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PCB Company Pty Ltd

www.pcbco.com.au

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Current Carrying Capacity of PCB tracks

The current carrying capacity of a PCB track is fundamentally determined by two parameters:
1. Heat generated by the track. This is a product of the current flowing though the track and the voltage drop across the track (P=IV), essentially the electrical resistance of the track.
2. The ability of the track to dissipate the heat generated in itself as a result of its electrical resistance.

The first parameter (Track resistance) can be easily calculated and compensated for by increasing copper thickness and track thickness.

The second parameter is more complex as a number of elements come into play. These include air moisture, mechanical orientation (which affects convection airflows), cumulative heating affects from adjacent tracks and components, and copper distribution adjacent to the track and on other parallel layers to the track..

For this reason calculating the exact current carrying capacity for a copper track, given a limiting temperature rise for the track, is complex.

The "Track Width Calculator" is based on data from IPC temperature charts. This calculator should only be used as a guide in determining track thickness for a given current load.

The calculator has a number of shortfalls which need to be understood by PCB designers in order to use this tool.

1) Copper thickness: The "track width calculator" uses the input parameter of "Copper thickness" to determine the cross section area of the copper track. PCB manufactures can produce a given thickness by electro-plating onto a base copper foil (for example a 2oz copper track may consist of a 1oz copper foil with 1oz of electro plated copper on top of the foil), or they may use a straight 2oz base foil without plating. The current carrying characteristic of the two are significantly different.
Where the 2oz pure foil track has a relatively flat surface finish, the 2oz "plated" track surface is uneven. The plating process inherently produces depressions and high points on the surface of the track. This not only reduces the current carrying capacity of the track due to the "skin effect" of electrical conductors, but also increase the DC resistance of the track.
The greater the amount of plating, the greater the problem is amplified.
The track width calculator assumes a pure 2oz thick copper conductor and does not compensate for plated conductors.
As a guide for practical track design and leaving a margin for error, plated copper thickness should be de-rated 20%. For example, if you intend to order/manufacture a PCB with 1oz copper foil, plated up an extra 1oz to give you a 2oz copper finish, you should assume a copper thickness of  2oz de-rated by 20% ( 1.6oz).

2) Thermal conductance of copper tracks: As mentioned above, the current carrying capacity of a track is related to the ability of the track to dissipate heat. If a track is connected to a large copper area (plane), or even a via or pad, heat will be conducted away from the track to the particular copper mass. This will increase the track's ability to dissipate heat and hence increase the current that the track can conduct, given the acceptable temperature rise of the track.
For example a track used as a thermal relief for a pad, joining the pad to a copper plane, will be able to conduct its heat into the pad and also the copper plane. As a result, the current that it will be able to conduct, given the limited temperature rise, will be significantly greater than that of a track standing on its own.
The "track width calculator" does not take into account thermal conduction into other copper masses and hence cannot be used where short tracks run into larger copper areas.

3) Cumulative temperature rise: The "track width calculator" uses the input parameter of "Ambient temperature" to determine the copper track resistance for a given temperature rise above this ambient.
Components and other tracks on the board will introduce a cumulative heating affect on the reference trace. The "track width calculator" does not take into account cumulative heating affects, which would exist in a typical PCB assembly.
In order to compensate for this, the "Ambient temperature" input parameter in the calculator should be set at the temperature the track will be exposed to on the board, taking into account the heat dissipation of the components and the other copper tracks affecting the reference track in question.

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Copyright © 2004 The PCB Company Pty Ltd   All rights reserved - Last modified: May 17th 2011

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