the present invention relates to a method to fabricate a flip chip substrate structure , which comprises : providing a carrier ; forming a patterned resist layer on the surface of the carrier ; forming sequentially a first metal layer , an etching - stop layer , and a second metal layer ; removing the resist layer , forming a patterned first solder mask , and then forming at least one first circuit build up structure thereon ; forming additionally a patterned second solder mask on the circuit build up structure ; respectively removing the carrier , the first metal layer , and the etching - stop layer ; and forming solder bumps on both sides of the circuit build up structure . the method increases integration and achieves the purpose of miniaturization . the method solves the problem of circuit layer multiplicity and process complexity .