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
##############################################################
|
