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December 2014 • The PCB Design Magazine 19 • Skip vias: This via model is effective for transitioning layers (because it emulates the stacked via); however it is not as efficient as stacked vias for route space due to the fact that the minimum diameter is 8th. • Buried vias: As a general rule, all unused pads on the buried vias should be removed. This will significantly reduce the crosstalk. • Microvia pad sizes: Although the pad size will vary by fabricator, using a pad .15mm/6th larger than the hole is adequate. • Via aspect ratio: Hole length to diameter for microvias is 5:1; for buried vias, 10:1. Alternative Via Spans Stacking Microvias and Buried Vias As seen in Figure 5, the microvias may be stacked upon each other, and/or with buried vias. Advantages • Using stacked vias enables the most flexible and efficient via configuration for routing. Disadvantages • Stacking vias generally costs more due to the additional steps required to ensure a good connection between the vias. Extending Buried Vias As seen in Figure 6, you can extend the bur- ied via into the first microvia layer. Advantages If you have power and ground nets that need to extend all the way through the board, using extended buried via uses less space. Disadvantages • Single-ended nets that use the extended blind via may suffer from additional via-stub effects; however, the additional stub length may be insignificant depending on the frequency. • Depending on the fabricator, the cost of extending the buried-via may be slightly more than just having the buried-via in the laminated core. Layer Count The number of buildup and core layers re- quired to route the board and fulfill the per- HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 5: examples of stacked microvias and buried vias. Figure 6: example of a buried via extended into the first microvia layer.