Brian, Unfortunately I must disagree with part of your first paragraph. Certainly in the three standards I work to voltage is applied during the entire test (not counting the initial chamber stabilization phase). These three standards and applicable sections are: 1) Nortel Networks "Corrosiveness of Soldering Fluxes" Section 2.5.4 states "A bias voltage of 45-50 volts is applied for 4 days." 2) Telecordia's GR-78-CORE, section 13.1.3.2.5 "After the stabilization measurement, apply a dc bias of 45V to 50V to all parallel conductors during the entire conditioning period." 3) IPC-TM-650, section 5.3.5 "Connect the 45-50V DC voltage source to the specimen points to apply the bias voltage to al specimens." It would seem to me that applying a voltage under stressful conditions (35 C., 85%) RH) could constitute an attempt to provoke a failure. Brian, one may not like it that people are calling what I described above as SIR, but since I think the majority of the electronics community does so means it is a fait accompli. I have never heard of an instrument that applies the test voltage (as opposed to the bias voltage) to all test points at once. For electromigration all of: the Nortel Networks spec, the Telecordia spec and the soon-to be-reissued IPC electromigration spec call for a bias voltage of 10 V DC, lower, not higher than that used in SIR. One other huge NA company that I know of claims that even a lower bias voltage (but still not zero) should be used. Back in the pre-nineties I think it was Bell Labs that did the work that showed that if one lets the test coupons acclimatize for 24 hours at 35C/85% or for 96 hours at 85C/85%RH it did not make a difference how the chamber ramped (temperature ahead of humidity or one or more dew point conditions on the way to the test condition). Another alternative is IPC-TM-650 suggests a controlled ramp where the chamber is set to low humidity, the chamber is ramped to temperature and then to humidity. regards, Bev Christian Nortel Networks > -----Original Message----- > From: Brian Ellis [SMTP:[log in to unmask]] > Sent: Friday, November 26, 1999 12:33 PM > To: [log in to unmask] > Subject: Re: [TN] SIR > > Hi! > > SIR is Surface Insulation Resistance which is a function, as the name > implies, of the surface quality of the insulation. There is no attempt > to provoke a failure mechanism, just to measure how good, or bad, the > electrical insulation quality is. It is an extremely important parameter > with Hi-Z circuits. It is measured at a low voltage applied for a > rigidly fixed period (often 30 secs) before the resistance value is > noted or recorded. The time interval is critical. At no other time is a > voltage applied. > > ECM is electrochemical migration. It is often determined in a similar > way to SIR, but with a constant bias voltage applied - often quite high > - between the tests. The idea is to provoke migration which will > manifest itself as a lowering of resistance. Many so-called SIR specs > are not SIR at all but ECM tests. Note that the bias voltage may or may > not be the same value or polarity as the test voltage. If the value or > polarity is different, then the time from applying the test voltage to > the resistance value recording is critical. Many test instruments apply > the test voltage to all of a number of points simultaneously, but then > measure the values sequentially, giving different intervals, increasing > with each test points: the results are therefore not comparable. > > It is quite normal for SIR to drop initially and then rise again after a > few hours. as the humidity stabilises within the laminate. You can > safely ignore the period before the minimum, provided it does occur > within a few hours. An important point is to jack the temperature up in > the test chamber and keep it at the test value for a couple of hours > before you start to jack up the humidity. If you put cold circuits into > a humid chamber or you allow the humidity to increase faster than the > temperature, then you may get condensation on the test coupons. If the > SIR drops after a few hours, you are in DEEP trouble, man! > > Brian > > wmlee1 wrote: > > > Hello, First of all, thanks for the valuable advice from all > > contributors. May be I had some misleading, (actually I was being > > misleaded after reading my previous post mail). I would like to > > clarify that:I have seen some test results of surface insulation done > > by other people (not me) that the resistance readings decrease > > initially in the first few hours (say 24 hours) and then increased > > slowly. Based on all of your comments, can I assume that there may > > have comtaminants on the board surface initially and caused decrease > > in resistance readings. The contaminants then evaporate out and > > resulted in gradually increase in resistance reading, is it? I have > > forgot the testing temperature of this test result, as per advise, the > > resistance will not increase if we decrease the testing temperature? > > please help to advise. I want to ask this because it is strangle for > > me that some test results showed decrease in resistance reading during > > the whole test period while some showed as above mentioned. Brian, > > would you please help to elaborate the difference between true SIR and > > ECM and what is ECM, as I am not familiar with these tests actually. > > What is bias voltage mean? Do we need to apply it on coupons > > throughout the test period in ECM or just when we need to measure > > resistance. Condensation on coupons shall be prevented because of > > ionize the contaminants? Thanks Leecp > > ############################################################## > TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c > ############################################################## > To subscribe/unsubscribe, send a message to [log in to unmask] with > following text in > the body: > To subscribe: SUBSCRIBE TECHNET <your full name> > To unsubscribe: SIGNOFF TECHNET > ############################################################## > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > additional > information. > If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or > 847-509-9700 ext.5365 > ##############################################################