An interposer substrate includes a first insulating layer having opposite first and second surfaces; a first wiring layer formed in the first insulating layer, with a surface of the first wiring layer exposed from the first surface; first conductive pillars formed in the first insulating layer; a second wiring layer formed on the second surface; second conductive pillars formed on the second wiring layer; a second insulating layer formed on the second surface and covering the second conductive pillars and the second wiring layer, with end surfaces of the second conductive pillars exposed from the second insulating layer; and immersion tin layers formed on the first wiring layer and the end surfaces of second conductive pillars. The immersion tin layers are used as surface processing layers to be applied to products having ball pads that need to be exposed extensively. A method for fabricating the interposer substrate is also provided.