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. >>> >>>--------------------------------------------------- >>>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 > ----------------------------------------------------- > > --------------------------------------------------- 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 -----------------------------------------------------