A circuit board assembly allowing a higher population density of electronic components on the board includes a first circuit board, a second circuit board, and an interposer electrically connecting the first circuit board and the second circuit board. The interposer includes a substrate sandwiched between the first circuit board and the second circuit board, the substrate includes a first surface facing the first circuit board, a second surface facing the second circuit board, and a lateral surface connecting the first circuit board and the second circuit board. It is the lateral surface which carries a test pad, allowing all testing of components to be done on the test pad.