Any discussion of plagues and parasites brings to mind thoughts of locusts, frogs, and biting insects.
Thankfully, such events are rare in 2017, but if you
work with high-speed digital and analog circuitry, there’s
one sure way to risk a disaster of Biblical proportions:
forgetting that the printed circuit board (PCB) is much
more than a benign way to hook up your components.
As we all learned in school and (if you’re like me)
promptly forgot, when frequencies increase, a PCB
becomes anything but harmless. In fact, it’s then a
component in its own right: a complex passive network
of parasitic resistors, inductors, and capacitors that can
wreak havoc with marginal designs.
The PCB As A Passive Network
PCB parasitics include inductors formed by package leads
and long traces; pad-to-ground, pad-to-power-plane, and
pad-to-trace capacitors; interactions with vias, and more.
Figure 1: The equivalent circuit per unit length of a PCB trace
contains resistive, inductive, and capacitive elements. (Source:
Models are a good way to simulate the electrical
contributions of PCB parasitics. Figure 1 shows the basic
block of the distributed element model (transmission
line model) of a PCB trace: R, L, and C are exactly what
you’d expect; G is the conductance of the PCB dielectric.
Quantifying PCB Parasitics
How can we get an idea on the magnitude of these PCB
parasitics? Let’s start with trace resistance.
The standard PCB uses 1-oz copper traces, which have
a thickness equal to one ounce of copper evenly spread
over a one square foot area, or 1.37 thousandths of an
inch (mils). The 1-oz copper has a resistivity of 0.5 m;
per square, which equates to 5 m; for a 10-mil trace per
100 mil of length.
Since most low-power designs do not carry more than
a few milliamps of current, the effect on trace resistance
is often negligible. For high-current paths, though, even
a few milliohms can cause voltage drops and excessive
temperatures, calling for wider traces.
PCB capacitance and inductance pose greater problems
because they can couple RF voltages and currents into
nearby traces and reduce the effectiveness of decoupling
capacitors and EMC filters.
The dimensions of a trace or via, plus the relative
permittivity, ;r, of the PCB material, determine its
inductance and capacitance. FR- 4 glass-reinforced epoxy,
the most common PCB material, has an ;r range from 4.0
to 5.0: 4. 5 is often used as a typical value.
Figure 2: The formulas for calculating trace inductance and
capacitance. (Source: Texas Instruments)
The formulas for PCB inductance and capacitance
are complicated: the ones given in Figure 2 and 3 are
approximations. A 0.8 mm (0.031”) trace on a 0.8 mm
(0.031”) thick FR- 4 PCB, for example, has about 4 nH
inductance and 0.8 pF capacitance per cm, or ˜ 10 nH
and 2.0 pF per inch.
Figure 3: The equivalent formulas for vias. (Source: Texas
A different set of approximations applies to PCB vias—
connections that route signals between different PCB
By Paul Pickering, Technical Contributor