A printed wiring board includes a first substrate having first pads and second pads such that the first pads are positioned to mount an electronic component on the first substrate and that the second pads are positioned to electrically connect a second substrate to the first substrate, and metal posts formed on the second pads, respectively, such that the metal posts are positioned to mount the second substrate on the first substrate. The first substrate and the metal posts satisfy that a ratio, b/e, is in a range from 0.3 to 1.0, where b represents a length of each of the metal posts and e represents a thickness of the first substrate.