Multilayer substrate and electronic device

A multilayer substrate comprising:

• a multilayer body including a plurality of resin layers including a first resin layer and stacked in a Z-axis direction;
• a signal conductor layer in the multilayer body;
• and
• one or more through conductors passing through the first resin layer in the Z-axis direction;
• wherein
• each of the plurality of resin layers includes a first main surface and a second main surface in the Z-axis direction;
• the first main surface of the first resin layer includes one or more hollow portions, the one or more hollow portions extend into the first resin layer and are not in contact with the signal conductor layer, and the one or more hollow portions overlap the signal conductor layer when viewed in the Z-axis direction;
• the one or more hollow portions do not pass through the first resin layer in the Z-axis direction;
• each of the one or more hollow portions and the one or more through conductors includes a tapered region in which each of a cross-sectional area of the one or more hollow portions orthogonal to the Z-axis direction and a cross-sectional area of the one or more through conductors orthogonal to the Z-axis direction increases toward the first main surface of the first resin layer in the Z-axis direction;
• and
• the tapered region of each of the one or more hollow portions and the one or more through conductors is in contact with the first main surface of the first resin layer.