PCB Impedance Control:
Formulas and Resources
Douglas Brooks, President
UltraCAD Design, Inc.
Many of the formulas we use for PCB impedance with one plane infinitely far away. Yet the formulas, while
calculations are readily available, but not all of them. In close, do not give identical results in this cases. This illus-
fact, while we are often quick to say that PCBs begin to trates the problem with all these formulas. They are approxi-
look like transmission lines when rise times are fast enough, mations and are based on simplifying assumptions. But they
and perhaps toss out a few formulas for our readers, we are only guides; none will give exact results. If exact results
miss at least three factors when we do so. are really important, get your board fabricator on board early
Not complete: It's not enough to say that Zo equals in the design process and rely on their expertise.
some complex formula. We may also want to know other Almost all references point out that for a transmission
parameters, such as propagation time, intrinsic capacitance, line, Zo is the square root of Lo/Co. (Figure 2a) Therefore,
intrinsic inductance, and the effects of device loading and the intrinsic inductance, Lo, can be calculated as shown in
terminations. I know of no single source where all of these Figure 2b. At least one source provides the formula shown in
formulas are made available. Figure 2c for calculating the inductance of a flat trace over a
Too complex: Even if the formulas are available, they plane.(footnote 2) The results of these two formulas for
can be so complex that even a person with a considerable microstrip can differ by as much as 10%, which illustrates
background in mathematics can't use them without an intol- again that all these formulas are approximations based on
erable probability of arithmetic error. simplifying assumptions, and different approaches can
Not right form: It is fine, for example, to say that Zo is sometimes lead to different results.
some complex function of width, thickness, and height
above the plane. But what if what you really want to know
Lo
is what width to use to hit a given Zo? Or what height is
Zo =
(a) Ohms
required? Most of our readers are not prepared to tackle Co
2
this type of formula manipulation.
Lo = Co * Zo
(b) pH/in
This article will do two things: (1) bring together all
F 2 * Pi * H I
the relevant formulas into one place, along with source L = 5 * Ln
G J
(c) nH/in
H W K
references, and (2) show you where there are several, free
resources available for dealing with them.
Figure 2
Formulas for trace impedance
The formulas
Figure 1 provides the basic formulas for Zo (in Ohms),
Co (in pf/in) and Tpd (ns/in) for the most common PCB
Devices placed along a trace add capacitance. This
trace configurations. While many sources have some of
capacitance "loading" has an effect on both Zo and on
these formulas, I only know of one source (footnote 1) that
Tpd. The correction factor is the square root of (1 +
contains all of them. One note, however, about embedded
Cd/Co*l) where Cd is the sum of the capacitive loads
microstrip. The adjustment for the relative dielectric coeffi-
and l is the length of the trace (Figure 3a). The correc-
cient is reasonable if the material above the trace is more
tion factors for Zo and Tpd are shown in Figure 3b and
than 4 mils thick and is of the same dielectric coefficient as
3c.
the material below the trace. If the thickness is less than
These adjustments are correct for parallel termina-
that, or if the dielectric coefficient is lower than the material
tions and they are found in most references. Motorola
below the trace, the actual result will be between those
has a complicated derivation that shows an additional
calculated for microstrip and for embedded microstrip.
effect from series termination. (Footnote 3). The theory
Note also that the embedded microstrip configuration,
is complex, but fundamentally involves the fact that the
for infinite embedding, is the same as asymmetric stripline
This article appeared in Printed Circuit Design Magazine, March, 1998
© 1998 Miller Freeman, Inc © 1998 UltraCAD Design, Inc.
Cd
1 + (a) Correction Factor illustrate various relationships, and solve for any variable
(Co * l )
(perhaps, though, through an interative process).
(b) Impact on Zo
Zo
Barry Olney has an interactive web site where you can
Zo' =
Cd
enter various parameters, and it will display the impedance of
1 +
Co * l
the various trace layers. The web site can be found at
www.icd.com.au/board/board.html.
Cd
(c) Impact on Tpd
t ' = t 1 +
pd pd
Polar Instruments, Ltd. has a freeware Windows calcula-
(Co * l )
tor that allows you to calculate several trace parameters given
îÅ‚ Å‚Å‚
Cd (d) Correction Factor for
ëÅ‚1 öÅ‚
the others. You can obtain this calculator at www.polar.co.uk
2 * + - 1śł + 1
ïÅ‚ ìÅ‚ ÷Å‚
Series Termination
Co * l
íÅ‚ Å‚Å‚
ïÅ‚ śł
and follow the link to the calculator. It is not readily apparent
ðÅ‚ ûÅ‚
that you need to enter the data in mils or mm or significant
round off error may result.
