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16 The PCB Design Magazine • December 2014 • There is a single microvia layer on at least one side. • Via holes are drilled in a laminated core and become buried when the dielectric material is added for the microvias. • Microvias are staggered from other microvias and may be stacked or staggered relative to the buried vias. Additional note: See Type I note on limit- ing the number of laminated core layers, which applies to all variations of Type II through and buried vias. Recommendations • In the context of large dense boards with multiple high pin-count BGAs, this stackup is better than Type I; however, it is not adequate for the more difficult designs. Using buried vias instead of the through vias is a significant advantage. Using only a single microvia layer will limit the ability to benefit from the smaller via and trace feature sizes. • The single microvia layer also restricts the viability of using the outer layers for a GND plane. Having only one buildup layer for routing traces isn't nearly as effective as two. HDI Type III This construction uses microvias, buried vias, and may have through vias. • There are at least two microvia layers on at least one side. • Via holes are drilled in a laminated core and become buried when the dielectric material is added for the microvias. • Microvias may be staggered or stacked with buried vias. Additional note: See Type I note on limit- ing the number of laminated core layers which applies to all variations of Type III through and buried vias. Recommendations • HDI Type III is the best stackup configuration for large dense boards with multiple high pincount BGAs. • With two microvia layers there is considerable routing area available using the smaller via and trace feature sizes. • Using the outer layers for a GND plane is feasible because there are still enough microvia layers available for signal routing. HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 2: various kinds of IPC Type II construction. Figure 3: example of IPC Type III construction.