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26 The PCB Magazine • November 2016 buried via to layer 5 (the 100 ohm differential- pair I/O). This arrangement forms a broad bou- levard between the buried vias four times larger than the device pitch. This boulevard is 4.0 mm and can permit the routing of up to 13 traces—a vast freeway for global routing. Under layer 5, the BGA disappears as there are only the buried vias creating this via-lined-boulevard for mass routing. Solid Paste Vias The purpose of this section is to examine a variety of advanced HDI fabrication processes that have evolved over time. It must be un- derstood that interconnect via hole (IVH) for- mation is just one element of fabricating HDI wiring boards and, although the laser drilling method is the most popular, a number of other methods for defining the IVH have taken root. The other two important factors are the various dielectric materials and the methods of metal- lization. Fabrication of HDI wiring boards with microvia holes involves many new processes not common to conventional board fabrication. Therefore, additional emphasis will be placed INNOVATIVE USE OF VIAS FOR DENSITY IMPROVEMENTS Figure 8: Close-up of Virtex 5 breakout of Figure 7 [7] . Figure 9: The HDI technologies in use today are made up of four important factors: The microvia stack- up architecture, the dielectric materials, the methods of IVH formation, and the method of metalliza- tion of those z-axis via connections. In recent years, 14 different HDI processes have been developed [8] .