Hi Steve,
This is just pulling a few things out of my memory that I read here on Technet
over time. I have not expeienced any of these myself.
I would look for a nearby place that has x-ray equip. You MAY, but probably will
not, be able to see something there. It should at lest give you an idea of the
clearances on the plane layers and how good the registratio is on the board. All
that depends on the number of layers, of course.
I wonder how close the clearances are on the plane layers. I often see designers
making the clearances the same size as the clearance for the solder mask. Even
for soldermask, it is usually pushing the capabilities of the board shops. A bit
of mis-registration and bingo, there's a short, but you usually see that pretty
quickly, rather than after a period of time.
I have read about something I think they called Cathodic (or Carbon) Filament
something or other.... I believe its caused by moisture getting into the fibers
inside the board. This, combined with some chemistry residues, finally causes a
voltage breakdown/arc-over, which carbonizes the fiber and they become a
conductor. That might fit in with your deterioration over time scenario. The
plating in the holes may not have been too good, allowing your process chemicals
or the bord shop's process chemicals to become trapped within the board (thru
pin holes in plating, etc.).
One lab I have used for cross sectioning and analysis is Robisan Labs. Susan
Mansilla usually hangs out on Technet, so you may hear from her. Her number is
317-353-6249. They are in Indianapolis.
Enjoy the cats and dawgs. We could use some of them down here. Been bone dry lst
half of July and most of August. Did have a fast moving, quite violent storm
come thru about two weeks ago tho'. Demolished several buildings under
construction. Killed one worker. Tore off numerous tree limbs. Blew down part of
my fence at the house.
Good luck with this one. Keep us posted.
Charlie B.
"Stephen R. Gregory" <[log in to unmask]> on 09/10/99 02:45:54 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
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To: [log in to unmask]
cc: (bcc: Charles Barker/US/I-O INC)
Subject: [TN] Internal fab shorts that happen over time...
Hi ya'll!
Hoo-boy! We be getting some weather today! They said that remnants of
hurricane "Greg" would be going through our area this weekend, and it's
comin' down like cats and dawgs now!
Anyway, we just an assembly back from one of our customers that has a short
internally in the fab, from a power connection on a terminal block that's
installed into it, to the ground plane of the fab. It's a board that handles
some high power. They said that the board powered-up fine, but over time the
short developed. They also said that they suspect that another place will
develop a similar short from some readings that they were getting when they
began to trouble-shoot the first short. There's 4, 6-pin terminal blocks on
this board.
They sent us back the board and suggested that we give the assembly back to
the fab vendor to look at it and find out what the problem is. There are
those here of the opinion here that we should find a third-party somewhere
and let them take a look at the problem, because they will be more objective
in the diagnosis...if you catch my drift. Problem is, I don't know where I
should start to look for somebody like that, do any of you have any
recommendations?
I'll admit that I don't know much about what can cause something like that.
First thing that came to my mind, was some sort of electromigration happening
internally to the fab...since it happened over time (I don't know the actual
time frame), could that be it? Could it be caused somehow by the way we
assembled it?
Thanks for any suggestions!
-Steve Gregory-
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