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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 8 Sep 2003 15:33:27 +0300
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text/plain (189 lines)
OOOh! Maybe if you were scared, you would run into my arms! That could
make me flip my chip, too. :-)  (not that you would want to associate
with a dogmatic, arrogant, old fogey like me!!!)

Brian

joyce wrote:
> Excellent point, Brian.  It scare me to death (not a pretty sight) when
> someone quote 10 ug/sq inch for flip chip/dca/csp etc.etc. without
> define the overall drive condition and spacing....
>                                                           jk
>
>
>>-----Original Message-----
>>From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
>>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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