Calculation of appropriate trace/ track width sizes for current carrying before fusing is an important requirement of circuit board development. Common materials used for PCB tracks layout are copper and aluminum which offer a small resistance when current flows through them. The resistance of a conductor at room temperature depends over three factors namely the conductivity of material, effective length and the cross sectional area of the conductor.
The dc resistance at room temperature with track’s length and cross sectional area is related by using the following formula:
Where R = Resistance of the conductor (Ω)
ρ = conductivity of conductor (Ω/m)
l= length of the conductor (m)
A = effective cross sectional area of the conductor (m2)
Fig. 1:- Track width and height (Thickness of copper sheet) over the base material
Fig. 1 shows common structure of a PCB laminate and width and height relationship of the copper foil. This relationship is for dc resistance calculation of the track without considering the copper on another layer.
The thickness of a track over a laminated sheet ranges from 17µm to 70µm unless specified otherwise. The resistance relations shows that the resistance is directly proportional to the length of the track i.e., the longer the track the higher the resistance and vice versa. The crosses sectional area is related to the width and height of the conductor. The conductor thickness is fixed by the manufacturer and the width is the parameter which is controlled and varied by the PCB designer or manufacturer. The track cross sectional is
The A = cross sectional area in (m2)
W = width of the track laid on the PCB
T = Height of the conductor i.e. copper sheet thickness
Actually these are the DC resistance parameters for a conductor. The voltage drop across a track due to dc resistance is:
This IR drop causes heat dissipation in the entire track and changes the track resistance significantly. A trach width calculator available online, offline in form of excel sheets or built in a design tool determines the width of the conductor at room temperature and elevated temperatures. Track width and current carrying capability chart is also available online and consulted by the designers. The chart shows the same things i.e., the cross sectional area or thickness of the conductor, current and temperature rise as in fig. 2.
For example, the IRDrop analysis tool built into allegro signal integrity tools analyzes the IRDrop analysis, width calculation, temperature analysis and current density in the respective layout design. Track width vs amperage charts are available easily but calculators which accompany the temperature effect have better results particularly in signal integrity analysis.
Fig. 2:- Screenshot of PCB track width calculator with temperature compensation