Current carrying capacity of PCB traces and vias

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Current carrying capacity of PCB traces and vias [Copy link]

 

Introduction: The electrical connection between various devices on the FR4 copper-clad PCBA is achieved through copper foil traces and vias on each layer.

Since different products and modules have different current sizes, in order to realize various functions, designers need to know whether the designed traces and vias can carry the corresponding current, so as to realize the functions of the product and prevent the product from burning out due to overcurrent.

This article introduces the scheme and test results of designing and testing the current carrying capacity of traces and vias on FR4 copper clad boards. The test results can provide designers with a certain reference in future designs, making PCB design more reasonable and more in line with current requirements.

1

introduction

At present, the main material of printed circuit boards (PCBs) is FR4 copper-clad board. Copper foil with a copper purity of not less than 99.8% realizes the electrical connection between various components on the plane, and plated through holes (VIA) realize the electrical connection in space between the copper foils of the same signal.

However, we have always relied on experience to design the width of the copper foil and define the aperture of the VIA.

In order to make the layout design more reasonable and meet the needs, the current carrying capacity of copper foils with different wire diameters was tested, and the test results were used as a reference for the design.

2

Analysis of factors affecting current carrying capacity

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

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

2.1 Copper foil thickness

In the early stage of product development, the copper foil thickness of the PCB is defined based on the product cost and the current state of the product.

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

If the product has some large current and the board size is sufficient, you can choose a copper foil with a thickness of about 35μm for the surface (inner) layer;

If most of the product's signals are high current, then a copper foil with a thickness of about 70μm must be selected for the (inner) layer.

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

Take the example of using 35μm copper foil on both the inner and outer layers of a PCB: after the inner layer circuit is etched, it is laminated, so the thickness of the inner layer copper foil is 35μm.

After the outer layer circuit is etched, drilling is required. Since the holes do not have electrical connection performance after drilling, chemical copper plating is required. This process is full-board copper plating, so the surface copper foil will be plated with a certain thickness of copper, generally about 25μm~35μm, so the actual thickness of the outer copper foil is about 52.5μm~70μm.

The capabilities of copper clad laminate suppliers are different, and the uniformity of copper foil will vary, but the difference is not large, so the impact on the current carrying capacity can be ignored.

2.2 Trace Width

After the copper foil thickness of the product is selected, the trace width becomes the determining factor for the current carrying capacity.

There is a certain deviation between the design value of the trace width and the actual value after etching, and the generally allowed deviation is +10μm/-60μm. Since the traces are etched, there will be residual solution at the corners of the traces, so the corners of the traces are generally the weakest places.

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

2.3 Temperature rise

When a continuous current passes through the PCB trace, the trace will heat up, causing a continuous temperature rise. When the temperature rises to the TG temperature of the substrate or higher than the TG temperature, it may cause deformation of the substrate such as warping and bubbling, thereby affecting the bonding strength between the trace copper foil and the substrate, and the trace warping and deformation may lead to breakage.

When a transient high current passes through the PCB traces, the weakest part of the copper foil traces will not be able to transfer heat to the environment in a short period of time, which is similar to an adiabatic system. The temperature rises sharply and reaches the melting point of copper, burning the copper wire.

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2.4 Plated through hole diameter

Plated through holes achieve electrical connections between different layers by electroplating copper on the hole wall. Since the entire board is copper plated, the copper thickness of the hole wall is the same for plated through holes of different apertures. The current carrying capacity of plated through holes of different apertures depends on the circumference of the copper wall.

3

Testing PCB Design

At present, the substrates with TG temperature >135℃ and >150℃ are used. Considering the requirements of ROHS for lead-free, PCB will gradually switch to lead-free, so the substrate with TG temperature >150℃ must be selected. Therefore, Shengyi S1000 is selected as the substrate for the test board this time.

The test board PCB size is 164mm wide and 273.3mm long. The PCB is made by Shenzhen Mutailai Technology Co., Ltd. The test board PCB is divided into three groups.

3.1 Group 1:

The outer copper foil is 17.5μm, the inner copper foil is 35μm. The first group of test boards PCB uses an outer layer of 17.5μm base copper and an inner layer of 35μm base copper.

The outer wire diameters are: 0.125mm0.16mm, 0.2mm, 0.25mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

The inner wire diameters are: 0.125mm, 0.16mm, 0.2mm, 0.25mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

The plated through hole diameters are: 0.15mm, 0.25mm, 0.3mm, 0.5mm, and 0.7mm.

Two samples were run for each pore size.

3.2 Group 2:

The outer copper foil is 35μm, the inner copper foil is 70μm. The second group of test boards PCB use an outer layer of 35μm base copper and an inner layer of 70μm base copper.

