Ten Printed Circuit Board  Myths and Facts

1. Myth: Power and signals flow in traces.

Fact: Power and signals flow in the spaces between traces and/or between traces and conducting planes.

2. Myth: Volts and amperes describe electrical behavior on circuit boards.

Fact: E and H fields are a better way to view how signals and energy move on a board. Volts and current are used because we can’t measure fields directly.

3. Myth: Ground bounce can be measured.

Fact: It can be calculated. There are several components to the fields present in any

measurement and this makes interpretation very difficult.

4. Myth: Transmission lines are order independent.

Fact: Order is critical. The characteristic impedance at the point of demand determines

the voltage drop.

5. Myth: Ground/power planes are good sources of "fast energy."

Fact: All connections to the ground/power plane capacitance require a via. These

connections are typically 50 ohms. This defines the initial power level.

6. Myth: A higher dielectric constant shortens the time it takes to obtain energy from a

ground/power plane.

Fact: Only the spacing between planes shortens the access time. This assumes that the via

connection is also a very low impedance.

7. Myth: Resistors terminate transmission lines.

Fact: Resistors have parasitic capacitance and in fact are lossy transmission lines. At one

GHz, a 1/16th inch resistor is just a lossy transmission line.

8. Myth: Surface charges are only important in electrostatics.

Fact: Surface charges exist for all voltage differences. These charges are the first to

move when there is a steep leading edge. 

9. Myth: Return current represents returned energy.

Fact: Return current represents the H field moving forward.

10. Myth: Transmission line energy can be returned to the power supply.

Fact: This energy can only be lost in heat or radiated.

11. Myth: Traces carrying balanced logic should have a controlled differential impedance.

Fact: Once a differential logic signal is on the board, the characteristic impedance of each

logic trace should be controlled. Control of differential impedance is required in a

connecting cable.

12. Myth: Ground planes act as shields.

Fact: They control the space used for carrying field energy. They do not stop fields from

propagating. They do reflect external fields.