Conductor Trace Sizing Charts

The board thickness is a part of defining a baseline for creating a trace-sizing chart and has a direct impact on conductor heating. The thickness of the board affects the heat transfer path, causing energy to flow away from the trace. As the board thickness increases, the heat transfer path away from the trace increases. When the heat transfer path is increased, the thermal resistance is lower and the temperature rise is lower. 

Several sets of charts for sizing traces are presented in this chapter. These charts are explained and additional charts are presented.Thermal management of printed circuits often requires an accurate estimate of trace heating. This chapter presents information about conductor sizing charts and trace heating information as an aid to printed circuit design. 

Charts have been in existence for many years. They have been adequate for most applications, although they have their limitations. The external and internal conductor-sizing charts in IPC 2221, Generic Standard on Printed Board Design, are the standard for sizing traces and are included as Figs. 16.1 and 16.2, respectively. 

The internal conductor chart is conservative for traces with cross-sectional area up to 700 sq. mil. It becomes less conservative at high current levels or with larger cross-sectional areas. The internal conductor-sizing chart represents half the current from the external conductor chart. It does not represent internal conductor test data. The internal trace chart has significant margin and should be used for sizing both internal and external conductors when possible. When that is not possible, a more detailed approach can be taken.The first step is to use new charts that focus on the baseline set of charts. 

The trace temperature rise derived from a conductor-sizing chart represents the increase in the trace temperature above the local board temperature surrounding the trace. For example, if the board temperature is 85°C as a result of component heating, and the trace is designed for a 10°C rise, then the trace will be 95°C.The temperature rise is the increase in temperature that the trace experiences when current is applied. 

Conductor sizing charts require knowing two of three variables current, trace temperature rise, or trace size. If you know any two of these variables, you can calculate the other. When a more accurate estimate of the trace temperature rise is needed, board thickness, copper thickness, and board material are also used. 

The predicted temperature rise of the same trace using the internal conductor-sizing chart in IPC-2221 is 266°C. 

Baseline charts.These are additional charts for sizing conductors and show an example of the effect of variables that impact trace temperature rise when current is applied. (Additional baseline charts can be obtained from Thermal Man, Inc., that take into account FR-4, BT, copper planes, and board thicknesses of 0.038 to 0.059 in.) Further discussion follows regarding the significance of the copper weight or thickness, board material, board thickness, and copper planes ... 

The internal conductor chart is best for sizing both internal and external traces that must operate in space or vacuum environments. 

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