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March 2005

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Subject:
From:
"Ingemar Hernefjord (KC/EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ingemar Hernefjord (KC/EMW)
Date:
Tue, 15 Mar 2005 08:27:47 +0100
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Is that a high voltage filter application, oil filled? Ceramic caps and stainless steel is otherwise odd combination as well as using 97.5PbAg2.5...

and what strange cap is this? Usually, there is  nickel barrier between the silver termination and next finish, which can be tinned or palladium plated etc. If you have a barrier, the silver should not be affected.

If you really got a just-silver termination and want solder that to stainless steel, I have only seen copper/silver alloys for that job, which is a real brazing.

Corrosion is family name of whole group of detrimental effects,like oxidation, attack from sulphuric contaminations, attacks from acidous or alcalic environment,  grain interstitial destruction, electromigration, various kind of other chemical attacks and so on. I tried once to go to the bottom with this word, but gave up because of the very confusing interpretation of the topic. David Douhit has written a lot about 'corrosion', ask for his work. Werner will probably send you megs of 'corrosion' papers.

I have seen 'darkened' silver in a lot of situations from my many years in the electronics industry, and most of the cases have just been a harmless superficial discoloration. On second place comes silver migration, which can go on until all silver is consumed. The result use to look frightening, can really be named 'corrosion'. On third place comes chemical attacks from the surrounding. I  have seen silver which has been completely dissolved. Also worth called 'corrosion' maybe.

Finally, silver in a alloy, especially the eutectic compounds, are not apt to 'corrosion'.

Looking forward to see what the expertise will say. Your case interests me...


Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]
Sent: den 15 mars 2005 01:12
To: [log in to unmask]
Subject: [TN] Capacitor Terminal Silver Braze Problem


March 14, 2005

Folks, the attached photographs show some type of problem with a silver to ceramic braze joint.  The joint provides a seal around a capacitor terminal.  The manufacturer rejected the terminal due to the noted conditions.  

In the photo titled top view, the white area on the outer diameter is a 90% Alumina ceramic.

Next to the ceramic is the silver braze connection that connects the ceramic to 304 stainless steel using Ag 2.5 silver solder (i.e., 97.5PbAg2.5).

In the middle of the top view photo is a copper terminal.

The other three photos show the top view enlarged in the 8 o'clock, 4 o'clock and 10 o'clock positions.

As you can tell from the photographs, the silver braze joint appears corroded.  Also, there appears to be poor or non-wetting to the 304 SST, or otherwise a crack formed.  

We used a knife in the area from the one o'clock position to about the 2 o'clock position and were able to easily scrape away the top surface of the braze down to about a 1/64 inch or > depth till we got to solid braze area.

We had an analysis performed that did not show any chlorides, just silver and lead.  It could be that the stuff we were able to scrape away is some form of lead oxide, not sure.

I note that the historical QQ-S-571 specification in section 6 indicates that Ag2.5 solder is subject to corrosion in a humid environment, as is Ag5.5.  However, the QQ spec indicates that Ag1.5, which is used interchangeably with Ag2.5, has better shelf life and will not develop a black surface deposit.  I don't know if the corrosion for Ag2.5 means just a black deposit, or if the solder joint will fail due to other type of corrosion.  I note that J-STD-006 does not include the same information as does section 6 of QQ-S-571.  

During our qualification testing, we subjected a capacitor to high humidity at high temp for an extended period of time with no evidence of corrosion.    
 
Can you folks please answer the following questions?

1.  What caused the conditions shown in the photographs?  What are the photographs showing?

2.  Would brazing (probably a furnace braze, not sure) at too high or too low a temperature cause the condition?

3.  Will Ag2.5 corrode in a humid environment? If so, what type of corrosion, and why can it occur?  Is it a black surface condition?, or can the corrosion ultimately cause failure of the braze joint?  What is the physical science basis for the corrosion?  Why does it only occur for certain % of silver content, and not for all % of silver?   

4.  Why doesn't J-STD-006 have the same info as QQ-S-571 regarding corrosion of Ag2.5 and Ag5.5 solder?

5.  Will the corrosion continue?

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