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Subject: Re: [Homebrew_PCBs] DIY Gold Fingers?

From: dgminala@...
Date: 2016-11-06


Silver migration is nothing new; it's been known to those in the antique radio restoration business for many years in the form of "thunderstorming".  In ye olde days, IF transformers for tube-type AM radios frequently were roughly tuned by small mica compression capacitors (The capacitors only put the IF transformers in the general frequency range; final tuning was done by tuning the ferrite slug in the coils).  The mica slabs were silver plated; the mica being the insulator and the silver plating being the conductors for the capacitors.

After years of aging, the silver plating would migrate toward the edges of the mica slabs, eventually causing a partial short.  Before the short caused a permanent failure, the current leakage across the capacitor would cause a noise in the receiver, noted at the speaker as sounding like the radio was being affected by a distant electrical thunderstorm; thus the term "thunderstorming"
Of course, the only cure was to replace the internal mica capacitors with new modern mica capacitors.

Dave M


---In Homebrew_PCBs@yahoogroups.com, <hrconsult@...> wrote :

The paper kinda, sorta, gets around to mentioning silvermigration at the very end.  The is  a very important failuremechanism for silver plated traces that have a DC voltagedifference between them. The paper mentions that this occurs underspecific conditions, but does not say what those conditions are.The conditions are easily met. A small potential differencebetween the conductors, and moisture. A film of water is notnecessary, but just plain humidity. If you want to see this verydramatically, and quickly, put 5 volts between two silver platedconductors and bridge them with a drop of water. Watch under about10X magnification. A fern like (dendritic) structure will grow outfrom the negative side toward the positive trace. If you don'tlimit the current, the the dendritic structure will "explode" whenit hits the positive trace. If you limit the current enough, theshort will be permanent, even after the water evaporates.

A good description is available here:

http://www.te.com/documentation/whitepapers/pdf/p313-89.pdf

This is a well known failure mechanism in the aerospace community.

Harvey

On 11/5/2016 8:44 PM,k5ess.nothdurft@... [Homebrew_PCBs] wrote:
 

I posted a file RE silver vs gold plating for contacts. I probably shouldn't take up file space for this so i willdelete the file and just supply the link.


ThePerformance Implications of Silver as a Contact ... - TEConnectivity

 

  Mike N.