Through-via substrate, mounting substrate, and method for manufacturing through-via substrate

A method for manufacturing a through-via substrate, the method comprising:

• a step of preparing a substrate including a first surface and a second surface located on a side opposite to the first surface, the substrate being provided with a through hole including a wall surface extending from the first surface to the second surface;
• and
• a through-via forming step of forming a through via in the through hole, wherein
• the through-via forming step includes
• a step of forming a seed layer located on the wall surface, the seed layer spreading along the wall surface from the first surface toward the second surface,
• a step of forming a first plated portion through a first plating process that involves use of a first plating solution, the first plated portion covering the seed layer, and
• a step of forming a second plated portion through a second plating process that involves use of a second plating solution, the second plated portion touching the first plated portion, and
• the second plated portion touches the first plated portion at an interface traversing the through hole.