A method for detecting the under-fill void of the flip chip ball grid array package structure is provided, which includes providing a substrate having an interconnect structure and a plurality of interposers therein; providing a chip having an active surface and a back side, and a plurality of first connecting elements on the active surface of the chip; mounting and electrically connecting the active surface of the chip on the substrate; performing at least once IR reflow to fix the plurality of first connecting elements on the substrate; filling an encapsulate material to cover the active surface of the chip and the plurality of first connecting elements; performing a detecting process to detect that void is not formed between the active surface of the chip and the plurality of first elements; and forming a plurality of second connecting elements on the back side of the substrate to obtain a flip chip ball grid array package structure.