A resin-impregnated substrate is obtained by a method comprising the steps of immersing a sheet comprising an aromatic liquid crystalline polyester fiber in an aromatic liquid crystalline polyester solution containing 100 parts by weight of a solvent and 0.5 to 100 parts by weight of an aromatic liquid crystalline polyester and removing the solvent, the solvent containing on a weight basis with respect to the solvent 30% or more of a phenol compound. The resin-impregnated substrate has a high heat resistance in soldering conditions.