Digital Signal Processing Reference
In-Depth Information
Width (mm)
0
.
05
0.10
0.15
0.20
0.25
0.30
0.36
0.
4
1
180
71
175
69
Stripline
170
67
165
65
Microstrip
160
63
50
Ω
155
61
60
Ω
150
59
2
4
6
8
10
12
14
16
Width (mils)
Figure 7.9
Approximate delay of half-ounce-thick stripline and microstrip on FR4 (
ε
r
=
4.2). The
stripline of all widths and impedances has a delay of 174 ps/inch (68.5 ps/cm), but the microstrip
trace width and impedance factor into the microstrip delay. The solder mask thickness is 1 mil (25.4
μ
m).
the 3-mil-wide 50
microstrip is about 8% faster than any of the shown stripline
traces, and is about 12% faster for a 15-mil-wide trace.
Wide, high-impedance microstrips have smaller delay because they are further
away from the ground plane and more of the field lines travel in air. However, the
effect is small. Changing the impedance by 20% from 50
Ω
Ω
to 60
Ω
reduces the
delay by just over 1%.
The data shown is for a 1-mil (25.4-
m)-thick solder mask, which has a dielec-
tric constant of 4.2. A thicker coating of solder mask causes the trace to be slower
(doubling the solder mask thickness from 1 to 2 mils causes the propagation time
to increase by about 3% on FR4). Increasing the microstrip thickness to 1-ounce
copper increases the delay by about 1%.
μ
7.8.2 How Sensitive Is Delay Time to Changes in the Dielectric Constant?
The delay for a 3-mil (0.08-mm)-wide and 10-mil (0.25-mm)-wide 50
microstrip
and a stripline is shown in Figure 7.10 plotted against different values for the circuit
board dielectric constant (
Ω
ε
r
). The traces are perfectly rectangular half-ounce copper
with a 1-mil (25.4-
ε
r
is 4.2.
The delay difference between stripline and microstrip traces grows as the di-
electric constant gets larger. Large dielectric constants also reduce the difference in
time of flight between wide and narrow microstrip traces.
μ
m) solder mask, whose
7.9 Main Points
The trace and return path form parallel plate capacitors and inductors.
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