I don't understand if you see transference caused by ions (existence of electrolytic "soup") or by electrons (electromigration). Maybe same problem as we had a few years ago: too high galvanic potentials between different elements and humidity and flux residues started movement of Copper, which also oxidized and became greenish, what a mess! By breaking that galvanic chain we got rid of the problem. Also, I have seen, under microscope, what happened across the gap under soldered ceramic capacitors. Flux residues locked in under the component and a little humidity, and silver (from end cap metallisation) started to migrate across, forming beautiful chains looking like leafs, all happening within seconds, depending on applied voltage. What you describe should not happen if the board is dry and cleaned. As you use the word dendrites, I recall my own experiences which were not electromigration (in terms of electron/vacancy exchange). Check galvanic chains, to start with, then look!
!
for inclusions which can serve as electrolytic. Remove both sources, and finally have an eye on what happens with the boards in environmental aspect. If noone can help you, last chance is Aussidoc. He makes a dowsing rod and tells you within one minute.
Ingemar Hernefjord
Ericsson Microwave Systems
PS. ASM No 13 is good, also 'Electronic Packaging and Corrosion in Microelectronics' by Morris E. Nicholson, also ASM.
-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: den 28 februari 2000 23:54
To: [log in to unmask]
Subject: [TN] Electromigration
Hi All!
I've learned about electromigration, and fortunately have never had a real
problem with (that I know about anyway). But recently, we've had some boards
returned to us that have failed for a short from VCC to Ground, usually at
the same area on the board.
It's beneath a keyed header that is hand soldered on the board. The first
couple of boards we got back, we couldn't find the problem. But we just got
back 3 more and I did see something. In trouble shooting we remove the header
(it's got a plastic body that has the plastic key molded into it) so we can
see if anything beneath the plastic.
Well, on one board today I think I saw something. There's traces that run
between the header pins and on one trace, it looked like the mask was not
adhered to the trace well. Then looking at it under higher magnification,
there was what appeared to look like these copper fern like structures
between the trace and the anular pad of the PTH on the laminate.
Will the dendrites be copper colored? All the pictures I've seen they have a
silvery gray to white appearance...I guess it would depend on which was the
cathode and which was the Anode in the scheme of things as to what the
dendrites would look like huh?
Has anybody else ever seen this?
Thanks ya'll!
-Steve Gregory-
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