Estimate PCB thermal relief width with a legacy external-trace approximation or a pad-edge allocation, or calculate copper-only resistance. Verify the actual layout before fabrication.
PCB Thermal Relief Width Formula
The following example problem outlines the steps and information needed to estimate a thermal relief spoke width using a simple pad-edge allocation heuristic.
Variables:
- TW is the thermal relief spoke width (trace width)
- X is the approximate pad edge length available for spoke attachments (use the same length units as TW)
- Y is the number of thermal connecting traces (spokes)
- f is a unitless allocation factor (0โ1). The illustrative example uses 0.60, meaning 60% of the chosen pad edge length is allocated to spoke attachment width in total.
To estimate the thermal relief spoke width with this heuristic, multiply the selected pad edge length (X) by the allocation factor (f), then divide by the number of spokes (Y). Note: thermal relief geometry is normally controlled by explicit design rules (spoke width/length and plane clearance), so treat this as a starting point and confirm it meets your PCB fab limits and soldering requirements.
How to Calculate PCB Thermal Relief Width?
Choose Pad-edge allocation in the calculator to use the formula above. This geometry-only method does not verify current capacity.
- First, determine the approximate pad edge length available for spoke attachments (X).
- Next, determine the number of thermal connecting traces/spokes (Y).
- Next, choose an allocation factor f (0โ1) and use the formula TW = (Xยทf)/Y.
- Finally, calculate the thermal relief spoke width (TW).
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge (using the default f = 0.60).
pad edge length available for spoke attachments, X = 4
number of thermal connecting traces/spokes, Y = 3
TW = (Xยท0.60)/Y = (4ยท0.60)/3 = 0.8 (in the same length units as X)
