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. >> >> --------------------------------------------------- >> Technet Mail List provided as a service by IPC using >LISTSERV 1.8e To >> unsubscribe, send a message to [log in to unmask] with >following text in >> the BODY (NOT the subject field): SIGNOFF Technet To >temporarily halt >> or (re-start) delivery of Technet send e-mail to >[log in to unmask]: SET >> Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the >> posts: send e-mail to [log in to unmask]: SET Technet Digest >Search the >> archives of previous posts at: >http://listserv.ipc.org/archives Please >> visit IPC web site >http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for >additional information, or contact Keach Sasamori at >[log in to unmask] or 847-509-9700 ext.5315 >> ----------------------------------------------------- >> >> > >--------------------------------------------------- >Technet Mail List provided as a service by IPC using LISTSERV >1.8e To unsubscribe, send a message to [log in to unmask] with >following text in the BODY (NOT the subject field): SIGNOFF >Technet To temporarily halt or (re-start) delivery of Technet >send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) >To receive ONE mailing per day of all the posts: send e-mail >to [log in to unmask]: SET Technet Digest Search the archives of >previous posts at: http://listserv.ipc.org/archives Please >visit IPC web site >http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for >additional information, or contact Keach Sasamori at >[log in to unmask] or 847-509-9700 ext.5315 >----------------------------------------------------- > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------