Wiring board and method of forming hole thereof

A wiring board includes a photosensitive insulating layer and a first wiring board. The photosensitive insulating layer has a hole, a first surface and a second surface opposite to each other. The hole has a first end opening formed in the first surface, a second end formed in the second surface opening, an axis, and a sidewall surrounding the axis. Part of the sidewall extends toward the axis to form at least one annular flange. The first wiring board is disposed on the first surface and includes a first pad, in which the hole exposes the first pad. At least one recessed cavity exists between the annular flange and the first pad. The minimum width of the annular flange is smaller than the maximum width of the recessed cavity.