Technically, the problem is not a cold solder joint. The solder HAS adhered to both the post and the pad on the board. The problem is mechanical, both in design of the board and often due to assembly errors.
In order for a solder joint to be effective, the connection must be mechanically stable and rigid before soldering. The problem in the picture occured because the post moved relative to the PCB hole because the hole was sized too large for a snug fit. This broke the solder joint, which then developed the black circle from arcing due to poor contact.
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