Hi Phil! Good summary! I have been through this issue on a couple of
projects. Use a SEM and confirm what elements comprise the dendrite. Under
typical conditions there is usually not enough ionics running around on an
adequately cleaned assembly to promote the electromigration of the tin/lead
capacitor metallization. However, there will be enough ionic material
present on the assembly to form dendrites provided you: have condensation
moisture, an electrical bias (5 volts is enough) AND there is access to
the silver/palladium capacitor end cap metallization. The crack could
provide access to the Ag/Pd metallization (many capacitors have a Ag/Pd
metallization layer as part of the end cap finish stackup) - Ag is a much
more "migratory" metal than tin or lead. If the dendrite is comprised
mainly of tin/lead then you have more ionic contamination than you think,
if the dendrite is comprised of mainly Ag/Pd then time to stop getting
components with cracks. And if its neither of those two possibilities I'll
have to go get another Coke and really do some brainstorming. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
phil bavaro <[log in to unmask]>@ipc.org> on 02/25/2002 11:53:06 AM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to phil bavaro <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [TN] Dendritic Growth...
OK, here is the background on this particular dendrite growth:
It is a field failure unit and upon closer inspection, this dendrite
follows a crack in the capacitor from one termination to the other. In my
opinion, the crack was there first, and was filled with contaminated
moisture while the cap had battery voltage (5V) across it, causing the near
perfect dendrite to form along the walls of the crevice.
This CCA was produced on our no clean assembly line, of which the
cleanliness is monitored on each lot of boards produced. We believe that
the solder flux residues are not related to this defect. The soldering
residues do not migrate to this area of the capacitor.
The condensation moisture alone would not seem to be enough to have
generated this dendritic growth so we are investigating other possible
contamination that may have been present such as moisture dripping off of a
dirty shelf above, and also the component cleanliness itself. If the crack
was there from the beginning then the contamination could have been as
well.
We have seen shorts across this capacitor before but usually only as random
faint carbon tracks that are nowhere as defined as this one was.
This one is so perfect, it almost looks like an organic plant like a small
fern.
This is an ongoing investigation so all comments are welcome and I will
provide an update at some point in the future.
Phil
At 06:51 AM 2/25/02 -0700, [log in to unmask] wrote:
>Doug,
>Isn't that known as the Mountain "Dew"lution Test?
>Dewey
>
> > -----Original Message-----
> > From: [log in to unmask] [SMTP:[log in to unmask]]
> > Sent: Monday, February 25, 2002 6:31 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Dendritic Growth...
> >
> > Sunny asks:
> > I'd like to know which type of Ion Chromatography can measure the
local
> > dot ionic cleanliness. Anyone can tell me which company produce this
> > equipment?
> >
> > Doug Pauls responds:
> >
> > No company that I know of builds ion chromatography equipment to do
spot
> > ionic checks. IC equipment merely analyzes extract solutions. If you
> > want
> > to measure localized ion concentrations, you have to develop a
localized
> > extraction technique. This might involve immersion of a portion of the
> > assembly in extract solution, or building some time of
non-contaminating
> > enclosure around the area to be extracted. Once you can localize the
> > extraction, collect the solution an analyze it with the IC equipment
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