PCB skill analysis, do you know?

Do you know PCB skills? The development and changes of electronic technology will inevitably bring many new problems and challenges to board-level design. Firstly, due to the increasing physical limits of high-density pins and pin sizes, resulting in low routing rates; secondly, due to the increase in system clock frequency, timing and signal integrity problems are caused; thirdly, engineers hope to be able to operate on the PC platform Complete complex, high-performance designs with better tools. From this, it is not difficult to see that there are the following three trends in PCB board design:

1. The design of high-speed digital circuits (ie, high clock frequency and fast edge rate) has become mainstream.

2. Product miniaturization and high performance must face the distribution effect problem caused by mixed-signal design technology (ie, digital, analog and radio frequency mixed design) on the same PCB board.

3. The increasing difficulty of design makes it difficult for traditional design processes and methods, as well as CAD tools on PCs, to meet current technical challenges.

The following describes techniques used in high-speed design.

1. High frequency circuit wiring skills

1) High-frequency circuits tend to be highly integrated and have high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference.

2) The less bending of the leads between the pins of high-frequency circuit devices, the better. The leads of high-frequency circuit wiring are best to use straight lines. If they need to be turned, they can use 45° folded lines or arc turns. This requirement is only used to improve the fixation strength of the copper foil in low-frequency circuits, but in high-frequency circuits, it is necessary to meet this requirement. One requirement can reduce the external emission and mutual coupling of high-frequency signals.

3) The shorter the leads of high-frequency circuit device pins, the better

4) The less alternation between lead layers between pins of high-frequency circuit devices, the better. That is, the fewer vias (Via) used in the component connection process, the better. It has been measured that one via hole can bring about 05pF distributed capacitance. Reducing the number of via holes can significantly increase the speed.

5) When wiring high-frequency circuits, pay attention to the crosstalk caused by close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of ​​ground can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. Parallel traces within the same layer are almost unavoidable, but the trace directions on two adjacent layers must be perpendicular to each other.

6) Measures to surround particularly important signal lines or local units with ground wires

7) Various types of signal line routing cannot form loops, and ground wires cannot form current loops.

8) At least one high-frequency decoupling capacitor should be installed near each integrated circuit block (IC), and the decoupling capacitor should be as close to the Vcc of the device as possible

9) When the analog ground wire (AGND), digital ground wire (DGND), etc. are connected to the public ground wire, a high-frequency choke must be used. In the actual assembly of high-frequency choke links, high-frequency ferrite beads with wires in the center are often used. They can be regarded as inductors in the schematic diagram, and a separate component package and wiring are defined for them in the PCB component library. before manually moving it to a suitable location near the convergence of the public ground lines.

2. Electromagnetic compatibility (EMC) design method in PCB

The choice of PCB base material and the setting of the number of PCB layers, the selection of electronic components and the electromagnetic characteristics of electronic components, component layout, and the length and width of interconnection lines between components all restrict the electromagnetic compatibility of PCB.

Integrated circuit chips (ICs) on PCBs are the main energy source of electromagnetic interference (EMI). Conventional electromagnetic interference (EMI) control technology generally includes: reasonable layout of components, reasonable control of connections, reasonable configuration of power lines, grounding, filter capacitors, shielding and other measures to suppress electromagnetic interference (EMI), which are all very effective. , is widely used in engineering practice.

1. Wiring rules in electromagnetic compatibility (EMC) design of high-frequency digital circuit PCB

1) High-frequency digital signal lines should be short, generally less than 2 inches (5cm), and the shorter the better.

2) The main signal lines are best concentrated in the center of the PCB board

3) The clock generation circuit should be near the center of the PCB board, and the clock fanout should use daisy chain or parallel wiring.

4) The power cord should be kept as far away from high-frequency digital signal lines as possible or separated by ground wires. The distribution of power supply must be low-inductive (multi-channel design). The power layer in the multi-layer PCB board is adjacent to the ground layer, which is equivalent to a capacitor and plays a filtering role. Power and ground wires on the same layer should also be as close as possible. The copper foil around the power layer should be retracted from the ground layer by 20 times the distance between the two plane layers to ensure better EMC performance of the system. The ground plane should not be divided. If the high-speed signal line is divided across the power plane, several low-impedance bridging capacitors should be placed close to the signal line.

5) The wires used at the input and output ends should be avoided to be adjacent and parallel as much as possible. It is best to add a ground wire between lines to avoid feedback coupling.

6) When the thickness of the copper foil is 50um and the width is 1~15mm, passing a current of 2A, the wire temperature is <3

7) PCB board wires should be as wide as possible. For integrated circuits, especially signal lines of digital circuits, wire widths of 4mi1~12mil are usually used. It is best to use wire widths greater than 40mil for power lines and ground wires. The minimum spacing between wires is mainly determined by the worst-case insulation resistance and breakdown voltage between wires. A wire spacing of 4mil or more is usually selected. In order to reduce crosstalk between wires, if necessary, increase the distance between wires and install ground wires as isolation between wires.

8) In all layers of the PCB board, digital signals can only be routed in the digital part of the circuit board, and analog signals can only be routed in the analog part of the circuit board. The ground of low-frequency circuits should be grounded in parallel at a single point as much as possible. If the actual wiring is difficult, some parts can be connected in series and then grounded in parallel. To achieve splitting of analog and digital power supplies, wiring cannot span the gap between split power supplies. Signal lines that must span the gap between split power supplies must be located on a wiring layer adjacent to a large area of ​​ground.

9) There are two main types of electromagnetic compatibility problems caused by power supply and ground in PCB, one is power supply noise and the other is ground wire noise. According to the size of the PCB board current, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of power lines and ground wires consistent with the direction of data transmission, which will help enhance noise immunity. Currently, the noise of the power and ground planes can only be measured on prototype products or experienced engineers can set the decoupling capacitance to a default value based on their experience.

2. Layout rules in electromagnetic compatibility (EMC) design of high-frequency digital circuit PCB

1) The layout of the circuit must reduce the current loop, shorten the connections between high-frequency components as much as possible, components susceptible to interference should not be too close, and input and output components should be as far away as possible.

2) Arrange the positions of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and the signals are kept in the same direction as possible

3) Center the core components of each functional circuit and lay out around it. Components should be arranged evenly, neatly and compactly on the PCB, and the lead connections between components should be shortened as much as possible

4) Partition the PCB into independent and reasonable analog circuit areas and digital circuit areas, and place the A/D converter across the partitions

5) One of the common practices in PCB electromagnetic compatibility design is to configure appropriate decoupling capacitors at various key parts of the PCB board. The above are some tips in the PCB drawing process, I hope they can help you.