My first stab at your situation would be that you're not getting a good time
above reflow to allow your intermetallic layer to form a good bond between your
solder and lead.  The intermetallic layer is hard and brittle, and should be
minimized, however it is necessary.  Do a cross section of the bond if you're
even getting good wetting and wicking to the lead.  Analysis under a scope is
probably going to answer your own question.  Appears that you either have too
much or not enough heat.  Your profile is the key in my opinion.

Jason Smith
Process Materials Engineer
Lexmark Electronics

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