Re: gEDA-user: Tin pest

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Sat, Feb 04, 2006 at 11:22:38AM -0500, Bob Paddock wrote:
> On Saturday 04 February 2006 09:31 am, Karel Kulhavy wrote:
> 
> > The pest takes 1.5 years of freezing temperature to happen.
> 
> If done wrong (right?) you can get dendrite growth within a few seconds.
> It is a cool thing to watch on a microscope as they seem almost alive
> as they branch out like small trees, hence them name 'Dendrites'
> 
> Dentrites have always been a problem with ultra-high impedance circuitry,
> not with just Lead Free.  Your photo diode section might be prone to dendrite 
> problems.  Stress to the builders of your project that ultra clean boards are a must.

I am not using boards in the amplifier, but airwire construction. Do I
get dendrites in airwire construction? Do I get dendrites with Sn63Pb37
solder?

CL<

> 
> What Are Dendrites:
> http://www.rpi.edu/locker/56/000756/dendrite.html
> 
> Google for 'dendrites solder' and you get a lot of hits.
> then Google for 'dendrites high impedance'.
> 
> This page explains "Dendrites" are NOT "Whiskers" and gives a good overview
> of the whisker problem:
> 
> http://nepp.nasa.gov/WHISKER/background/index.htm
> 
> I do take exception to this advice at that page:
> 
> "Conformal Coat or foam encapsulation over the whisker prone surface appears to
> be beneficial but the limitations are not completely understood."
> 
> There is a significant limitation to Conformal Coating.  I used Conformal Coating 
> a lot in the coal mines to keep the caustic dust of the boards.
> 
> Since conformal coating is not a hermetic seal what real happens is
> the impurities in the water are kept away from the circuit, but the water
> itself reaches the traces, eventually.  Since the water is now fairly devoid of 
> contaminates the water acts more like a dielectric insulator.  You never
> notice in a digital circuit, but unless debugging is an obsession don't let it
> get near a RF tuning circuit or a high impedance sensor circuit.
> 
> 
> -- 
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