Multi-layered, organic build-up semiconductor package substrates have build-up layers with layers of both fibrous organic dielectric material and non-fibrous organic dielectric material. Non-fibrous dielectric material layers are positioned below the signal metal layers and fibrous dielectric material layers are positioned below the power/ground plane metal layers. The package substrate combines in a single package substrate the advantages of rigidity, strength and relatively low CTE of a fibrous material with the capacity of a non-fibrous material to achieve fine resolution signal metal lines.