To provide a thermosetting flux, which can surely carry out a solder joining operation, and does not need to remove the remaining flux by washing after the solder joining operation, and can keep the electric insulation resistance even in a hot and moist atmosphere, and is excellent in the dielectric characteristic, and can achieve a highly reliable solder joining, and further to provide a flux sheet and a multilayer printed circuit board.

The thermosetting flux serving as a flux in the solder joining operation contains a resin (A) having a phenolic hydroxy group, a resin (B) serving as a hardening agent for the resin (A), and a cyclo-olefinic resin (C). The multilayer printed circuit board has interlayer joined portions joined by the solder joining operation through the thermocompression bonding of heating and pressing a joining layer and a layer to be joined via a flux layer composed of the thermosetting flux, the joining layer having wiring patters, conductor posts formed on the wiring patterns, and solder layers formed on the surfaces of the tip ends of the conductor posts, and the layer to be joined having interlayer joining lands for the conductor posts.