Hi Victor!
     
     
     How about neither method! We do double sided surface mount with one 
     solder alloy (Sn63) with very few problems. This practice only 
     requires that you profile your assemblies carefully to not "drop" 
     components off the back/bottom side during the second reflow. You 
     don't need glue, just good equipment thermal characterization. Yes, 
     some assemblies are hard to do both sides due to design issues (# of 
     layers, etc.) but nothing is impossible. I would not advocate the use 
     of different solder alloys on the different sides of the assembly 
     because of the confusion it can end up generating in manufacturing 
     operations, repair, etc.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Two sided SMA
Author:  [log in to unmask] at ccmgw1
Date:    12/9/96 11:35 AM


Hi,
     
When there is a need to solder SMT components on both sides of PCB, I 
understand that 2 approaches are taken generally.
     
i) use low temp. solder paste on one side and high temp solder on the other 
ii) use conductive adhesive on one side and solder on the other side
     
My question are:
     
1) Which one is more popular in practice?
2) What are the pros and cons for the two methods?
3) For i), what are the melting points for the 2 kinds of solder paste? 
4) What are the precautions/possible defects one should bear in mind?
5) Can I use the same operating procedures for making 2 sided BGA assembly?
     
Any literature that may describe the above issues?
     
Thank you very much for any kinds of inputs.
     
Best regards,
     
Victor
     
     
     
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