Figure 3
UltraCAD Design, Inc. has a freeware Windows calcula-
Impact of Capacitive Loading
tor that has all the formulas contained in this article, and that
can solve for virtually any parameter given the others. It also
capacitive loads are charged through the series load,
has a help file that discusses the formulas and their sources.
somewhat slowing down the rise time, and therefore the
You can obtain it at www.ultracad.com and follow the links to
propagation time, of the trace. The adjusted correction
"calculators."
factor for series termination is shown in Figure 3d. Note
Note: The article, as published, contained illustrations of
that in all these cases the adjustment factor collapses to 1.0
these last three resources. The illustrations have been omitted
if Cd = 0.
here because the pictures added almost 900k to the .pdf file!
Finally, the voltage reflection coefficient, Á, of a
transmission line is given by
Summary
Á = (RL - Zo)/(RL + Zo)
Articles and seminars have shown you the significance,
One characteristic of a transmission line is that if it is
importance, and impact of transmission lines on PCBs. The
terminated in a resistive load equal to its characteristic
formulas and their sources are summarized here, but can
impedance (Zo), a signal traveling down the trace will be
admittedly be difficult to use. There are, however, several
completely absorbed by the load and not reflect back. If
freeware resources available to you on the web and elsewhere
the trace is left open at the far end (RL is infinite), the
that can simplify this task. The only thing left is for you to
signal reflects back with the same polarity and magnitude.
take advantage of these resources.
But if the trace is shorted at the far end (RL = 0 and
therefore Á = -1), the signal reflects back with the same
magnitude but the opposite polarity.
Footnotes
1. IPC-D-317, April, 1990, Design Guidelines for Electronic
The Resources
Packaging Utilizing High-Speed Techniques , pp. 17-25.
It is really not too difficult to build your own spread-
2. Ott, Henry, Noise Reduction Techniques in Electronics,
sheet to make these calculations. An example is shown in
Wiley Interscience, 1988, p. 281
Figure 4a and the formulas for row 14 are shown in
3. MECL System Design Handbook, Rev. 1, Motorola
Figure 4b.(footnote 4) These formulas can simply be
Semiconductor Products, Inc. 1988, p. 157
copied to additional rows to show the effects of a wide
4. These formulas are correct for Quattro Pro and for Lotus
range of relative dielectric coefficients. Building a spread-
1-2-3. They might need some very minor adjustments for
sheet has the added benefit that you can build graphs to
Excel spreadsheets.
B14 2
C14 87*@LN(5.98*$D$5/(0.8*$D$7+$D$6))/(B14+1.41)^0.5
D14 0.67*(B14+1.41)/@LN(5.98*$D$5/(0.8*$D$7+$D$6))
E14 1.017*(0.475*B14+0.67)^0.5
F14 12/E14
G14 (C14^2)*D14/1000
I14 60*@LN((4*(2*$D$5+$D$6))/(0.67*3.14159*(0.8*$D$7+$D$6)))/((B14)^0.5)
J14 1.41*B14/@LN(3.81*$D$5/(0.8*$D$7+$D$6))
K14 1.017*(B14)^0.5
L14 12/K14
M14 (I14^2)*J14/1000
O14 80*(1-$D$5/(4*($D$5+$D$8+$D$6))))*@LN(1.9*(2*$D$5+$D$6)/(0.8*$D$7+$D$6))/(B14)^0.5
P14 2.82*B14/@LN(2*($D$5-$D$6)/(0.268*$D$7+0.335*$D$6))
Figure 4b
Formulas used in spreadsheet example
1 B C D E F G H I J K L M N O P
2 This program is set up to calculate various impedances and capacitances associated
3 with strip lines, microstrips, etc. Enter data in inches.
4
5 let: h = 0.009 Zo is in ohms
6 t = 0.0008 Co is capacitance (pf)/inch
7 w = 0.01 Tp is propagation delay in ns/ft
8 c = 0.0076
9
10 Microstrip Stripline Dual Stripline
11 dialectric Zo Co Tp ns/ft Tp in/ns Inductance Zo Co Tp ns/ft Tp in/ns Inductance Zo Co
12 coeff (Ohms) (pf/in) (nH/in) (nH/in)
13
14 2.0 85.317 1.262 1.294 9.270 9.183 59.446 2.073 1.438 8.343 7.327 69.002 3.286
15 2.1 84.093 1.299 1.313 9.137 9.183 58.013 2.177 1.474 8.142 7.327 67.339 3.451
16 2.2 82.920 1.336 1.332 9.010 9.183 56.679 2.281 1.508 7.955 7.327 65.791 3.615
17 2.3 81.795 1.373 1.350 8.888 9.183 55.433 2.384 1.542 7.780 7.327 64.345 3.779
Figure 4a Spreadsheet can be used for impedance calculations
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