Copper is an excellent conductor with a high melting temperature, but it should be kept at a low temperature. You must adjust your power rails to keep the temperature in a certain range. It would be helpful to consider the current flow in a particular alignment. You can use the PCB alignment width vs. ammeter to calculate the power alignment size you should use when working with high-voltage parts, power rails, and other heat-sensitive areas of the board.
You can also use a calculator that is based on IPC-2152 and IPC-2221. Learning how to read the equivalent alignment width charts in the IPC standard is important since the PCB alignment tables are not very detailed. This article will go over the resources that you’ll need.
Maintains low temperature in high-current designs
The PCB routing and design process is often plagued by the problem of determining how wide the power lines should be to maintain a certain temperature for a device at a specific current or vice versa. The typical operational goal is to keep the temperature of the conductors on the board between 10-20 degrees Celsius. A high-current design aims to size the alignment width and copper weight to keep the temperature rise within a specific limit of the required operating current.
IPC standards have been developed for testing and calculating the temperature rise of PCBs aligned to specific input currents. The IPC-2221 and IPC-2152 standards contain an abundance of information about these topics. These standards are so comprehensive that it takes a lot of time for designers to analyze the data and determine alignment width in comparison to ammeters.
PCB alignment width and ammeter
IPC 2152 is a good starting point for determining alignment and aperture size. These standards provide formulas that can be used to calculate the current limits for a temperature rise. However, they do not take into account controlled impedance cables. When determining the PCB cross-sectional size and alignment width, it is best to start by comparing PCB alignment with an ammeter. You can then choose a maximum current for the alignment. This will allow you to size your alignment to fit controlled impedance routing.
The electrical properties of the substrate change at high temperatures when circuit boards are used at high currents. When the temperature rises, both the electrical and mechanical properties change. If the board is run at high temperatures over a long period of time, it will become discolored and weakened. Designers often adjust the alignment to maintain a temperature rise of less than 10 °C. This is also done to allow for a range of ambient temperatures instead of a single operating temperature.
The PCB Power Supply Alignment Widths and Currents table below shows some alignment widths and corresponding current values that will limit the temperature rise to 10°C at 1 oz./sq. ft. of copper weight. This will give you an idea of how to adjust the alignment of your PCB.
| Currents Table (A) | Alignment Width (Mil) |
|---|---|
| 1 | 10 |
| 2 | 30 |
| 3 | 50 |
| 4 | 80 |
| 5 | 110 |
| 6 | 150 |
| 7 | 180 |
| 8 | 220 |
| 9 | 260 |
| 10 | 300 |
The table above applies to PCBs typically manufactured using a standard procedure that targets a temperature rise of 10 °C. This table applies to laminates that contain standard copper foil (1 oz./sq. ft.).
You may have noticed two things in this table:
Different alignment thicknesses and copper weights
- Calculate the alignment thickness based on how much copper is on the board. The standard value of 1 oz/sq. ft. is above. Those boards that run at high currents require heavier copper to handle the higher temperature increase.
Alternative Substrates
- These data have been compiled using FR4 and cover a large range of PCBs used in production. Advanced applications may need aluminum-core PCBs or ceramic substrates. They might also require advanced high-speed laminations using alternative resin systems. The temperature of an alignment will decrease if you use a thermally conductive substrate. This is because the heat from the hotter alignment will escape more quickly.
- The temperature rise can be approximated in the first order by multiplying the thermal conductivity ratio of the FR4 with the thermal conductivity coefficient of the substrate.
Using the IPC 2152 Column Line Chart
The IPC 2152 column line atlas is a handy tool for using different copper weights in the outer or inner layers. This table is handy for determining conductor sizes based on specific currents or temperature increases. If you select a current for power PCB alignment, you can also choose a current that produces a specific temperature rise. This tool allows you to visually verify the current limits of an alignment design without needing to consult an IPC-2152 Calculator or look it up.
The following two examples will illustrate this. The following figure only defines internal alignments.
The red arrows indicate how to calculate the maximum current for the desired alignment width (140 mils), copper weight (i.e. alignment cross-sectional surface area) and temperature rise. In this example, the width of the conductor (140 mils), is first selected. Then, the red arrows will be traced horizontally up to the desired weight of copper (1 oz/sq. ft.). Then, we trace vertically up to the temperature increase (10°C) and back down to the Y axis to find the current limit (2.75 A).
The orange arrow indicates the opposite direction. Start at our desired current (1A) and trace a horizontal line to the expected temperature rise (30 °C). The alignment is then measured by tracing vertically. Let’s say that we want to specify 0.5 oz/sq. ft. copper weight. We trace back horizontally to the Y axis after tracing to this line to find a width of approximately 40 mil. We’ll use a weight of copper equal to 1 oz/sq. In this case, we would need a power supply alignment of 20 mils.
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