MPCB Design Tips: Power Plane Processing

Current carrying capacity

When doing power processing, the first thing to consider is its current carrying capacity.

It contains 2 aspects:

Is the width of the power cord or copper foil sufficient? To consider the power line width, you must first understand the copper thickness of the layer where the power signal is processed. The copper thickness of the outer layer of the PCB (TOP / BOTTOM layer) under normal processes is 1OZ (35um). The copper thickness of the inner layer will be based on actual conditions. 1OZ or 0.5OZ. For 1OZ copper thickness, under normal circumstances, 20mil can carry a current of about 1A; 0.5OZ copper thickness, under normal circumstances, 40mil can carry a current of about 1A.

Whether the size and number of holes meet the power supply current carrying capacity when changing layers. First of all, we must understand the flow capacity of a single via. Under normal circumstances, the temperature rise is 10 degrees.

A single 10mil via can carry a current of 1A, so when designing, if the power supply is 2A, when using a 10mil via to drill and change layers, at least two vias must be punched. Generally, when designing, you will consider punching a few more holes in the power channel to maintain a little margin.

Power path

Secondly, the power path should be considered. Specifically, the following two aspects should be considered:

The power path should be as short as possible. If the power path is too long, the voltage drop of the power supply will be serious. If the voltage drop is too large, the project will fail.

The plane splitting of the power should be kept as regular as possible. Slim bars and dumbbell-shaped splitting are not allowed.

When splitting the power supply, keep the power supply and power plane split distance as close as 20mil. If in the BGA area, you can maintain a 10mil split distance. If the power supply plane is too close to the plane, there may be a risk of short circuit.

If you are processing power on adjacent planes, try to avoid copper or parallel wiring. The main purpose is to reduce the interference between different power sources, especially between some power sources with very different voltages. The problem of overlapping power planes must be avoided. If it is difficult to avoid, you can consider the middle interval ground.

When splitting the power supply, try to avoid cross-segmentation of adjacent signal lines. Signals that cross-segment (the red signal line has a cross-segment phenomenon as shown below) will have impedance abrupt changes due to the discontinuity of the reference plane, which will cause EMI and crosstalk When doing high-speed design, cross-segmentation will greatly affect signal quality.