PCB Tech - PCB circuit board reflow and related calculations

For a purely resistive load, the crosstalk current is proportional to dI/dt=dV /(T¬10%-90%*R). In the formula, dI/dt (rate of current change), dV (interference source swing) and R (interference source load) all refer to the parameters of the interference source (if it is a capacitive load, dI/dt is the same as T¬10 The square of %-90% is inversely proportional.). It can be seen from the formula that the low-frequency signal of the PCB is not necessarily smaller than the crosstalk of the high-speed signal. That is to say: 1KHz signal is not necessarily a low-speed signal, we must consider the situation of the edge comprehensively. For a signal with a steep edge, it contains a lot of harmonic components, and has a large amplitude at each frequency multiplication point. Therefore, pay attention when choosing PCB devices. Don't blindly choose chips with fast switching speeds, which will not only increase the cost, but also increase crosstalk and EMC problems.

Any adjacent power plane or other plane, as long as there is a suitable capacitor at both ends of the signal to provide a low-reactive path to GND, then this plane can be used as a return plane for this signal. In normal applications, the corresponding chip IO power supply is often the same for receiving and sending, and there are generally 0.01-0.1uF decoupling capacitors between each power supply and ground, and these capacitors are also at the two ends of the signal, so the The reflow effect of the power plane is second only to the ground plane. However, if other power planes are used for return flow, there is often no low reactance path to the ground at both ends of the signal. In this way, the current induced in the adjacent plane will find the nearest capacitance and return to ground. If the "nearest capacitor" is far away from the start or end, the return will have to travel a long distance to form a complete return path, and this path is also a return path for adjacent signals, and this same return flow The effect of road and common ground interference is the same, which is equivalent to crosstalk between signals.

For some unavoidable cross-supply divisions, a high-pass filter (such as a 10-ohm resistor string 680p capacitor) formed by a capacitor or RC series connection (such as a 10 ohm resistor string 680p capacitor) can be connected across the division. The specific value depends on the type of signal. To provide a high-frequency return path, but also to isolate the low-frequency crosstalk between the mutual planes). This may involve the problem of adding capacitors between the power planes, which seems a bit funny, but it is definitely effective. If some specifications do not allow it, you can lead capacitors to the ground on the two planes of the division.

In the case of borrowing other planes for return flow, it is best to add a few small capacitors to the ground at both ends of the signal to provide a return path. But this approach is often difficult to achieve. Because most of the surface space near the terminal is occupied by the matching resistor and the decoupling capacitor of the chip.

Return noise is one of the main sources of noise on the reference plane. Therefore, it is necessary to study the path and flow range of the return current.

Theoretical knowledge of PCB reflow path.

It is a circuit in the PCB printed board, and there is current through the wire. Usually, we only see the wires used to transmit signals on the surface, from the driving end to the receiving end. In fact, the current can only flow on the loop. The transmission line is what we can see, and the path of current return is usually invisible. They usually flow back by means of the ground plane and the power plane. Since there is no physical circuit, the loop path It becomes difficult to estimate, and it is difficult to control them.

Each wire on the PCB and its loop form a current loop. According to the principle of electromagnetic radiation, when a sudden current flows through the wire loop in the circuit, it will generate an electromagnetic field in space and affect other wires. This is what we are Generally speaking, in order to reduce the influence of radiation, we should first understand the basic principles of radiation and the parameters related to radiation intensity.

Differential mode radiation on PCB printed board

These loops are equivalent to working small antennas, radiating magnetic fields into space. We use the radiation generated by a small loop antenna to simulate it. Set a small loop with current I and an area S. The electric field strength measured in the far field of r in free space is:

E――Electric field (V/m)

f――Frequency ()

S――Area ()

I――Current (A)

r――distance (m)

――Measure the angle between the antenna and the radiation plane ()

It is suitable for small loops placed in free space and no reflection on the surface. In fact, our products are carried out on the ground instead of free space. The radiation is proportional to the loop current and the loop area, and is proportional to the square of the current frequency.

The path of the return current in the printed PCB is closely related to the frequency of the current. According to the basic knowledge of the circuit, DC or low-frequency current always flows in the direction of least impedance; and high-frequency current always flows in the direction of least inductive reactance with a certain resistance.

If the influence of the holes and trenches formed on the copper plane by the vias is not considered, the path with the least impedance, that is, the path of low-frequency current, is composed of arc lines on the ground copper plane, and the current on each arc The density of is related to the resistivity on this arc.

High frequency current path on PCB copper plane

For transmission lines, the return path with the smallest inductance, that is, the high-frequency current return path, is on the copper-clad plane directly below the signal wiring. This return path minimizes the space area enclosed by the entire loop, which makes this The loop antenna formed by the signal has the smallest magnetic field intensity (or ability to receive space radiation) radiated to space.

For relatively long, straight wiring, it can be regarded as an ideal transmission line. The signal return current flowing on it is a band-shaped area with the signal wiring as the central axis. The longer the distance from the central axis of the signal wiring, the smaller the current density.

Is the original signal current, the unit is "A, ampere";

It is the distance between the signal wiring and the copper plane, the unit is "in., inch";

It is the vertical distance from the point on the copper plane to the signal line, the unit is "in., inch";

It is the current density at this point, the unit is "A/in., ampere per inch".

PCB transmission line return current density distribution

Lists the percentage of the return current flowing through the belt-shaped region centered on the center of the transmission line and the width is.

Assuming inches, the current returning through the area 0.035 inches away from the transmission line only accounts for 13% of all the return current, and the specific distribution to one side of the transmission line is only 6.5%, and the density is very small. So it can be ignored.

Summarize:

1. When there is a continuous, dense and complete copper-clad plane under the PCB signal wiring, the noise interference of the signal return current to the copper-clad plane is local. Therefore, as long as the principle of localization of layout and wiring is followed, that is, the distance between digital signal lines, digital devices and analog signal lines, and analog devices is artificially extended to a certain extent, which can greatly reduce the impact of digital signal return current on analog circuits. interference.

2. The high-frequency transient return current flows back to the drive terminal via the plane (ground plane or power plane) adjacent to the signal trace. The terminal load of the driver signal trace is connected across the signal trace and the plane (ground plane or power plane) immediately adjacent to the signal trace.

3. The larger the surrounding area of the power and ground wires on the PCB printed board, the greater their radiation energy. Therefore, by controlling the return path, we can minimize the surrounding area and control the degree of radiation.