The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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54 Chapter 7 The PCB Design Stackup N ow that your parts are placed and your test strategy defined, perhaps you are not seeing enough room left over to route traces and create power paths—don't panic. This is a very common condition caused by today's dense packaging. Your design requirements have transitioned into a need for multi-layer PCB design technology, and with that it is time for some vertical thinking; time to consider some important Z axis strategy for routing and power planes. The term "topology" in the context of a PCB layer refers to geometric properties—copper shapes, clear areas—left over for routing or flooding with more copper. After placement of the components, which may have taken up one or both outer sides of the layout, a designer can see more clearly how much space remains for routing. With some experience and a keen eye to see enough remaining room between part footprints, a designer will be able to begin thinking about whether the connections will be able to be reasonably routed on the two outer layers. Quite often, advanced designers will set up and utilize the software's auto-routing capability to run a feasibility check for routing density. If it appears there is enough space to complete the routing, the PCB will most likely become a two-layer board (IPC class 2) design. However, if the component footprints occupy most of the area defined by the outline, the layout is most likely too dense to complete the routing on the outer layers. At this point, a multi-layer design configuration must be considered to provide additional layers of copper to complete the routing. Click to Enlarge

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