This tool, based on the formulas and graphs contained in the standard document [1], calculates the maximum allowable current that can flow through a copper printed circuit board external trace (also called as microstrip), keeping the temperature increase of the trace itself below the specified input value. As shown in the picture, the microstrip has width W and thickness T and is separated by an insulating dielectric from a large conductive ground plane.

By providing additional input parameters (ambient temperature and trace length), it is possible to calculate the trace total temperature, resistance, voltage drop and power dissipation (power loss).

Max Current calculation

First, calculate the area according to the following formula:

A = (T · W · 1.378 [mils/oz/ft2])                 (I)

Then, calculate the maximum current:

I_MAX = (k · T_RISE^b) · A^c                                       (II)

Where:

A is the cross-section area [mils²] T is the trace thickness [oz/ft²] W is the trace width [mils] I_MAX is the maximum current [A] T_RISE is the maximum desired temperature rise [°C] k, b and c are constants. According to IPC-2221A Par. 6.2 (“Conductive Material Requirements”), their values for inner layers are as follows: k = 0.048 b = 0.44 c = 0.725

Equation (II) is based on a curve fit to the charts provided in [1] (par. 6.2, Figure B and Figure C).

Trace temperature calculation

The overall trace temperature can be calculated as follows

T_TEMP = T_RISE + T_AMB

Where:

T_TEMP is the trace temperature [°C] T_RISE is the maximum desired temperature rise [°C] T_AMB is the ambient temperature [°C]

Resistance calculation

First, convert the cross-section area from [mils²] to [cm²]:

A’ = A * 2.54 * 2.54 * 10^-6

Then, calculate the resistance:

R = (ρ * L / A’) * (1 + α * (T_TEMP – 25 °C))

Where:

T is the trace thickness [oz/ft²] W is the trace width [mils] R is the resistance [Ω] ρ is the resistivity parameter, whose value for copper is 1.7E-6 [Ω · cm] L is the trace length [cm] α is the resistivity temperature coefficient, whose value for copper is 3.9E-3 [1/°C] T_TEMP is the trace temperature [°C]

Voltage drop calculation

Voltage drop can be calculated as follows:

V_DROP = I * R

Where:

V_DROP is the voltage drop [V] I is the maximum current [A] R is the resistance [Ω]

Power dissipation calculation

Power dissipation, or power loss, can be calculated according to the following formula:

P_LOSS = R * I²

Where:

P_LOSS is the power loss [W] R is the resistance [Ω] I is the maximum current [A] </span

Example 1

Inputs W = 12 mil T = 5 mil T_RISE = 30 °C T_AMB = 25 °C L = 12 inch

Output Cross-section Area = 60.00 mils² I_MAX = 4.17 A

Additional output Trace Temperature = 55 °C Resistance = 0.150 Ω Voltage Drop = 0.626 V Power Dissipation = 2.608 W

Example 2

Inputs W = 10 mil T = 3 oz/ft² T_RISE = 20 °C T_AMB = 18 °C L = 25 cm

Output Cross-section Area = 41.34 mils² I_MAX = 2.66 A

Additional output Trace Temperature = 38 °C Resistance = 0.167 Ω Voltage Drop = 0.444 V Power Dissipation = 1.182 W

Reference

[1] IPC-2221A “Generic Standard on Printed Board Design”

[2] Trace Current Calculator

[3] Omicalculator