Wiring substrate, composite substrate, and manufacturing method of wiring substrate

A manufacturing method of a wiring substrate, comprising:

• preparing a ceramic substrate comprising an upper surface, a lower surface opposite to the upper surface, and a lateral surface between the upper surface and the lower surface, the lower surface comprising a first lower surface located on a first outermost side of the ceramic substrate and a second lower surface located inward from the first lower surface, the lateral surface comprising a first lateral surface located on a second outermost side of the ceramic substrate and a second lateral surface located inward from the first lateral surface;
• forming a first conductive paste layer by disposing a first conductive paste on the second lower surface;
• forming a first wiring intermediate by firing the first conductive paste layer;
• and
• forming a first wiring such that a main surface of a lower surface of the first wiring is substantially parallel to the second lower surface, the main surface being 50% or more of a surface area of the lower surface of the first wiring, the first wiring being disposed inward from the first lower surface, by polishing or grinding the first wiring intermediate, wherein
• in the forming of the first wiring, the first wiring has a thickness of 80% or more of a height from the second lower surface to the first lower surface, the first wiring being disposed in contact with the second lower surface and the second lateral surface, and a lateral surface of the first wiring is exposed from the first lateral surface of the ceramic substrate.