In PCB design, line width and line spacing are two very important parameters, which directly affect the performance and reliability of the circuit.
1. Basic concepts of line width and line spacing
1.1. Line width: Line width refers to the actual width of the wire on the PCB board, usually in mil (one thousandth of an inch). The size of the line width will affect the resistance, current carrying capacity and signal transmission performance of the conductor.
1.2. Line distance: Line distance refers to the distance between adjacent wires on the PCB board, also in mil. The size of the line spacing will affect the crosstalk, electromagnetic compatibility and heat dissipation performance of the circuit.
2. line width and line spacing setting principle
2.1. Current carrying capacity: The line width and line spacing should be set to meet the current carrying capacity requirements of the circuit. According to IPC-2221 standards, the relationship between line width and line spacing and current carrying capacity is as follows:
▪ Line width: Each 1mil line width can carry about 1A current.
▪ Line spacing: Each 1mil line spacing can reduce the current carrying capacity by about 1A.
2.2. Signal transmission performance: The setting of line width and line spacing should meet the performance requirements of signal transmission. The larger the line width, the smaller the loss of signal transmission; The larger the line spacing, the smaller the crosstalk between the signals.
2.3. Electromagnetic compatibility: The line width and line spacing must meet the requirements of electromagnetic compatibility. The larger the line spacing, the smaller the electromagnetic interference. The smaller the line width, the smaller the electromagnetic radiation.
2.4. Heat dissipation performance: The cable width and cable spacing should meet the requirements of heat dissipation performance. The larger the line width, the better the heat dissipation performance; The greater the line spacing, the worse the heat dissipation performance.
2.5. Cost and process: The setting of line width and line spacing also needs to consider the limitations of cost and process. Line width and line spacing are too large, which will increase the manufacturing cost of PCB; The line width and line spacing are too small, which will increase the difficulty of PCB manufacturing.
3. Calculation method of line width and line spacing
3.1. Calculate line width according to current carrying capacity:
▪ Determine the maximum current value in the circuit.
▪ Calculate the required line width according to IPC-2221 standard: Line width (mil) = maximum current (A).
3.2. Calculate line width according to signal transmission performance:
▪ Determine the signal frequency and transmission rate.
▪ Calculate the required line width based on the characteristic impedance of signal transmission and transmission line theory.
3.3. Calculate line spacing according to electromagnetic compatibility:
▪ Determine the source of electromagnetic interference and sensitive devices in the circuit.
▪ Calculate the required line spacing according to the design principles of electromagnetic compatibility.
3.4. Calculate the cable width and cable spacing according to the heat dissipation performance:
▪ Determine the heat dissipation requirements and environmental conditions of the circuit.
▪ Calculate the required line width and line spacing according to the basic principle of heat dissipation.
4. line width and line spacing setting precautions
4.1. The setting of line width and line spacing requires comprehensive consideration of multiple factors, such as current carrying capacity, signal transmission performance, electromagnetic compatibility and heat dissipation performance.
4.2 In the design process, it is necessary to flexibly adjust the value of line width and line spacing according to the specific requirements and conditions of the circuit.
4.3. When designing high-speed signal transmission circuits, the setting of line width and line spacing needs to be more stringent to reduce signal loss and crosstalk.
4.4. When designing high-power circuits, the setting of line width and line spacing needs to be more relaxed to meet the requirements of current carrying capacity and heat dissipation performance.
4.5. When designing circuits with high electromagnetic compatibility requirements, the setting of line spacing needs to be more stringent to reduce electromagnetic interference.
4.6 When designing a circuit with limited cost and process, it is necessary to minimize the value of line width and line spacing under the premise of meeting the performance requirements to reduce the cost and improve the feasibility of the manufacturing process.
5. Setting examples of line width and line spacing
5.1. Assume that the maximum current in a circuit is 5A, the signal frequency is 100MHz, the electromagnetic compatibility requirements are high, the heat dissipation performance requirements are general, and the cost and process requirements are moderate. According to the above calculation method, the following setting values of line width and line spacing can be obtained:
▪ Line width: 5A / 1A/mil = 5mil
▪ Line pitch: Considering the signal frequency and electromagnetic compatibility requirements, it can be set to 10mil.
5.2 Suppose that the maximum current in a circuit is 10A, the signal transmission rate is 1Gbps, the electromagnetic compatibility requirements are general, the heat dissipation performance requirements are high, and the cost and process requirements are low. According to the above calculation method, the following setting values of line width and line spacing can be obtained:
▪ Line width: 10A / 1A/mil = 10mil
▪ Line pitch: Considering the signal transmission rate and heat dissipation performance requirements, it can be set to 6mil.
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