The wiring board includes an insulation layer made of ceramic and a conductor layer extending in a planar direction inside the insulation layer. The conductor layer is constituted by a sintered body of a plurality of crystallites containing a metal as a main component, and has a layered structure in which a dense layer, a non-dense layer, and a dense layer are layered in layers in this order in a thickness direction.

Provided is a coating agent capable of forming a coating film having high heat insulating properties. The coating agent according to the present invention is a coating agent used for forming a coating film in an electronic component module in which an electronic component is mounted on a circuit board via a conductive adhesive portion, the conductive adhesive portion being coated with a coating film, the coating agent includes a thermoplastic resin, a solvent that dissolves the thermoplastic resin, and hollow particles, in which a content of the thermoplastic resin is 1.0 wt % or more and 22.0 wt % or less based on 100 wt % of a total content of the thermoplastic resin and the solvent, and a content of the hollow particles is 15.0 parts by weight or more and 50.0 parts by weight or less based on 100 parts by weight of the total content of the thermoplastic resin and the solvent.

A method of making an interconnect substrate includes forming a first insulating layer containing a filler and covering a first interconnect layer, forming a via hole in the first insulating layer by laser processing, the via hole exposing the first interconnect layer, performing a heat treatment, plasma processing, and a desmear process in this order with respect to the first insulating layer, and forming, after the desmear process, a second interconnect layer including both an interconnect pattern formed on an upper surface of the first insulating layer and a via interconnect formed in the via hole.