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 internal trace (also called as stripline), keeping the temperature increase of the trace itself below the specified input value. As shown in the picture, the stripline has width W and thickness T and is totally inside the dielectric material. For this reason, a stripline structure is less likely to radiate radiofrequency (RF) signals and is therefore widely used in microwave cicuits.

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:

Eq. 1

A = (T · W · 1.378 [mils/oz/ft2])

Then, calculate the maximum current:

Eq. 2

I_MAX = (k · T_RISE^b) · A^c

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 = 0.024, b = 0.44, c = 0.725

Eq. 2 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_TEMPis the trace temperature [°C]

Voltage drop calculation

Voltage drop can be calculated as follows:

V_DROP= I * R

Where:

V_DROPis 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]

Example 1

Inputs W = 9 mil  T = 2 oz/ft²  T_RISE= 10 °C   T_AMB= 25 °C  L = 10 inch

Output Cross-section Area =24.804 mils²  I_MAX=0.678 A

Additional output Trace Temperature =35 °C  Resistance =0.280Ω  Voltage Drop =0.190 V  Power Dissipation =0.129 W

Example 2

Inputs W = 25 mil  T = 4 mil  T_RISE= 20 °C  T_AMB= 27 °C  L = 8 inch

Output Cross-section Area =100.00 mils²  I_MAX=2.53 A

Additional output Trace Temperature =47 °C  Resistance =0.0581Ω  Voltage Drop =0.147 V  Power Dissipation =0.372 W

Reference

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