Copper Thickness and Resistance

Reduce copper thickness for improved yields

Outer layer copper thickness requirements should be reviewed as trace width and spacing decrease below 5-6 mils. The designer should try to utilize a copper callout to meet the electrical requirements as well as consider manufacturability of the PCB. A general rule is (1 oz. copper being nominal) as circuit density increases, copper weight should decrease. For example, a 4 mil trace and space design should utilize half ounce copper for external layers wherever possible.

Copper weight is specified in ounces per square foot.

Nominal .125oz. = .00015″ (5 microns)
0.25oz. = .00035″ (9 microns)
0.5oz. = .0007″ (12.5 microns)
1.0oz. = .0014″ (35 microns)
2.0oz. = .0028″ (70 microns)

If current carrying capacity permits, the specification of .025 and 0.5 ounce copper should be taken into consideration for improved manufacturability. The finished copper trace will be thicker than the starting copper because it includes the electroplated copper which is deposited in the holes and on the surface. During the etching process, only the starting copper thickness is etched away, not the plated surface copper.

Calculating copper resistance by thickness and length:
R=(0.679×10-6 ohm/inch)
width x thickness inches x length)

In fine line technology, using 0.5oz. copper, a .005″ trace, 5 inches the resistivity will be:
((.679x 10-6)/(5x 0.7 x10-6)) x 5 = 0.97 Ω

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