Method of manufacturing treated sheet and method of manufacturing resin multilayer substrate

A method of manufacturing a resin multilayer substrate, the method comprising:

• preparing a resin layer composite including a resin sheet, a thermoplastic resin layer supported on one surface of the resin sheet, and a conductive pattern disposed on a surface of the thermoplastic resin layer opposite to the resin sheet;
• applying a dispersion liquid including a powdery thermoplastic resin raw material onto a surface of the thermoplastic resin layer opposite to the resin sheet so as to cover the conductive pattern;
• fusing the thermoplastic resin raw material to the thermoplastic resin layer by irradiating, with light from a flash lamp, a coating of the thermoplastic resin raw material formed in the applying step;
• removing the resin sheet from the resin layer composite after the fusing step;
• stacking another resin layer on a surface of the resin layer composite subjected to the step of removing the resin sheet, the surface being located at a side at which the thermoplastic resin raw material is fused;
• and
• integrating a stack including the resin layer composite and the other resin layer by applying heat and pressure to the stack;
• wherein
• the thermoplastic resin raw material has a melting point higher than a glass transition point of a resin that is a primary material of the resin sheet;
• and
• a portion of the dispersion liquid including the powdery thermoplastic resin raw material that covers the conductive pattern is irradiated with light from the flash lamp in the step of fusing the thermoplastic resin raw material to the thermoplastic resin layer.