Peter,
The "contamination" should "bridge" adjacent traces, pins, sockets leads, ect.
Contaminants will also attract water vapor, converting it into a liquid film.
It is this aqueous film that becomes the growing field for dendrites.
Residues from the manufacturing process will speed up this process.
Exposure to air will also allow deposition of contamination. It just takes
longer.
Conformal coatings can delay dendrites if there is good coverage
(over sharp points, edges, under components, the back side of leads, etc.).
Verifying coating integrity can be a difficult process.
This is one reason circuitry becomes humidity sensitive over a period of time.
David a. Douthit
Manager
LoCan LLC
[log in to unmask] wrote:
> Hi, David,
>
> So is it true that contamination has to be on a copper trace to start with
> in order to grow? If contamination is only on the solder mask or other
> non-metallic surface, there's no harm?
>
> Hmmm! Further light is glimmering on this contamination business (or is it
> yet another train coming through the tunnel?)
>
> Peter
>
> David Douthit <[log in to unmask]> 06/02/2003 12:49 PM
> Sent by: TechNet <[log in to unmask]>
>
> Please respond to "TechNet E-Mail Forum."; Please respond to David Douthit
>
> To: [log in to unmask]
> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> Subject: Re: [TN] Dendrites
>
>
>
>
> Peter,
>
> They "grow" by chemically desolving the metal in the traces and
> redepositing
> the metal ions. The reason they have that shape (fern like)
> is because it is a metallic based crystal structure.
> You are "growing" new traces!
>
> David A. Douthit
> Manager
> LoCan LLC
>
> [log in to unmask] wrote:
>
> > Hi, Wee Mei,
> >
> > Dendrites can be (and usually are) on top of Solder mask but underneath
> > conformal coating. Solder mask, of course, leaves exposed metal where
> > components are soldered, test pads are located, etc. It just takes a bias
> > voltage to make microscopic "seeds" of ionic contamination on the surface
> > grow between the points of differential voltage.
> >
> > On top of conformal coating, if dendrites happen at all, it doesn't
> matter.
> > The metal they would short to is on one side of the coating and the
> > dendrites would be on the other side, so they never come into contact.
> >
> > I've always been curious to know, though - where do these beautiful and
> > quite substantial fern-like growths get all the material from to grow
> like
> > that? Their botanical look-alikes draw nutrition from the soil they grow
> in
> > - what do dendrites draw on to grow? It's not like they're "alive" or
> > anything. What are dendrites composed of (in general terms, not specific
> > chemical analysis, which will differ from specimen to specimen)?
> >
> > Peter
> >
> > Lum Wee Mei <[log in to unmask]> 06/02/2003 07:22 AM
> > Sent by: TechNet <[log in to unmask]>
> >
> > Please respond to "TechNet E-Mail Forum."; Please respond to Lum Wee Mei
> >
> > To: [log in to unmask]
> > cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> > Subject: Re: [TN] Dendrites
> >
> > Susan,
> >
> > I am not a reliability gal but curious to learn. While it is
> understandable
> > that dendrites can formed under moisture condition between conductive
> > patterns, how dendrites can be present on top of solder mask or conformal
> > coating? Just my curiousity?
> >
> > Regards,
> > Wee Mei
> >
> > [log in to unmask] wrote:
> > It would be good to know where the dendrites are located. You
> might
> > be able
> > to determine this with stereo microscropes. You need to determine
> if
> > they are under the solder mask or on top of the solder mask.
> >
> > The main way to get the dendrites to grow would be to add moisture
> > and a bias.
> >
> > Are the assemblies conformally coated? Have you done a simple
> ionic
> > cleanliness test?
> >
> > Susan Mansilla
> > Robisan Lab---------------------------------------------------
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