42 The PCB Design Magazine • May 2015
Typical traces usually have a trapezoidal
cross-section after etching due to etch fac-
tor. Since the tool does not handle trapezoidal
cross-sections in the impedance calculation, an
equivalent rectangular trace width was deter-
mined based on a 2:1 etch-factor (60
degree ta-
per). The as designed nominal trace width of 11
mils, and a 1oz trace thickness of 1.25 mils per
isoStack was used in the analysis.
The default foil used on FR408HR core lami-
nates is MLS, Grade 3, controlled elongation RT
foil. The roughness parameters were easily ob-
tained from Oak-Mitsui
[11]
. Reviewing the data
sheet, 1 oz. copper roughness parameters R
z
for
drum and matte sides are 120 μin (3.175 μm)
and 225 μin (5.715 μm) respectively. Because
this is RT foil, the drum side is the treated side
and bonded to the core laminate.
An oxide or micro-etch treatment is usual-
ly applied to the copper surfaces prior to final
lamination. This provides enhanced adhesion
to the prepreg material. CO-BRA BOND®
[12]
or
MultiBond MP
[13]
are two examples of oxide
alternative micro-etch treatments commonly
used in the industry. Typically 50 μin (1.27 μm)
of copper is removed when the treatment is
completed. But depending on the board shop's
process control, this can be 70-100 μin (1.78-
2.54 μm) or higher.
The etch treatment creates a surface full
of micro-voids which follows the underlying
rough profile and allows the resin to squish in
and fill the voids providing a good anchor. Be-
cause some of the copper is removed during the
micro-etch treatment, we need to reduce the
published roughness parameter of the matte
Figure 8: example seM photos of Mls rT foil. left is the treated drum side and center is untreated
matte side. seM photo on the right is the matte side after etch treatment. (source Oak Mitsui)
Table 1: CMP-28 test board parameters obtained
from manufacturers' data sheets and design ob-
jective.
CANNONBALL STACk FOR CONDuCTOR ROuGHNESS MODELING continues
article
