1. IPC-2221 6.1.2 Power Distribution Considerations A predominately important factor that should be considered in the design of a printed board is power distribution. The grounding scheme can be used as a part of the distribution system. It provides not only a DC power return, but also an AC reference plane for high-speed signals to be referenced. The following items should be taken into consideration.

2. IPC-2221 6.1.2 Power Distribution Considerations Maintain a lower radio-frequency (RF) impedance throughout the DC power distribution. An improperly designed ground can result in RF emissions. This results from radiated field gradients developed across the uneven board impedance and its inability of decoupling capacitors to efficiently reduce the boards EMI.

3. IPC-2221 6.1.2 Power Distribution Considerations Decouple the power distribution at the printed board connector using adequate decoupling capacitance. Distribute adequate individual power/ground decoupling capacitors evenly throughout logic device board areas. Minimize the impedance and radiation loop of the coupling capacitor by keeping capacitor leads as short as possible, and locating them adjacent to the critical circuit.

4. IPC-2221 6.1.2 Power Distribution Considerations In a multilayer printed board, planes should be used for power and ground distribution techniques. When utilizing planes for power and ground distribution, it is recommended that the incoming power and ground signals terminate at the input decoupling network, prior to connecting to the respective internal planes. If external power busses are required, commercially available bussing schemes may be employed.

5. IPC-2221 6.1.2 Power Distribution Considerations When using power conductors, as shown in Figure 6-1, power traces should be run as close as possible to ground traces. Both power and ground traces shall be maintained as wide as possible. The power and ground planes virtually become one plane at high frequencies, and should, therefore, be kept next to each other.

6. IPC-2221 6.1.2 Power Distribution Considerations Figure 6-1A shows a poor layout, giving high inductance and few adjacent signal return paths; this leads to crosstalk. Figure 6-1B is a better layout and reduces power distribution, logic-return impedances, conductor crosstalk and board radiation. The best layout is shown in Figure 6-1C, which has further EMI problem reductions.

7. IPC-2221 6.1.2 Power Distribution Considerations In digital power distribution schemes, the grounding and power should be designed first, not last, as is typically done with some analog circuits. All interfacing, including power, should be routed to a single reference edge, or area. Opposing end interconnections are to be avoided. When unavoidable, care should be taken to route the power and ground away from active circuits (see Figure 6-2). At the interconnection reference edge, all ground structures are to be made as heavy as possible.