Werner Engelmaier wrote:
>
> Hi Phil,
> The "feathery migration tracks" you are describing are most likely dendritic
> growth, sometimes called 'Conductive Anodic Filaments (CAF)'. Dendritic
> growth starts as the migration of metal, and other salts, along paths created
> by structural imperfections. This migration requires moisture, water in most
> cases, but the presence of an electrical potential is not necessary. When
> electrical potential is applied, the migration is more rapid and organizes
> itself into CAF. CAF leads to a reduction in insulation resistance, and
> depending on the currents present can create localized shorts and burn-out.
>
CAF and electroCHEMICAL migration are two different phenomena.
I suspect what Phil is observing is electrochemical migration,
which results in the formation of metal dendrites, which frequently
have a feathery or tree-like appearance. Terry gave a pretty good
description of electrochemical migration, which is sometimes called
simply electromigration, which gets it mixed up with non-electrochemical
electromigration, as described by Bill Davis, another phenomenon
which requires no humidity.
In electrochemical migration on circuit boards, moisture is absorbed
by hygroscopic contaminants on the surface of the circuit, and if
these contaminants are ionic or other contaminant which are ionic
are present, a layer of electrolyte is formed. If this electrolyte
is present between two adjacent conductors with a dc voltage difference
between them, 1.5 V can be enough, metal, usually tin, copper, or silver,
can be electrochemically dissolved by oxidation to metal
cations or metal complex cations at the anode or positive
conductor. If the metal species formed are soluble, they will
which move through the film of elctrolyte under the
influence of the electric field between the conductors to the cathode,
or negative conductor. There, the metal cations are
electodeposited at the cathode, but unlike electroplating, which
this failure mechanism resembles, a nice smooth deposit is not obtained.
The metal redeposits as needle-like or tree-like structures called
dendrites. No dc voltage, no dendrite formation. These dendrites
like to grow along the little cracks on the substrate
surface. Sometimes these dendrites grow from the cathode all the way
to the anode and short the circuit. Not good. If one is lucky, the current
flowing through the very fine dendrite is high enough for resistive heating
to volatilize the dendrite. Chloride and some other anions tend to exacerbate
the metal dissolution at the anode because they are specifially adsorbed, that
is they form a covalent bond to the metal at the surface and facillitate
corrosion.
CAF occurs when an electolytic conductive path is formed from the anode
to the cathode, usually in voids along glass fibers in epoxy glass laminates.
It usually involves higher electrical fields than electrochemical migration,
does not involve metal redeposition a the cathode. The conductive path is the
metal salt solution, usually copper, not a metal filiment.
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