Power Planes Ch 6

6.9 Symmetry and Board Warping
In designing stackup, we should keep the it symmetrical with respect to the copper and non copper planes. For example of second layer from the top is a power or ground layer, then the second layer from the bottom should also be a ground or a power layer. This requirement has to do more with the mechanical requirement than the electrical requirement. An unsymmetrical stack up will create warping during any process that applies heat the PCB. A warped PCB will decrease yield especially if it has BGAs. The heights of the solder balls will be different in a warped board leading to open conditions and reducing yield. Keep your stack up symmetrical. If a power plane is partially filled, fill up the remaining are with copper, even if they are not connected to a net.


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