With the increase of integrated circuit output switching speed and PCB board density, Signal Integrity has become one of the issues that must be concerned in high-speed digital PCB design. The parameters of components and PCB board, the layout of components on PCB board, the wiring of high-speed Signal line and other factors, Can cause problems with signal integrity.

For PCB layouts, signal integrity requires a board layout that does not affect signal timing or voltage, while for circuit wiring, signal integrity requires termination elements, layout strategies, and wiring information. High signal speed on a PCB, incorrect placement of end components, or incorrect wiring of high-speed signals can cause signal integrity problems, which may cause the system to output incorrect data, the circuit to work improperly or not work at all. How to take signal integrity into full consideration and take effective control measures in PCB design has become a hot topic in PCB design industry.

Signal integrity Problem Good signal integrity means that the signal can respond with the correct timing and voltage level values when required. Conversely, when the signal does not respond properly, there is a signal integrity problem. Signal integrity problems can lead to or directly lead to signal distortion, timing errors, incorrect data, address and control lines, and system misoperation, or even system crash. In the process of PCB design practice, people have accumulated a lot of PCB design rules. In PCB design, the signal integrity of PCB can be better achieved by carefully referring to these design rules.

When designing PCB, we should first understand the design information of the whole circuit board, which mainly includes:

1. The number of devices, device size, device package, chip rate, whether PCB is divided into low speed, medium speed and high speed area, which is the interface input and output area;

2. The overall layout requirements, device layout location, whether there is a high power device, chip device heat dissipation special requirements;

3. Type of signal line, speed and transmission direction, impedance control requirements of signal line, bus speed direction and driving situation, key signals and protection measures;

4. Type of power supply, type of ground, noise tolerance requirements for power supply and ground, setting and segmentation of power supply and ground plane;

5. Types and rates of clock lines, source and direction of clock lines, clock delay requirements, longest line requirements.

PCB layered design

After understanding the basic information of the circuit board, it is necessary to weigh the design requirements of the circuit board cost and signal integrity, and choose a reasonable number of wiring layers. At present, the circuit board has gradually developed from single layer, double layer and four layer to more multi-layer circuit board. Multi-layer PCB design can improve the reference surface of signal routing and provide backflow path for signal, which is the main measure to achieve good signal integrity. When designing PCB layering, follow the following rules:

1. The reference plane shall preferably be the ground plane. Both power supply and ground plane can be used as reference plane, and both have certain shielding function. However, the shielding effect of the power supply plane is much lower than that of the ground plane because of its higher characteristic impedance and larger potential difference between the power supply plane and the reference ground level.

2. Digital circuit and analog circuit are layered. Where design costs permit, it is best to arrange digital and analog circuits on separate layers. If must want to arrange in same wiring layer, can use ditch, add earthing line, the method such as dividing line to remedy. Analog and digital power and ground must be separated, never mixed.

3. The key signal routing of adjacent layers does not cross the segmentation area. Signals will form a large signal loop across the region and generate strong radiation. If the signal cable must cross the area when the ground cable is divided, a single point can be connected between the ground to form a connection bridge between the two ground points, and then the cable can be routed through the connection bridge.

4. There should be a relatively complete ground plane below the component surface. The integrity of the ground plane must be maintained as far as possible for the multilayer plate. No signal lines are normally allowed to run in the ground plane.

5, high frequency, high speed, clock and other key signal lines should have adjacent ground plane. In this way, the distance between signal line and ground line is only the distance between PCB layers, so the actual current always flows in the ground line directly below the signal line, forming the smallest signal loop area and reducing radiation.

How to design the signal of integrity PCB

PCB layout design

The key of signal integrity design of printed board is layout and wiring, which is directly related to the performance of PCB. Prior to layout, the PCB size must be determined to meet the function at the lowest possible cost. If the PCB is too large and distributed, the transmission line may be very long, resulting in increased impedance, reduced noise resistance, and increased cost. If the components are placed together, heat dissipation is poor, and coupling crosstalk may occur in adjacent wiring. Therefore, the layout must be based on the functional units of the circuit, while considering electromagnetic compatibility, heat dissipation and interface factors.

When laying out a PCB with mixed digital and analog signals, do not mix digital and analog signals. If analog and digital signals must be mixed, be sure to line vertically to reduce the effect of cross-coupling. The digital circuit, analog circuit, and noise-generating circuit on the circuit board should be separated, and the sensitive circuit should be routed first, and the coupling path between the circuits should be eliminated. In particular, consider the clock, reset and interrupt lines, do not parallel these lines with the high current switch lines, otherwise easily damaged by electromagnetic coupling signals, causing unexpected reset or interrupt. The overall layout should follow the following principles:

1. Functional partition layout, analog circuit and digital circuit on PCB should have different spatial layout.

2. According to the circuit signal process to arrange the functional circuit units, so that the signal flow to maintain the same direction.

3. Take the core components of each functional circuit unit as the center, and other components are arranged around it.

4. Shorten the connection between high-frequency components as much as possible and try to reduce their distribution parameters.

5. Easily disturbed components should not be too close to each other, input and output components should be far away.

How to design the signal of integrity PCB

PCB wiring design

All signal lines should be classified before PCB wiring. First of all, clock line, sensitive signal line, and then high-speed signal line, in order to ensure that this kind of signal through the hole is enough, distribution parameters of good characteristics, and then general unimportant signal line.

Incompatible signal lines should be far away from each other and do not parallel wiring, such as digital and analog, high speed and low speed, high current and small current, high voltage and low voltage. Signal cables on different layers should be routed vertically to each other to reduce crosstalk. The arrangement of signal lines is best arranged according to the flow direction of the signal. The output signal line of a circuit should not be retraced back to the input signal line area. High-speed signal lines should be kept as short as possible to avoid interfering with other signal lines. On the double panel, if necessary, the isolation ground wire can be added on both sides of the high-speed signal line. All high-speed clock lines on the multilayer board should be shielded according to the length of clock lines.

The general principles for wiring are:

1. As far as possible to choose low density wiring design, and signal wiring as far as possible thickness consistent, conducive to impedance matching. For rf circuit, the unreasonable design of signal line direction, width and line spacing may cause cross interference between signal transmission lines.

2. As far as possible to avoid adjacent input and output wires and long-distance parallel wiring. To reduce crosstalk of parallel signal lines, the spacing between signal lines can be increased, or isolation belts can be inserted between signal lines.

3. The line width on PCB shall be uniform and no line width mutation shall occur. PCB wiring bend should not use 90 degrees corner, should use arc or 135 degrees Angle, as far as possible to maintain the continuity of line impedance.

4. Minimize the area of the current loop. The external radiation intensity of current-carrying circuit is proportional to the current passing through, the loop area and the square of signal frequency. Reducing the current loop area can reduce the ELECTROMAGNETIC interference of PCB.

5. As far as possible to reduce the length of the wire, increase the width of the wire, is conducive to reducing the impedance of the wire.

6. For switch control signals, the number of SIGNAL PCB wiring that changes the state at the same time should be reduced as far as possible.