Hi Greg - 
     
     Oh boy, did you ask a question that will have the TechNet hopping for 
     awhile!!! When you have gold finished components & gold plated PCBs 
     and solder them using 63/37 you run the risk of having gold 
     embrittlement. Gold embrittlement is one of those topics that 
     metallurgists and material engineers love to discuss because: (a) it 
     is a very well documented phenomena, (b) gold-tin intermetallics are 
     interesting to look at.  A couple general rules of thumb to use:
     
     a) Is there enough solder volume for the gold to diffusive into?
     
        The answer to this question is almost always yes. The published 
     literature consensus is that gold embrittlement occurs at/around 4 
     wt.%. 
     
     b) Is the soldering operation long enough and hot enough to allow #1 
     to occur?
     
        This is the question that is the problem. Many times the soldering 
     process is too short to allow the gold to totally diffuse into the 
     solder thus leaving a thin, heavy concentration of gold-tin 
     intermetallic where cracks will originate.
     
     
     I don't have any experience with using a different alloy selection to 
     avoid the problems but hopefully some of the other TechNetters will 
     have some good suggestions. The grainy appearance of your solder 
     joints may be the gold-tin intermetallic particles. A good paper to 
     read on the subject is:
     
     "The Effect of Gold on the Reliability of Solder Joints", Dr. Judy 
     Glazer, 1991 Surface Mount International Conference proceedings.
     
     
     Good Luck.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
      


______________________________ Reply Separator _________________________________
Subject: ASSY: Gold-to-Gold Soldering
Author:  [log in to unmask] at ccmgw1
Date:    9/30/96 9:12 PM


          Does anyone have experience soldering gold-plated SMT components to 
          gold-plated PCB's using a eutectic (Sn63/Pb37) solder? The solder 
          joints exhibit a dull, grainy appearance. Although I know that longer 
          liquidus dwell times and a maximum reflow temperature of 220 C are 
          typically recommended when soldering to gold,  will other solder 
          alloys perform better (i.e,, Indium alloys)?
     
          Thanks in advance,
     
          Greg Kilinski
          Acuson Corp. 
          [log in to unmask]
     
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