Notes from the Board Room - Placement in a Multi-Board System

There are definitely limits on how much circuitry can fit on a single board. The board itself has an upper limit around the size of an extra-large pizza box for the sake of the fabrication equipment and standard panel sizes. The number of transistors on a device continues to balloon while the resistors and capacitors all but disappear to the unaided eye. Even with all of that integration and miniaturization, circuit complexity can grow to an unmanageable scale. One pizza isn't going to get the job done.

Another consideration related to board thickness is via stubs in the Z-axis. If you want to route from the top to layer 4 of a 24 layer board, those 20 other layers form a dangling line of sorts.

Expansion beyond the single PCB takes numerous forms. Telecom and other networking equipment often wind up in rack systems. The familiar 19 inch wide by 1.75 inch high rack unit is the default enclosure size for smaller circuits. This is what you picture when you read the words server farm or data center. From there, the baseline expands to multiple rack-units. Although they come in all sizes, a popular one is a seven rack-unit shelf; still 19 inches wide but it takes just over a foot of rack space. The shelf is populated with PCBs loaded in the vertical orientation, and they are connected using a 19" x 12" backplane that is assembled at the rear of the shelf.

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