  An earlier study of a high layer-count test board using plated-through-hole
(PTH) vias and a limited quantity of laser vias was shown to be capable of
supporting 112 Gb/s PAM-4 links (or equivalent signaling having 28 GHz
(Nyquist) bandwidth). This original board design was then rebuilt using a
different fabricator, and the test results revealed a significant decrease in
the bandwidth of the vias. These results led to the development of a set of
design specifications that PCB vendors can easily validate, which will ensure
that the use of high layer-count boards with PTH technology are viable for
emerging 112 Gb/s PAM-4 links.
