Unless your board fabricator put an unusual amount of solder during
there hasl process I cant see how just applying flux can give you a
reliable solder joint. This is certainly not the norm. Normally you apply
solder paste that has flux in it, place the parts and then reflow. And you
said there were cold solder joints on the sockets. Was that the sockets
only. If it was the sockets only then your cold solder joints were
probobly due to something else like maybe reflow temperature or board
handling after reflow.
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