First relation can be used also for the calculus of the current through a PCB structure of 18μm but the precision is considered to be smaller.

At this moment it must be made a very important remark: all that it was presented above have referred at interconnection tracks situated on external layers (“top layer” or “bottom layer”). In the case in which the tracks are situated on internal layers (“inner layers”) the formulas presented above are not anymore valuable. IPC offers the formula:

 

but many testers sustain, without making too many mistakes, that for the same heating degree of the track (the same supra-temperature) the current can be “taken” as being a half of the current that passes through an external track.

The specialists consider that the separation of the transversal section in its component measures and the usage of different coefficients drive to a better precision of calculus of the current intensity through the interconnection track.