Ultra-thin multilayer (UTM) is the term used to describe a construction class for extremely thin printed circuit boards in microfine conductor technology that is much thinner than normal multilayer printed circuit boards. The vias
also have a much smaller diameter.prepreg, a four-layer UTM PCB is less than 300 µm, a six-layer less than 400 µm and an eight-layer less than 500 µm. In contrast, a standard 8-layer PCB is 1.5 mm thick. The vias of UTM PCBs have a diameter of only 150 µm.