Circuit substrate and method for manufacturing the same

A method for manufacturing a circuit substrate, comprising:

• forming a cavity in a core substrate such that the cavity penetrates through the core substrate;
• accommodating a metal block in the cavity of the core substrate;
• filling a filling resin into a gap formed between the cavity and the metal block such that the metal block is positioned in the cavity of the core substrate;
• forming a first build-up layer comprising an insulating resin layer on a first side of the core substrate such that the first build-up layer covers the cavity on the first side of the core substrate;
• and
• forming a second build-up layer comprising an insulating resin layer on a second side of the core substrate such that the second build-up layer covers the cavity on the second side of the core substrate,
• wherein the metal block has a plurality of roughened surfaces on the first and second sides of the core substrate respectively such that the roughened surfaces are in contact with surfaces of the insulating resin layers in the first and second build-up layers on the first and second sides of the core substrate, respectively, the metal block has a plurality of roughened side surfaces between the roughened surfaces such that the roughened side surfaces are in contact with the filling resin, and the filling resin comprises resin derived from at least one of the insulating resin layers in the first and second build-up layers.