Copper thickness and resistance on the outer surface layer is very critical and in most cases over looked by many customers. If possible and the design will allow this, the designer should try to utilize he copper call out to not only meet his electrical requirements, but offer the manufacturer every opportunity to build a reliable printed circuit board. A good rule to work with would be (1 oz. copper being nominal) as you increase the density of the trace and space on the outer layers the copper thickness should decline. A 4mil trace and space design should be designed with .05oz. copper for external layers.
The copper clad is usually specified by its ounce weight per foot.
Nominal
125oz.
=
.00015"
(5 microns)
0.25oz.
=
.00035"
(9 microns)
0.5oz.
=
.0007"
(12.5 microns)
1.0oz.
=
.0014"
(35 microns)
2.0oz.
=
.028"
(70 microns)
If current carrying capacity permits, the specification of .25 and 0.5 ounce copper needs to be considered in all advanced cases. This will allow extreme enhanced fine line etching capabilities.
Resistively of copper is the measurement
This measurement verifies line width and copper thickness of actual etched line width (i.e. process control)
R=(0.679x10-6 ohm/inch)
(width x thickness inches x length)
Example:
In fine line technology, using o.5oz. copper, a .005" trace, 5 inches the resistively will be:
((.679x 10-6)/(5x 0.7 x10-6)) x 5 = 0.97 Ω
Log onto the Streamline Circuit's Extranet to perform a copper resistance calculation
