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December 2014 • The PCB Design Magazine 23 minimizing the bypass capacitors needed for the BGA (assuming you use a relatively thin dielectric, less than .05 mm). • This is also an opportunity to use embedded capacitors and pull-up resistors effectively. Considerable routing space would be opened on all signal layers. 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. Split Planes: Often large BGAs require mul- tiple voltage supplies. You can use split planes or dedicated voltage layers for this power dis- tribution. It would be best to add a couple of voltage supply layers in the center of the board, surrounded by GND planes to avoid having sig- nal layers affected by crossing the splits or dif- ferent voltages. Signal Integrity This is a subject that has many dependen- cies, variables and thousands of articles. No at- tempt here to do anything but point out a few design methods related to large HDI stackups that will positively affect signal integrity. • Remove unused pads on buried vias. This will reduce crosstalk significantly. • Route the high-speed single-ended nets on the buildup layers closest to the component. Stub effects are eliminated because buried vias are not used. • Route diff pairs on the laminated core layers. The via stubs affect the diff pairs less than the single-ended nets and the crosstalk between the diff pair vias (if the unused pads are removed) is likely to be insignificant. • A stripline configuration where pairs of signal layers are sandwiched between plane layers not only provides the best return paths but also reduce crosstalk. This supports the notion that using a GND plane on the outer layers is a good practice. Recommended HDI Type III Stackups What are the best HDI stackups? It depends on your priorities. These stackups were analyzed for relative cost, route density, power integrity and signal integrity. The following three are rat- ed A, B, and C, with the priority given to route density with good power and signal integrity. Stackup A comments: • This is a good average of the variables and a great stackup to start with if this is your first attempt at HDI. HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 8: Typical stackup for gnD and vCC assigned to outer layers. Figure 9: stackup a, the best overall HDI stackup of the group.