48 The PCB Design Magazine • March 2017
With reference to Figure 2, using D
keff
with
rough copper model, as shown on the left, is
equivalent to using D
keff_rough
, with smooth cop-
per model, as shown on the right. Therefore, all
you need to do is use D
keff_rough
for impedance
calculations, and any other numerical simula-
tions based on surface roughness, instead of D
k
published in data sheets.
It is as simple as that.
For example, one case study I presented used
measurements from a CMP28 modeling plat-
form from Wild River Technology.
[4]
The PCB
was fabricated with FR408HR material and re-
verse treated foil (RTF). Keysight EEsof EDA ADS
[5]
software was used for modeling and simula-
tion. The results are shown in Figure 3.
The left graph in Figure 3 shows results
when data sheet values for core and prepreg
were used. D
keff
measured (red) was 3.761, com-
pared to simulated D
keff
(blue) of 3.626, at 10
GHz. This gave a delta of ~ 4%. But when the
D
keff_rough
was used for core and prepreg the delta
was within 1%.
The paper shows in more detail how Equa-
tion 1 was derived, based on Gauss' Law. In ad-
dition, I show how IL and phase delay is also
improved when D
keff_rough
is used instead of data
sheet values. You can download "A Practical
Method to Model Effective Permittivity and
Phase Delay Due to Conductor Surface Rough-
ness" and other papers on modeling conduc-
tor loss due to roughness from my website. "A
Practical Method to Model Effective Permittiv-
ity and Phase Delay Due to Conductor Surface
Roughness," DesignCon 2017, Lambert Simo-
novich, Lamsim Enterprises.
PCBDESIGN
References
1. "Transmission Line Design Handbook,"
Brian C. Wadell.
2. IPC-TM-650, section 2.5.5.5, Rev C.
3. Wild River Technology.
4. Keysight EEsof EDA ADS.
Bert Simonovich is an electronic
engineering technologist. Over
a 32-year career, working at Bell
Northern Research/Nortel, in
Ottawa, Canada, he helped pioneer
several advanced technology solu-
tions into products. After leaving
Nortel in 2009, he founded Lamsim Enterprises
Inc., where he provides innovative signal
integrity and backplane solutions as a consultant.
You can contact Bert through his web site at
Lamsimenterprises.com.
OBSESSING OVER CONDUCTOR SURFACE ROUGHNESS: WHAT'S THE EFFECT ON DK?
Figure 3: Measured vs simulated D
keff
using FR408HR data sheet values for core and prepreg (left) and
using D
keff_rough
(right). Modeled and simulated with Keysight EEsof EDA ADS software.
