Dave, 

You are correct to question the rework of lead-free with eutectic solder.  This is the very same question currently facing the Department of Defense repair depots.  They do not have the resources to determine the exact alloy coming in for repair - from the COTS equipment now in use. The current approach is to repair everything with tin-lead solder as the "least bad" answer.  To confirm this approach, an additional set of circuit boards is in test within the NASA/DoD test matrix.  Roughly 30 boards with various SAC solders have had components repaired once or twice with eutectic solder.  The early test returns indicate little loss of assembly life through harsh environment vibration and drop shock testing.  Thermal cycle data is incomplete.  Unfortunately for your answer here - "the jury is still out" but the facial expression of the jury looks favorable,  The final data should be totally compiled within a year as themal cycle testing takes that long.  

My answer - if you can determine what the original SAC composition is - use that. 
If you cannot, use eutectic because that is the least bad alternative. 

Denny Fritz


-----Original Message-----
From: Dave Schaefer <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, Oct 27, 2009 11:52 am
Subject: [TN] Reworking LF(SAC) Assemblies with SnPb Solder



Working on a product that is exempt from RoHS requirements but does require
F(SAC) processing because of BGA ball solder composition. Most other
roducts are standard SnPb process, and rework is performed with SnPb solder.
Am I correct to question the use of SnPb solder to rework the LF(SAC)
ssemblies?
It would seem that mixing SnPB and SAC solder would result in an unknown
older composition.
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