Right now, I feel like a moth circling around the blue light of a bug
zapper. I know I shouldn't go near the light, I know I will get zapped,
but I just can't help myself.....................
Looking for a simple definition of SIR is like looking for a simple
definition of reliability. Good luck finding one that everyone agrees with
other than in the broadest possible terms. Many of the IPC SIR and
Electromigration task group meetings start with a good 20-30 minutes on
arguing about terms. Longer if Brother Brian Ellis attends. Far longer if
Phil Wittmer gets into the act. And if I'm involved, God knows when you
get out of the meeting.
Conceptually, surface insulation resistance refers to a materials system
characteristic. In theory, you measure the resistance between two
conductors, such as fingers of a comb pattern, across the intervening
surface. The resulting measurement is the surface insulation resistance.
In reality, you have several parallel resistances. A resistance across the
surface, such as through a flux residue, a resistance through the bulk of
the resin, perhaps a resistance through the bulk of solder mask if a mask
fills the space. Hence the existing wording in 9201. I would direct your
attention to an article I wrote in the November 2000 Process Rx column that
talks about all of the parallel resistances and capacitances you need to
consider if you are truly going to characterize the system. I can make
that column available to anyone who wants it.
Ever look at a comb pattern? Two conductors separated by a dielectric
material. Sounds like a capacitor to me. A metrologist would be a good
person to consult in SIR testing. They always ask, are you measuring what
you think you are measuring?
Further confusing the issue is how SIR testing, as a term, has been used.
I think of SIR as a materials system characteristic. I can measure SIR at
ambient, at hot/dry conditions, at hot/humid conditions etc. I can look at
how SIR changes with changes in environment. So, SIR and SIR testing mean
subtly different things.
The 9201 handbook is starting its revision cycle this spring and will be
headed up by Curt Lustig of Morton Specialty Chemicals. If there is
something you want better explained, now is the time to say so.
Is SIR considered an alternative to ionic cleanliness? Maybe. Depends on
your criteria. Depends on who is doing the testing (in either case).
Depends on your definition of cleanliness. Depends on what ionic
cleanliness test you are talking about.
Is there any correlation between the two? A very loose one. As ionic
residues increase, the SIR levels will decrease, but a good scientific
correlation? Nope. It also depends on what measures of ionic cleanliness
you are talking about. If you are talking about SIR vs. ROSE, the
correlation is very very broad and loose. If you are talking about SIR vs.
an individual ion as measured by ion chromatography, the correlation is a
little more scientific, but not by much.
I suggest you go back over the archives of Technet, searching on SIR,
electromigration and Ellis. Brian has given some wonderful dissertations
on the physics of the measurement and what goes on in electromigration, far
better than I can. Must be the sunshine in Cyprus or the clean air in
Switzerland.
What other benefits are available with SIR? Well, it helps out the local
economies near test labs...... Personally, if I had a choice between ionic
contamination testing and SIR testing (assuming both competently done), I
would choose the SIR testing as the bottom line of whether a residue was
harmful. Ion chromatography is a good tool for determining what residues
are present and in what amounts, but past clients still come back with the
"so what" question. What does 5 micrograms of chloride mean to me? On the
other hand, if you had SIR testing, you can show no detrimental leakage
currents under humid conditions, no corrosion, no metal migration, which
are the big factors in figuring out electrochemical reliability. People
often have a greater comfort with passing SIR than they do with passing
ROSE tests. Of course, SIR testing takes a while and so it is best to do
both ion chromatography and SIR at the same time so you can determine the
correlation between SIR performance and cleanliness levels for
troubleshooting the process later.
Enough for now.
Doug Pauls
Rockwell Collins
Thats for starting the conversation. I have been trying to find time to
look into SIR. Is it considered an alternative to ionic cleanliness? If
so, is there any correlation to ug/sq. cm. We have some medical implant
customers, that are concerned about any conductive surface contaminate.
What other benefits are available with SIR?
>>> [log in to unmask] 04/27/01 10:17AM >>>
OK, let's see what I can stir up today. :)
I am looking for a clear, succinct definition of surface insulation
resistance. If I look in IPC-9201 "Surface Insulation Resistance
Handbook",
I don't find what I am looking for. (Sorry, Joe, Doug et al). What I find
on page 2, section 2.1, Definitions is the following: "It represents the
electrical resistance between two electrical conductors separated by some
dielectric material(s). This property is loosely based on the concept of
sheet resistance, (see ASTM-D-263), but also contains elements of bulk
conductivity, leakage through electrolytic contaminants, multiple
dielectric
and metallization materials and air." Whew!
Any takers? Hmmm?
regards,
Bev Christian
Research in Motion
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