Q: I’ve read some articles about Normal map baking, and they all said that in order to have a clean result, I have to have Smoothing split, and UVs split on every hard edge. For example like this box, below. Sometimes I can not have UVs split on every hard edge and then the normal seam show up. What then?
A: By adding more edges to your low poly (like, chamfers) to support the extreme change in angle on the edges.