The first step in DFM: PCB board wiring reliability design

wire width and spacing are important design parameters that affect both the electrical performance and electromagnetic compatibility of the PCB, as well as the manufacturability and reliability of the PCB. The width of the printed wire is determined by the load current of the wire, the allowable temperature rise and the adhesion of the copper foil. The width and thickness of the wire determine the cross-sectional area of ​​the wire. The larger the cross-sectional area of ​​the wire, the greater the current-carrying capacity. However, the current flowing through the wire will generate heat and cause the temperature of the wire to rise. The magnitude of the temperature rise is affected by the current and heat dissipation conditions. The allowable temperature rise is determined by the characteristics of the circuit, the operating temperature requirements of the components and the environmental requirements of the whole machine, so the temperature rise must be controlled within a certain range. The printed wire is attached to the insulating substrate, and excessive temperature will affect the adhesion of the wire to the substrate. Therefore, when designing the wire width, it should be considered on the basis of the selected copper foil thickness of the substrate, the load current required by the wire, The width of the printed conductor shall be determined under the condition that the allowable temperature rise of the conductor and the adhesion of the copper foil can meet the requirements. For example, for copper foil with a wire width of not less than 0.2mm and a thickness of 35um or more, when the load current is 0.6A, the temperature rise will generally not exceed 10°C. Due to the small load current of SMT printed boards and high-density signal wires, the wire width can reach 0.1mm, but the thinner the wire, the greater the processing difficulty and the smaller the load current capacity. Therefore, when the wiring space allows, a wider wire should be appropriately selected. Generally, the ground wire and the power wire should be designed to be wider, which is not only conducive to reducing the temperature rise of the wire, but also conducive to manufacturing.

The spacing of the printed wires is determined by the insulation resistance, withstand voltage requirements and electromagnetic compatibility, as well as the characteristics of the substrate, and is also limited by the manufacturing process. The insulation resistance between the wires on the surface layer of the board is determined by factors such as the distance between the wires, the length of the parallel sections of adjacent wires, the insulating medium (including the substrate and air), the processing quality of the printed board, temperature, humidity and surface pollution. decided. Generally speaking, the higher the insulation resistance and withstand voltage requirements are, the wider the wire spacing should be. When the load current is large, the small distance between the wires is not conducive to heat dissipation, and the temperature rise of the printed board with a small wire spacing is also higher than that of the board with a large wire spacing. For adjacent wires with large difference, the wire spacing should be appropriately increased when the wiring space allows, which is not only conducive to manufacturing, but also conducive to reducing the mutual interference of high-frequency signal lines. Generally, the width and spacing of the ground lines and power lines are larger than those of the signal lines. Taking into account the requirements of electromagnetic compatibility, the distance between the edges of adjacent conductors of high-speed signal transmission lines should not be less than twice the width of the signal lines, which can greatly reduce the crosstalk of the signal lines and is also conducive to manufacturing.

When designing a printed circuit board, the appropriate trace width and trace should be selected according to the signal quality, current capacity and the processing capability of the PCB manufacturer spacing, and the following process reliability requirements should be considered:

1. According to the current processing capability of most pcb manufacturers, the line width/line spacing is generally required to be no less than 4mil; 2. go Right angle turning points are not allowed at line bends;

< span style="font-size: 16px;">3. In order to avoid crosstalk between the two signal lines, the distance between the two lines should be widened when running parallel lines. Add a ground wire between lines;

4. The wiring on the board should be properly dense, and when the density difference is too large, it should be filled with mesh copper foil; & lt;/p>

6. When leading from an SMT pad whose pin width is thinner than the trace, the trace cannot be traced from the pad Overlay, should be lead from the end of the pad.

7. When the fine-pitch SMT pad leads need to be interconnected, the connection should be made outside the pad, and direct connection in the middle of the pad is not allowed.

8. Avoid crossing wires between pads of fine-pitch components as much as possible. If it is really necessary to cross wires between pads, it should be reliably shielded by solder mask.

9. No wiring is allowed in the area where the metal shell is in direct contact with the PCB. Metal bodies such as radiators and horizontal voltage regulators cannot be in contact with the wiring. Various screws and rivets are installed in holes. Wiring is strictly prohibited within the forbidden area to avoid potential short circuit.