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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 8 Sep 2003 06:27:58 -0700
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        Brian:
                Besides biomedical electronics in which boards need to be
super clean,  it has been my experience that any thing under 10 micro grams
of NaCl per square inch is passable. So far I have not heard of a case,
while tallking to my co-workers in several companies, where there has been
any problems releted to salt contamination. Could you expand on this matter.
        Regards,
        Ramon

> -----Original Message-----
> From: Brian Ellis [SMTP:[log in to unmask]]
> Sent: Saturday, September 06, 2003 3:29 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Ionograph
> 
> 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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