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22 The PCB Design Magazine • December 2014 Route Density Microvias and smaller trace widths on build- up layers can significantly improve route den- sity. Via-in-pad and stacking vias is also very good for improving route density. Using micro- vias and blind vias in certain patterns can open considerable space for routing on the laminate core layers. Ultimately, increasing route density will reduce your layer count and overall fabrica- tion costs. Power Integrity In the context of an HDI Type III stackup, location of the planes will impact your power distribution and integrity along with signal in- tegrity. The appropriate location of planes is a much deeper and more complicated subject than can be addressed in this article; however, certain methodologies are recognized as effec- tive and will be described at a high level here. In the stackups shown in Figures 7 and 8, the number of layers in the laminated core is variable; 16 layers is just convenient for the purpose of depicting plane layer assignments graphically. Outer Layer GND: A stackup such as this is typical when GND is assigned to the outer layers. Advantages • GND on the outer layers provide an excellent EMI shield. • If you can place the bypass capacitors for the BGA on the same layer as the BGA, then you can minimize the number of vias used for GND underneath the BGA. This will open routing channels which may be critical for an extremely dense board. You may still want vias for some of the GND pins to improve the return paths. Disadvantages • If you manage your return paths with an appropriate but not excessive number of GND vias under the BGA, then there really isn't a downside to using this method. • Some may say that using the outer layers for GND will limit the number of buildup (smaller features) layers for routing signals. Although this is true, it is also important to consider that controlling the signal integrity of those nets will be more difficult and burying the first GND plane in the laminate structure will result in the routing on the microvia layers not having a good reference plane. Outer Layer GND & VCC: A stackup such as this one is typical when GND and VCC are as- signed to the outer layers. Note: Using stacked vias would be good for this kind of stackup if it can be cost-justified. Advantages Same advantages as listed for Outer Layer GND, plus these additional benefits: • The capacitive coupling between the GND and VCC layers will be excellent, HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 7: Typical stackup for gnD assigned to outer layers.