X-ray tomographic view of microvias and a buried via

X-ray tomographic view of microvias and buried vias, laser-drilled holes just a small fraction of a millimetre across, used to link different layers of high-density interconnect printed circuit boards, HDI PCBs, but subject to reliability issues, known in the PCB industry as the 'hidden reliability threat'. This shows the PCB footprint for a grid array EEE component with HDI routing, showing two microvia layers underneath the solder pads, as well as a long buried via in the core of the PCB.