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18 The PCB Design Magazine • December 2014 • Using stacked vias will allow for greater route density; however, the cost will be higher. HDI Type IV, V, VI These additional HDI Types are defined in the IPC-2315 specification; however, they are not presented here simply because they are more expensive to fabricate and are probably not necessary for large, dense PCBs with BGA breakout and routing challenges. Via Models HDI Type III accommodates numerous via models and spans. Ultimately the via model that suits your design best will be driven by finding the least expensive method that will still enable adequate route density within the constraints of signal integrity. The graphic in Figure 4 presents some of the via models that may be used in HDI Type III. HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 4: examples of Type III construction.