At the very minimum, a good board stackup has solid reference planes for signal routing.\u00a0 Power demands now pretty much require that you have at least one power\/ground sandwich in your stackup as well.\u00a0 Ideally, you would have all the power and ground planes on the top layers in a big PWR-GND-PWR-GND alternating “Big Mac” where they were nice and close to the ICs and all the caps were mounted on the top.\u00a0 But that would also require symmetry to the bottom of the board and that’s a lot of layers just for power.\u00a0 But you should really have at least one power\/ground sandwich somewhere in the board, especially if you are running some very low voltages with high current demands.\u00a0 The reason is that embedded capacitance in the board really helps reduce the higher-frequency pieces of the power distribution network (PDN) impedance.\u00a0 More-so, it provides a low-impedance path for energy to propagate between the decoupling caps and the IC power pins.<\/p>\n
You might\u00a0think that as long as\u00a0your caps\u00a0are connected to solid planes you are fine, but in reality you need to have a plane pair in order for the caps to have a nice low-inductance connection.\u00a0 If they are not connected to a plane pair, they are rendered almost useless.\u00a0 Kind of like routing a trace without a reference plane.<\/p>\n
To read more about stackup design, check out my article in PCD&F:
\nhttp:\/\/pcdandf.com\/cms\/magazine\/171-current-issue\/7993-designers-notebook<\/span><\/span><\/a><\/p>\n