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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