Abstract: Methods for reducing warpage and increasing the effectiveness of hybrid bond void testing are disclosed. A semiconductor package has a first side and a second side, with a dummy bond pad located on the second side, and at least one metal-containing layer between the first side and the second side (for example a redistribution layer). A warpage control structure is provided in the semiconductor package that extends from the first side into the semiconductor package, and is aligned with the dummy bond pad. The warpage control structure is made of a low-density filling. This relieves stress that causes/increases warpage. When a hybrid bond is formed between the semiconductor package and another semiconductor package, the warpage control structure maximizes acoustic wave penetration for testing the quality of the hybrid bond at the dummy bond pad.