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How does the PCB carry large current?

How does the PCB carry large current

The design current of the PCB is usually small, so how to make the PCB bear more current? The electronic products used in daily life also vary from tens of milliamperes to a few amperes. If one comes down to tens of amperes or more than 100A, it should not directly cause board burning or safety accidents. What can be done to address such concerns?

The ways PCB can carry large current

The ways PCB can carry large current

wiring on PCB

This method is the most used method in PCB design, because the wider the wiring on the PCB, the greater the current carrying capacity. However, the line width and current carrying capacity are not linear. It is assumed that under the same conditions, 10mil wiring can withstand 1A current, but 50mil wiring cannot withstand 5A current.

Therefore, we need to understand the structure of PCB at this time. Take a simple two-layer board as an example. This kind of circuit board is usually three-layer structure: copper sheet, plate, and copper sheet. The copper sheet is the main path for current and signal in PCB. The common design and operation methods of high current PCB are as follows:
(1) Increase the line width, generally 1A 1mm (40mil);
(2) Thickness of copper coating, 1oz, 2oz++;
(3) Open windows on the wire and add tin, silver, copper, etc;
In practice, there is no strict standard for the length of wiring. In engineering, the current carrying capacity of PCB is usually measured by three indicators: copper thickness, temperature rise and wire diameter. We can also learn how to increase copper thickness and wire diameter through the following two tables to improve PCB heat dissipation and current carrying capacity of PCB:

terminal

In addition to wiring on the PCB, wiring can also be done by means of wiring posts. Its main functions are as follows:
(1) Fixation: it is a screw post with thread, which requires that the metal must have certain hardness and be easy to manufacture. So our common materials with suitable hardness and low cost are: 1 brass, 2 aluminum alloy, 3 steel.
(2) Grounding: For grounding, any metal can be used, because the grounding will not have a large load, and the screws are relatively thick and multiple, and any metal can meet the requirements.

copper bar

If we use it in industry, we need to customize copper bars to carry large current. For example, transformers, server cabinets and other applications use copper bars to carry large current.

Other special processes

In addition to the above three methods, we sometimes apply some special processes, but these special processes can be processed by fewer manufacturers in China, which is generally not easy to find.

For example, Infineon has a kind of PCB, which is designed with three layers of copper. The top and bottom layers are signal wiring layers, and the middle layer is 1.5mm thick copper layer, which is specially used for arranging power supply. This kind of PCB can easily achieve small volume overcurrent of more than 100A.

Matters needing attention:
(1) Excessive current causes serious PCB heating, or local heating, because the current does not flow evenly through the conductor. The TG of the commonly used FR4 is generally 140~170 degrees, which may not burn if it exceeds the temperature, but will directly lead to the softening of the plate and serious degradation of the electrical properties.
(2) The reliability of the plate will be reduced.

Factors affecting PCB current carrying capacity

Factors affecting PCB current carrying capacity
Closup detailed shot of a printed circuit board

Due to the different module functions of the product PCBA, its current size is also different, so we need to consider whether the wiring that acts as a bridge can carry the passing current. The main factors that determine the current carrying capacity are:

Copper foil thickness, wiring width, temperature rise, plating through-hole diameter. In the actual design, it is also necessary to consider the product use environment, PCB manufacturing process, plate quality, etc.

Copper foil thickness

Generally, for products without large current, the table (inner) layer can be selected about 17.5 μ M thick copper foil:

If the product has some large current and the board size is enough, you can select about 35 meters (inner) layer μ M thick copper foil;

If most of the signals of the product are high current, about 70 (inner) layers must be selected μ M thick copper foil.

For PCB boards with more than two layers, if the surface layer and the inner layer of copper foil use the same thickness and the same wire diameter, the current carrying capacity of the surface layer is greater than the inner layer.

wiring width

There is a certain deviation between the design value of wiring width and the actual value after etching, and the general allowable deviation is+10 μ m/-60 μ m。 Because the wiring is formed by etching, there will be liquid medicine residue at the wiring corner, so the wiring corner will generally become the weakest place.
In this way, when calculating the current-carrying value of the line with corners, the current-carrying value measured on the straight line should be multiplied by (W-0.06)/W (W is the line width, unit: mm).

temperature rise

The PCB wiring will heat up after passing the continuous current, thus causing continuous temperature rise. When the temperature rises to or higher than the TG temperature of the substrate, it may cause warping, blistering and other deformation of the substrate, thus affecting the binding force of the copper foil and the substrate, and the warping deformation of the wiring leads to fracture.
After the PCB wiring passes through the transient large current, the weakest part of the copper foil wiring will not be able to conduct heat transfer to the environment in a short time, which is similar to the thermal insulation system, and the temperature will rise sharply, reaching the melting point temperature of copper and burning the copper wire.

plating through-hole diameter

The plated through hole realizes the electrical connection between different layers by electroplating copper on the wall of the through hole. Because the whole plate is plated with copper, the thickness of the hole wall copper is the same for the plated through hole of each aperture. The current carrying capacity of plated through holes with different apertures depends on the circumference of the copper wall.

PCB CURRENT

Conclusion

Generally, PCB design current will not exceed 10A, or even 5A. Especially in household and consumer electronics, the continuous working current on the PCB usually does not exceed 2 A. However, some products need a continuous current of about 80 A. Considering the instantaneous current and the margin left for the whole system, the continuous current of power wiring should be able to withstand more than 100 A.

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