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April 1997

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From:
"ddhillma" <[log in to unmask]>
Date:
Mon, 28 Apr 97 17:37:33 cst
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     Hi Jim!
     
     Yes, metallurgical questions are always fun! (especially since we 
     TechNetters are get to throw all sorts of different possibilities to 
     you without exactly knowing the perfect answer). Silver diffuses into 
     solder quite fast so your component would have to have a reasonably 
     thick silver plate or very little solder paste available. If you did 
     get a copper/silver intermetallic it would be very hard to "melt". The 
     silver/copper eutectic point is nearly 800 C. I suggest you put a 
     cross section of one of the solder joints (cut it out of the board) 
     into the Scanning Electron Microscope and do a elemental mapping 
     analysis. If you see silver migration into or at the copper pad 
     interface then silver diffusion is the cause. If not, then look at 
     component/soldermask interactions. Good Luck.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
      


______________________________ Reply Separator _________________________________
Subject: ASSY: DE-SOLDERING PROBLEM
Author:  [log in to unmask] at ccmgw1
Date:    4/28/97 1:30 PM


Hello Technetters:
     
(Put on your metallurgical hats for a moment.)
     
I ran into an unusual circumstance the other day that has me baffled.  We 
soldered down a surface mount glass diode using conventional processes (Sn63 
water soluble solder paste, convection reflow).  The components were 
mis-oriented therefore had to be removed.  To our dismay, the components 
wouldn't separate from the board (polyimide multilayer).  Even though the 
solder was molten, the components wouldn't budge.  Finally, the pads lifted off 
the board so I tried to remove the pads from the component, to no avail.  
     
It turns out that the component's base metal is pure silver which is then 
solder coated.  The cross section looks like the silver dissolved with the 
solder, with the resultant alloy in contact with the copper pad (which was 
originally solder coated).
     
Could the soldering process have created a metallurgical bond between the 
silver and the copper?
     
Thanks for your assistance,
     
Jim Marsico
(516) 595-5879
[log in to unmask]
     
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