Nine Rules for PCB Wiring (2)

4. Maximum allowable working current of printed wire

Formula: I=KT0.44A0.75

K is the correction factor. Generally, the copper-clad wire is 0.024 for the inner layer and 0.048 for the outer layer.

T is the maximum temperature rise, the unit is ℃.

A is the cross-sectional area of ​​the copper clad wire, the unit is mil (not mm, pay attention).

I is the maximum allowable current, and the unit is A.

5. Principles of Environmental Effects

Pay attention to the application environment, for example, in a vibration or other environment that is easy to deform the board, it is easy to peel and break when using a copper film wire that is too thin.

6. Safe Working Principles

To ensure safe work, for example, to ensure that the minimum distance between the two wires must withstand the applied voltage peaks, the high-voltage wires should be smooth, and there should be no sharp chamfers, otherwise it is easy to cause circuit breakdown.

7. Convenient Assembly And Standard Principles

The wiring design should consider whether the assembly is convenient. For example, when there is a large area of ​​ground and power lines on the printed circuit board (with an area of ​​more than 500 square millimeters), windows should be partially opened to facilitate corrosion.

In addition, the design of assembly specifications should be considered. For example, the solder joints of the components are represented by pads. These pads (including vias) will automatically not be painted with solder mask. However, if filling blocks are used as surface pads or line segments Gold finger plugs, without special treatment, (draw areas without solder mask oil on the solder mask layer), the solder mask oil will cover these pads and gold fingers, which is easy to cause misunderstanding errors. When the pins of the SMD device are connected to a large area of copper, thermal isolation is required. Generally, a track is made to the copper foil to prevent the stress concentration caused by uneven heating and lead to false soldering. If there are vias with Φ12 or square 12mm or more on the PCB, it must be make a hole cover to prevent the solder from flowing out, etc.

8. Economic Principles

Following this principle requires the designer to have sufficient knowledge and understanding of the processing and assembly process. For example, it is more difficult to corrode a 5mil wire than 8mil, so the price is higher, the smaller the via, the more expensive, etc.

9. The Principle of Thermal Effect

The following methods can be considered when designing the printed board, evenly distributing the heat load, installing radiators for the parts, and forced air cooling locally or globally.

From the perspective of conducive to heat dissipation, the printed board is best installed upright, the distance between two boards should not be less than 2cm, and the arrangement of the devices on the printed board should follow certain rules.

The devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed in the cooling air flow. At the top (at the entrance), devices with large heat or heat resistance , such as power transistors, large-scale integrated circuits, etc., are placed at the bottom of the cooling airflow.

In the horizontal direction, high-power devices are arranged as close as possible to the edge of the printed board to shorten the heat transfer path. In the vertical direction, high-power devices are arranged as close as possible to the top of the printed board to reduce the influence of these devices on the temperature of other devices during operation.

The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

The heat dissipation of the printed board in the equipment mainly relies on air flow, so the path of air flow should be studied during the design, and the device or printed circuit board should be reasonably configured. Using reasonable device arrangement can effectively reduce the temperature rise of the printed circuit.

In addition, methods such as derating and isothermal treatment are also frequently used methods in thermal design.

XPCB Limited is a manufacturer specializing in the production of high-precision double-sided, multilayer and impedance, blind buried vias, thick copper circuit boards, products covering HDI, thick copper, backplanes, rigid-flex combined, buried capacitance and buried resistance, Golden Finger and other types of circuit boards, which can meet the needs of customers for various products.