Hi TechNet!
     
     Oh boy, another question for the metallurgists!! A eutectic 
     transformation is when a liquid converts to two or more intimately 
     mixed solids! Many metals have eutectic points on their phase 
     diagrams. The reason to use a eutectic alloy of solder is that in 
     manufacturing you can use the convenient liquid/solid switch as the 
     on/off switch to facilitate handling, processing etc. The true 
     eutectic for tin/lead is 61.9%tin/38.1%lead (183C)- everyone calls 
     63/37 or 60/40 eutectic but they are really near eutectic alloys. As 
     for 62Sn, 36Pb, and 2Ag there is some literature published that shows 
     the alloy is better in reliability than 63/37 - there is also data 
     that says 62Sn, 36Pb, and 2Ag is no different than 63/37 so you will 
     have to review the data to form an opinion. When chip capacitors 
     didn't have barrier metals  the 62Sn/36Pb/2Ag was used to help slow 
     down leaching of the PdAg alloy off the chip capacitor end cap. 
     Barrier metals are used by most chip capacitor fabricators today so 
     leaching isn't an issue any more. Your "pasty" range of the 62Sn, 
     36Pb, and 2Ag alloy isn't very big so I don't think any preheater 
     adjustments are necessary. Hope this is helpful.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Sub: Solder Paste Plastic or Eutectic
Author:  [log in to unmask] at ccmgw1
Date:    11/21/96 4:49 PM


     Question:
     If I have a solder paste that is 62Sn, 36Pb, and 2Ag, is 
     this a Eutectic or Plastic range solder paste?  63Sn/37Pb is 
     a Eutectic solder with a freeze point at 361 degrees "F".
     
     I am using a ESP paste at the above mix.  The range of this 
     solder is 354 - 372 degrees.  If so it would seem to be a 
     plastic solder.  If this is true then I need to readjust my 
     preheat temperatures at Wave Solder to compensate for the 
     354 degree low end of the solder paste to prevent reflow.
     
     Is this paste Eutectic or Plastic?  Has anyone experienced 
     this?  Do I need a 62/36/2 paste like this?  Why would a 
     regular 63/37 paste work?
     
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