Tool Description

Trace width is a requirement that designers specify to ensure that the trace can handle the required current capacity. This tool calculates the trace width based upon the following design specifications:

Max Current

Trace Thickness

Max Desired Temperature Rise

This tool also calculates the following additional valuable information related to the trace:

Resistance

Voltage Drop

Trace Power Dissipation

Explanation

This tool, based on the formulas and graphs contained in the standard document [1], calculates the thickness of a copper printed circuit board trace required to conduct a given current, keeping the temperature increase of the trace itself below the specified input value.

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).

It should be noted that external PCB layers achieve better heat transfer than internal layers, due to the heat dissipation through air convection. The other way around, internal dielectric does not conduct heat very well and that explains why internal traces are wider than external traces.

Trace Width calculation

First, calculate the area according to the following formula:

A = (I / (k * T_RISE^b))^1/c                                     (I)

Then, calculate the trace width:

W = A / (T * 1.378 [mils/oz/ft²])                 (II)

Where:

A is the cross-section area [mils²], I is the maximum current [A], T_RISE is the maximum desired temperature rise [°C], W is the trace width [mils], T is the trace thickness [oz/ft²], 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]

Example 1

Inputs

I = 10 A

T = 2 mil

T_RISE = 20 °C

T_AMB = 25 °C

L = 10 inch

Output

Cross-section Area = 256.27 mils²

Trace Width = 128.13 mil

Additional output

Trace Temperature = 45 °C

Resistance = 0.0282 Ω

Voltage Drop = 0.282 V

Power Dissipation = 2.82 W

Example 2

Inputs

I = 8 A

T = 3 oz/ft²

T_RISE = 86 °F

T_AMB = 27 °C

L = 10 inch

Output

Cross-section Area = 147.29 mils²

Trace Width = 35.63 mil

Additional output

Trace Temperature = 57 °C

Resistance = 0.0511 Ω

Voltage Drop = 0.409 V

Power Dissipation = 3.27 

Reference

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