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You cannot see a good solder joint from the outside. You can see the fillet, you can see the shine, you can see whether the part is sitting flat. But what is happening inside the barrel of a through-hole, under the pad of a BGA, or at the interface where the lead meets the copper — that is invisible to the naked eye. X-ray is the only inspection method that shows you the truth. And the truth is usually worse than what you hoped.