Electronic Design - Reliability design of PCB circuit board wiring

Reliability design of PCB circuit board wiring

The width and spacing of printed conductors are important design parameters, which not only affect the electrical performance and electromagnetic compatibility of the PCB, but also affect 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, but the current flowing through the wire will generate heat and cause the temperature of the wire to rise. The size 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. Excessive temperature will affect the adhesion of the wire to the substrate. Therefore, when designing the wire width, you should consider the selected copper foil thickness of the substrate. The width of the printed wire can be determined when the allowable temperature rise of the wire and the adhesion of the copper foil can meet the requirements. For example, for a 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. Because the load current of SMT printed boards and high-density signal wires is very small, the wire width can reach 0.1mm, but the thinner the wire, the more difficult it is to process, and the smaller the load current capacity. Therefore, when the wiring space permits, a wider wire should be selected appropriately. Generally, the ground wire and the power wire should be designed to be wider, which will not only help reduce the temperature rise of the wire, but also facilitate manufacturing.

The spacing of printed wires is determined by the insulation resistance, withstand voltage requirements, electromagnetic compatibility and the characteristics of the substrate, and is also limited by the manufacturing process. The insulation resistance between the wires on the surface of the printed board is determined by the distance between the wires and the parallel sections of adjacent wires. The length, insulating medium (including base material and air), the quality of the processing technology of the printed board, temperature, humidity and surface pollution are determined by factors. Generally speaking, the higher the insulation resistance and withstand voltage requirements, the longer the wire spacing should be. When the load current is large, the small distance between the wires is not conducive to heat dissipation. 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. Adjacent wires with large differences, if the wiring space permits, the wire spacing should be appropriately increased, which is not only beneficial to manufacturing, but also helpful to reduce the mutual interference of high-frequency signal lines. Generally, the wire width and spacing of the ground wire and the power wire are larger than the width and spacing of the signal wire. Taking into account the requirements of electromagnetic compatibility, the edge spacing between adjacent wires of high-speed signal transmission lines should not be less than twice the width of the signal line, which can greatly reduce the crosstalk of the signal line and is also beneficial to manufacturing.

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

1. According to the current processing capabilities of most PCB manufacturers, the line width/line spacing is generally required to be no less than 4mil;

2. No right-angle turning point is allowed at the turn of the routing;

3. In order to avoid crosstalk between the two signal lines, the distance between the two lines should be extended when parallel routing, it is best to adopt a vertical cross method or add a ground wire between the two signal lines;

4. The wiring on the board surface should be properly dense, and when the difference in density is too large, it should be filled with mesh copper foil;

5. The traces drawn from the SMT pad should be drawn vertically as far as possible to avoid diagonal pulling lines;

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

7. When the fine-pitch SMT pad leads need to be interconnected, they should be connected outside the pad, and direct connection between the pads is not allowed.

8. Avoid crossing wires between pads of fine-pitch components as much as possible. If it is necessary to cross wires between pads, solder masks should be used to reliably shield them.

9. No wiring is allowed in the area where the metal shell is in direct contact with the PCB. Metal bodies such as heat sinks and horizontal voltage regulators cannot be in contact with the wiring. All kinds of screws and rivet mounting holes are strictly prohibited within the forbidden area. Wiring to avoid potential short-circuit hazards.