Interview question: general layout and wiring principles of high-speed PCB

During the interview, about high speed PCB Layout 、 Questions about wiring principles can be asked about layout Take some consideration of your skills , In this regard, the author summarizes 、 Record the following , For reference only ——

High speed PCB Layout

High speed PCB The layout basically determines the general direction and structure of wiring 、 Separation of power supply and ground plane , And to noise and EMI Control of . The general layout principle is ——

1. Understand the system schematic , Divide the numbers in each circuit 、 simulation 、 Mixed numbers / Analog components , Pay attention to the positioning of the power supply and signal pins of each chip ;

2. According to the proportion of each part in the circuit , Preliminary division of digital circuits 、 Analog circuits are PCB Wiring area on , Let digital components 、 Analog components and their corresponding wiring shall be as far away as possible and limited in their respective wiring areas , After the division, the general order is hybrid devices -->  Simulators -->  Digital devices -->  Bypass capacitance

3. Digital analog hybrid components are placed at the junction of digital signal area and analog signal area , In the direction of placement, note that the digital signal and analog signal pins of the chip face their respective wiring areas . Pure digital or analog components are placed within their respective specified ranges , The crystal oscillator circuit should be as close to its driver as possible ;

4. Devices sensitive to noise should be far away from high-frequency signal wiring , Such as feedback voltage Fb. Time sequence requires limited signal wiring , The layout needs to be adjusted according to the length and structure . The bypass capacitor shall be placed as close to the power pin as possible , Especially high-frequency capacitors . Place a large capacity capacitor near the power interface , To keep the power supply stable 、 Reduce low-frequency noise interference .

High speed PCB wiring

High speed PCB Wiring involves more details and is more flexible , On the premise of reasonable layout , Basic principles of coordination wiring , It can help us avoid unexpected signal integrity problems or timing problems ——

1. Choose the number of layers reasonably . High frequency circuit integration , High wiring density , Reasonable use of multilayer boards can use the middle layer to set shielding , Better realize grounding , Effectively reduce parasitic inductance , Shorten the signal transmission length , Reduce cross interference between signals .

2. Reduce the bending of leads between pins of high-speed circuit components . It is better to use full straight line for high-frequency line , When bending is needed , You can use 45° Broken line or arc line , Avoid signal refraction ;

3. Shorten the length of high-frequency lead ;

4. Reduce the overlap between layers of high-frequency leads . That is to reduce the vias used in the wiring process , A through hole can bring about 0.5pf Distribution capacitance of , Reducing the number of vias can reduce the impact on signal speed ;

5. Pay attention to the cross interference that may be introduced when the signal line is routed in a short distance , If parallel distribution cannot be avoided , It can be on the opposite side of the parallel line 、 A large area of ground wire is arranged between lines . In practice , Parallel wiring in the layer is almost inevitable , However, the wiring directions of two adjacent layers must be perpendicular to each other , That is, parallel horizontal and vertical wiring shall be carried out in the routing direction of the adjacent two layers ;

6. Take ground covering measures for particularly sensitive signal lines or local units . Such as packaging the clock unit ;

7. All kinds of signal wiring cannot form a loop , Nor can a current loop be formed ;

8. Correctly select single point grounding and multi-point grounding . In low frequency circuit , The operating frequency of the signal is usually less than 1MHz, At this time, the influence of wiring and inductance between devices is small , The circulation formed by the grounding circuit has a great influence on the interference , Therefore, single point grounding is adopted ; In high frequency circuits , For example, when the signal is greater than 10MHz when , The ground impedance will become very large , At this time, multi-point grounding should be adopted , Reduce grounding impedance . About the design of ground wire , Also pay attention to thicken the ground wire as much as possible , If the ground wire is too thin, the ground potential will change with the change of current , If possible , Earth wire home village can pass 3 Twice as much as PCB Allowable current of , another , Forming a closed loop of the grounding system can also reduce the potential difference .

reference ：

《Candence Design and Simulation of high speed circuit board 》 Zhou runjing 、 Edited by Wang Hongyan

Recommended reading ：

Original collection of hardware bear (2022/05 to update )

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