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June 2014 • The PCB Design Magazine 49 microvias, only the space on layer 3 is needed, and because microvias have smaller diameter holes, there is more space to fanout the signals on layer 3. Why use Through-Hole vias for Closest groups to Address, Command, and Control group? Signals from the data banks can be organized into "closest groups" and "outside groups," making it easier to distinguish between routing in the center of the board (on the X-Y plane), and the outer edge of the board. Some signals of the address, command, and control group will need the space below the closest groups. When a signal from the address, command, or control group is routed through a microvia on layer 3, there will be free space left under this group on layer 10. This space can be used to fanout the closest groups. Why do outside groups use Microvias? From Figure 7, it's clear that there won't be space left on layer 10 to fanout the outside groups. So placing the outside groups on layer 3 and using microvias is the result of the fanout planning. Note: The same layer/via/microvia article RoUTINg DDR3 MEMoRY AND CPU FANoUT continues Figure 7: Free space left under groups routed with microvias can be used to fanout the closest groups. fanout planning technique can also be applied to the other interfaces (e.g., PCI, ISA, etc.). This way, even really packed and very dense designs can be routed. Conclusion With a bit of care and planning ahead, rout- ing and length tuning DDR3 fanouts can be a stress-free process, even on the most compact and densely packed designs. The iMX6 Rex is a terrific example of this care and planning, designed in part as a tool for showing how it's done. By following these steps, any DDR3 de- sign can be completed in far less time and with much greater likelihood of designing it right the first time. The complete project reference design including schematics and PCB documents with the DDR3 layout from the screenshots can be downloaded at PCBDESIgN robert Feranec is a contract design engineer and training instructor. he can reached through his website,

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