Hi Peter -
     
     The impact of the bismuth/indium elements (or others) in the solder 
     joint would depend on what percentage of the final solder joint those 
     elements constituted. I would recommend "processing" a statistical 
     sample of solder joints using the proposed process and then conducting 
     either chemical or electron microscopy analyses to determine just what 
     you have for a solder joint makeup. Some elemental additions can alter 
     the solder joint reliability at very low percentages and the 
     possibility of having elemental segregation which could result in 
     embrittlement or a low melting point alloy would be a concern. Good 
     Luck.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: ASSY: Rework Method
Author:  [log in to unmask] at ccmgw1
Date:    6/2/97 1:18 PM


((Sent this on Friday, but it didn't appear to come through. Apologies if it 
repeats.))
     
I would be interested in TechNetters' comments about a rework system we have 
been asked to consider.
     
The process involves adding a low temperature solder alloy 
(tin/lead/bismuth/indium/? - not sure of the %ages) to the existing 
solder joints at low temperature; the alloys mix and then 
allow the SMD to be removed at the low temperature. The lands are cleaned and 
then the device replaced as normal (soldering iron and conventional solder).
     
The pros are rework at low-to-zero capital cost (maybe good for field 
service), and at low temperature. The cons are that it's actually about a 10 
step process, and I believe takes longer than conventioanl methods. Couldn't be 
easily used for BGAs, best for QFPs etc where you can access the joints.
     
My initial worry was the idea of remnants of the low temp alloy would be left 
in the new joint - what effect would that have on joint reliability? But, 
probably only a low concentration of bismuth might be introduced to new solder 
joint.
     
Any other thoughts? Comments? I know that it's not a newly invented process - 
why isn't it used more commonly?
     
Peter
-- 
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Peter Swanson                                              Oxfordshire, England 
INTERTRONICS
[log in to unmask]                      http://www.cygnetuk.demon.co.uk
     
Suppliers of materials and consumables to the electronics & related industries 
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