Every PCB layout engineer is familiar with resistors. But far too few engineers consider that all the wires and PCB traces with which their systems and circuits are assembled are also resistors (as well as inductors as well, as will be discussed later). In higher precision systems, even these trace resistances and simple wire interconnections can have degrading effects. Copper is not a superconductor—and too many engineers appear to think it is!
1 ounce PCB copper foil is 0.036 mm (0.0014″). Using the relations shown, the resistance of such a standard copper element is therefore 0.48 mÙ/square. One can readily calculate the resistance of a linear trace, by effectively “stacking” a series of such squares end to end, to make up the line’s length from Printed Wiring Board Reverse Engineering. The line length is Z and the width is X, so the line resistance R is simply a product of Z/X and the resistance of a single square, as noted in the figure.
For a given copper weight and trace width, a resistance/length calculation can be made. For example, the 0.25 mm (10 mil) wide traces frequently used equates to a resistance/length of about 19 mÙ/cm (48 mÙ /inch), which is quite large. Moreover, the temperature coefficient of resistance for copper is about 0.4%/°C around room temperature.
Figure 12.2 illustrates a method of calculating the sheet resistance R of a copper square, given the length Z, the width X, and the thickness Y.
