The failure mode that I worry about and would need X-ray to detect is that temporary misalignment will merge/bridge some of the solder balls.
I like to "wiggle" the chip with tweezers (under hot air) to see surface tension in action and judge whether the balls have all melted, but a tiny bit too much wiggle causes that problem.
I like to "wiggle" the chip with tweezers (under hot air) to see surface tension in action and judge whether the balls have all melted, but a tiny bit too much wiggle causes that problem.