PURPOSE: To improve interlayer bonding and hence solder heat resistance by previously roughening internal layer copper with chemicals, reducing the surface of tie internal layer copper, and subjecting the resulting sample to multilayering bonding.

CONSTITUTION: A copper-clad laminate containing a plating catalyst is etched to form a circuit, and thereafter a copper surface is chemically roughened and treated by a reducing agent. A prepreg containing a plating catalyst, and a bonding sheet or copper foil containing a plating catalyst are superimposed on said circuit, and further heated, pressurized and molded in a united manner. Hereby, interlayer bonding and hence solder heat resistance are improved.