BUM boards provide circuitry in a three-dimensional array and are therefore advantageously space-saving, relative to conventional printed circuit boards, which provide at most two layers of circuitry on a two-sided board. 积层板的电路是三维排列的,与传统印制电路板相比极大地节省了空间,因为后者最多只能排布两层电路(即双面板)。