The outer wire diameters are: 0.125mm, 0.16mm, 0.2mm, 0.25mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

Since the existing suppliers' capacity for the copper foil thickness of 70μm is 0.2mm for the minimum inner layer wire diameter, the inner layer wire diameters are: 0.2mm, 0.25mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

The plated through hole diameters are: 0.15mm, 0.25mm, 0.3mm, 0.5mm, and 0.7mm.

Two samples were run for each pore size.

3.3 Group 3:

The outer copper foil is 70μm, the inner copper foil is 105μm. The third group of test boards PCB uses an outer layer of 70μm base copper and an inner layer of 105μm base copper.

Since the existing suppliers' capacity for the copper foil thickness of 70μm is 0.3mm for the minimum outer layer wire diameter, the outer layer wire diameters are: 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

Since the existing suppliers' capacity for the copper foil thickness of 105μm is 0.3mm for the minimum inner layer wire diameter, the inner layer wire diameters are: 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8 mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 2.8mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm.

Two samples were taken for each wire diameter.

The plated through hole diameters are: 0.15mm, 0.25mm, 0.3mm, 0.5mm, and 0.7mm.

Two samples were run for each pore size.

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4

Test plan

According to the 2.5.4 multi-layer circuit board current withstand section of IPC-TM-650 TEST METHODS MANUAL, the test plan is designed as follows.

At room temperature, for the test of inner and outer layer traces: stick the temperature sensor in the middle of the copper foil trace to be tested, apply current to both ends of the copper foil trace to be tested, and keep it for 3 minutes after the temperature rise ΔT stabilizes, and record ΔT. Gradually increase the current until the copper foil trace is destroyed.

At room temperature, for the test of plated through holes: attach the temperature sensor to the VIA, apply current to both ends of the VIA lead-out line to be tested, and keep it for 3 minutes after the temperature rise ΔT stabilizes, and record ΔT. Gradually increase the current until the VIA is destroyed.

The current value range is 0~100A. Sampling values: 0.1A, 0.2A, 0.3A, 0.4A, 0.5A, 0.6A, 0.7A, 0.8A, 0.9A, 1A, 1.2A, 1.5A, 1.8A, 2A, 2.3A, 2.5A, 2.7A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 55A, 60A, 65A, 70A, 75A, 80A, 85A, 90A, 95A, 100A.

5

Test results analysis

Here, only the first set of test data results are analyzed.

5.1 Analysis of wire diameter test results

Taking the outer copper foil with a wire diameter of 2.8 mm as an example, the measurement data is shown in Table 1.

According to the measured data in Table 1, a trend graph can be made, as shown in Figure 1:

Figure 1. Temperature rise and current trend of 2.8mm outer copper foil wire diameter

After taking the average value based on the measured values, we can get 2.8mm. The outer copper foil routing can carry about 8A current when the temperature rise is ΔT=20℃; about 10.8A current when the temperature rise is ΔT=40℃; about 13A current when the temperature rise is ΔT=60℃; about 16A current when the temperature rise is ΔT=100℃; the maximum continuous current is about 20A.

According to the above method, we can obtain the current carrying capacity of different wire diameters of 17.5μm outer copper foil and the current carrying capacity of different wire diameters of 35μm inner copper foil.

5.2 Analysis of Plated Through Hole Test Results

Since the temperature measurement of the plated through hole cannot be achieved on the copper layer of the hole wall, we actually measured the temperature of the plated through hole pad surface, so the following test data is for reference only.

Figure 2 Trend of temperature rise and current of 0.15mm diameter plated through hole 0.15mm diameter plated through hole measurement value

The graphs of the measured values of plated through holes with apertures of 0.25 mm, 0.3 mm, 0.5 mm, and 0.7 mm are omitted here, but after summarizing, Table 2 can be obtained.

Table 2 Current carrying capacity of different apertures on PCB with 17.5μm outer layer/35μm inner layer copper foil

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Summarize

Through this experiment and the analysis of the experimental data, we have a more intuitive understanding of the current carrying capacity of the traces and gaps on the copper-clad PCB.

However, on the one hand, since the test board is not made by a mass production supplier, the different manufacturing processes affect the differences in trace width and the thickness and circumference of the plated through-hole arms; on the other hand, there are certain differences in the heat dissipation status of each sample during the experiment.

In addition, the design of the test board and the experimental plan are ideal. The actual product installation location is different, the distribution of components on the product is different, the density of wiring and the use of different substrates are all things that the test board cannot simulate, so the analysis data cannot directly guide the design.

However, we can refer to the data of this experiment in future development and design. At the same time, we can also correct the experimental data based on the design situation and practical verification in future products to guide the design more accurately.

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