Ave netters,
interesting stuff to read. We studied such dendritic growth under
microscope and took photos. The objects were small ceramic capacitors
mounted on a Duroid board. With addition of halogenated water (small
droplets) the dendrites (looking like nice leaves) growed from one pad
to the other in seconds! And a short was made. After we blowed these
'shorts' with a current pulse, the dendrites did not come back,
curiously enough. We found that dendritic growth seems to be comparable
with bacteria and similar organics: they love humidity, unspecified soil
and warmth. When we cleaned the boards more carefully from flux
residues, increased the space between component and lands, and also
minimized the possibility of humidity to condensate as droplets (once
there, small drops of contaminated water are inclined to become
aggressive and hide themselves with an innocent face, looking like dry
material, but are camouflaged batteries!),after that there were no more
dendrites. Another important factor seemed to be the evidence of
microscopical metal particles in the insulator surface, located between
the lands or the conductors. These particles seemed to be the result of
a less good plating control: the PWB board manufacturer had no idea
about the myriades of less-than-micrometer sized metal particles that
were left in the insulators rough surface. That was also overcome. So,
today, no troubles at all./

PS. There is a good book about it all, edited by Morris E.Nicholson.
Titled 'Electronic Packaging and Corrosion in Microelectronics' done by
ASM as a Library of Congress book with ISBN: 0-87170-291-6. It it's not
in the bookstore, try ASM and ask about a conference 28-30 April -87 in
Minneapolis. Good Luck. DS.

Ingemar Hernefjord
Ericsson Microwave Systems




---------------------------- ------------------------------------

>      It's interesting and logic in the explanation below. But more
>      important is how to prevent it. Any suggestion?
>
>      Regards,
>      KenF
>
>
> ______________________________ Reply Separator
_________________________________
>
> Subject: Re: [TN] Electromigration of Tin Lead?
> Author:  "TechNet E-Mail Forum." <<a href="/scripts/wa.exe?LOGON=A3%3Dind9905%26L%3DTECHNET%26E%3D7bit%26P%3D4038598%26B%3D--%26T%3Dtext%252Fplain%3B%2520charset%3DUS-ASCII%3B%2520name%3D%2522%5BTN%5D%2522%26N%3D%5BTN%5D" target="_parent" >[log in to unmask]</a>>  <a href="/scripts/wa.exe?LOGON=A3%3Dind9905%26L%3DTECHNET%26E%3D7bit%26P%3D4038598%26B%3D--%26T%3Dtext%252Fplain%3B%2520charset%3DUS-ASCII%3B%2520name%3D%2522%5BTN%5D%2522%26N%3D%5BTN%5D" target="_parent" >[log in to unmask]</a>
at
> Internet
> Date:    5/26/99 2:26 PM
>
>
> 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.
>
> Werner Engelmaier
> Engelmaier Associates, L.C.
> Electronic Packaging, Interconnection and Reliability Consulting
> 7 Jasmine Run
> Ormond Beach, FL  32174  USA
> Phone: 904-437-8747, Fax: 904-437-8737
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