Method for fabricating a flip chip substrate structure

A method for fabricating a flip chip substrate, the steps comprising:

• providing a carrier;
• forming a resist layer on the carrier, wherein plural first openings are formed in the resist layer;
• forming a first metal layer, an etching-stop layer, and a second metal layer in the first openings of the resist layer, sequentially;
• removing the resist layer;
• forming a first solder mask on the carrier surface and the second metal layer;
• forming plural second openings on the first solder mask and the surface of the second metal layer, and second openings correspond to the second metal layer;
• forming at least one circuit build up structure on the surface of the first solder mask;
• forming a second solder mask upon the circuit build up structure, wherein plural third openings are formed on the second solder mask to expose the portions of circuits in the circuit build up structure that are to be electrically conductive pads;
• and
• removing the carrier, the first metal layer, and the etching-stop layer to expose the second metal layer in the second openings of the first solder mask as the electrically conductive pads of the other side.