What is the relationship between PCB wiring, through hole and current carrying capacity?

The electrical connection between the components on the PCBA is achieved through copper foil wiring and through-holes on each layer.

The electrical connection between the components on the PCBA is achieved through copper foil wiring and through-holes on each layer. Due to the different products, different modules of different current size, in order to achieve each function, designers need to know whether the designed wiring and through hole can carry the corresponding current, in order to achieve the function of the product, prevent the product from burning when overcurrent.

Here introduces the design and test of the current carrying capacity of wiring and passing holes on FR4 copper-coated plate and the test results. The test results can provide certain reference for designers in the future design, making PCB design more reasonable and more in line with the current requirements.

The electrical connection between the components on the PCBA is achieved through copper foil wiring and through-holes on each layer.

At the present stage, the main material of printed circuit board (PCB) is the copper coated plate of FR4. The copper foil with copper purity of no less than 99.8% realizes the electrical connection between each component on the plane, and the through hole (VIA) realizes the electrical connection between the copper foil with the same signal on the space.

But for how to design the width of the copper foil, how to define the aperture of VIA, we always design by experience.

In order to make layout design more reasonable and meet the requirements, the current carrying capacity of copper foil with different wire diameters is tested, and the test results are used as the reference for design.

Analysis of factors affecting current carrying capacity

The current size of PCBA varies with the module function of the product, so we need to consider whether the wiring that acts as a bridge can bear the current passing through. The main factors that determine the current carrying capacity are:

Copper foil thickness, wire width, temperature rise, plating through hole aperture. In the actual design, we also need to consider the product environment, PCB manufacturing technology, plate quality and so on.

1.Copper foil thickness

At the beginning of product development, copper foil thickness of PCB is defined according to product cost and current status on the product.

Generally, for products without high current, you can choose the surface (inner) layer of copper foil about 17.5μm thickness:

If the product has part of the high current, the plate size is enough, you can choose the surface (inner) layer of about 35μm thickness of copper foil;

If most of the signals in the product are high current, the inner layer of copper foil about 70μm thick must be selected.

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

Take the use of 35μm copper foil for both the inner and outer layers of PCB as anexample: the inner circuit is laminated after etching, so the thickness of the inner copper foil is 35μm.

After the etching of the outer circuit, it is necessary to drill holes. Because the holes after drilling do not have electrical connection performance, it is necessary to electroless copper plating, which is the whole plate copper plating process, so the surface copper foil will be coated with a certain thickness of copper, generally between 25μm and 35μm, so the actual thickness of the outer copper foil is about 52.5μm to 70μm.

The uniformity of copper foil varies with the capacity of copper plate suppliers, but the difference is not significant, so the influence on current load can be ignored.

2.Wire line

After the copper foil thickness is selected, the line width becomes the decisive factory of current carrying capacity.

There is a certain deviation between the designed value of the line width and the actual value after etching. Generally, the allowable deviation is +10μm/-60μm. Because the wiring is etched, there will be liquid residue in the wiring corner, so the wiring corner will generally become the weakest place.

In this way, when calculating the current load value of a line with a corner, the current load value measured on a straight line should be multiplied by (W-0.06) /W (W is the line width, the unit is mm).

3.Temperature rise

When the temperature rises to or higher than the TG temperature of the substrate, it may cause deformation of the substrate, such as warping and bubbling, so as to affect the binding force between the copper foil and the substrate. The warping deformation of the substrate may lead to fracture.

After the PCB wiring passes the transient large current, the weakest place of copper foil wiring cannot heat to the environment for a short time, approximating the adiabatic system, the temperature rises sharply, reaches the melting point of copper, and the copper wire is burned.

4. Plating through hole aperture

Electroplating through holes can realize the electrical connection between different layers by electroplating copper on the hole wall. Since it is copper plating for the whole plate, the copper thickness of the hole wall is the same for the plated through holes of each aperture. The current-carrying capacity of plated through holes with different pore sizes depends on the perimeter of copper wall