Hi Gaby - You are not going to like this but the cracking phenomena 
     you are seeing has been documented and is going to be a problem! The 
     NCMS has/is completing a VERY large study on Lead-Free solder alloy 
     evaluation. They covered a huge amount of work - everything from cost 
     analysis, manufacturability and several flavors of solder joint 
     reliability investigations. I am not allowed to give out specific 
     details but they did find that with some of the Sn/Pb/Bi alloys that 
     on thru hole technology assemblies due to solidification physics that 
     cracking occurs. They will be publishing a portion of this data at the 
     TMS meeting in Orlando in February and at the IPC Expo in March. There 
     is some really neat data (well, from a metallurgist standpoint anyway) 
     going to be coming out of this effort. I suggest you contact Jerry 
     Rosser at Hughes (or is it Raytheon now?) for more info. His email is 
     [log in to unmask] . Good Luck.
     


Dave Hillman
Rockwell Collins
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Reflow Soldering of PTH Connectors
Author:  [log in to unmask] at ccmgw1
Date:    1/23/97 7:35 PM


Hello,technetters!
We perform double reflow on boards populated with SM LEDs on one side 
and on the other side SMCs and PTH connectors.
In order to prevent the LEDs to go twice through hgh temperature 
profiles -the second one being even loger-we tried to use a solder 
paste with a lower melting point:Sn43Pb43Bi14 which is not eutectic. 
The cross section of the pin in hole showed some solder voids-which
were present also when using Sn63 paste and are accptable from our point 
of view-but also cracks which originated from them along the pin or 
along the hole wall.Our boards are 0,1microns gold plated.
Could anyone explain why it happed and give us some advice?
     
Gaby
P.S.Thanks to CONDOR and DAVE HILLMAN for their former replies!
     
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