure our calculation is correct, and I was wondering
> what other companies pass/fail limits are for comparisons. I called about
> three places in my area and got three different answers. If this helps most
> of our boards are double sided. I would appreciate any feed-back. Thanks in
> advance.
>
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