PCB Layout Design

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PCB Design, Selecting the Right PCB Trace Widths
PNC Inc
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Every PCB designer has a series of decisions to
make as they translate an abstract schematic into
a functional, reliable, and manufacturable PCB
assembly. Placing the components on the PCB is
usually the first step, connecting those
components with copper conductors to create the
circuit is the next. To connect the components,
the layout designer must interpret the circuit
netlist and turn that netlist into actual copper
traces, subject to constraints of both
manufacturing technology and the laws of physics.
One of the most important considerations for the
designer is the appropriate trace width for each
of those connections. The width of each trace
determines both the real-world performance of the
circuit and the overall size and number of layers
of the PCB.
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To balance circuit performance and PCB size, the
designer needs to balance four considerations

• The manufacturers minimum trace width and
  spacing
• The size and pitch of the component pads that the
  trace will connect
• The amount of current flowing through the trace
• Whether the trace is part of a controlled
  impedance circuit

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The manufacturers minimum trace width and trace
spacing will define the smallest trace width that
can be used for all signal traces that do not
carry significant current or have impedance
constraints. The minimum trace width is typically
used as the default for the layout, since using
the minimum trace width will result in the
smallest possible PCB and the most flexibility in
routing.
Minimum Trace Width and Spacing
For a standard Printed Circuit Board, fabrication
minimum trace widths/spacingis typically 5 mil
(.127mm). PNCs High Density Interconnect (HDI)
PCB trace width/spacing can be as narrow as 3 mil
(.076mm)
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Trace Width vs Pad Width
High Current Traces
Once a designer has placed the components in the
layout, they will often focus next on creating
the power and ground traces to the active
components. This is because the current
carrying traces need to be appropriately sized
and routed. Signal traces, which are typically
at the minimum trace width, can be more easily
routed around the larger power traces.
Another consideration when selecting trace widths
is that the trace should be smaller or equal to
the pad width. For the most part, if working
with the minimum trace widths, this will not be
an issue, however, care must be taken when laying
out the traces and pads for high current
applications.
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Copper PCB traces, like any conductor, have an
internal resistance that is proportional to the
conductor length, and inversely proportional to
its cross-sectional area. Since the copper on a
layer is of a uniform thickness, the width of the
trace determines its cross-sectional area. There
will be both a voltage drop along the trace as
well as heating of the trace due to the power
dissipation. If a PC Board trace is not sized
appropriately to carry the current required by
the circuit, the trace can fail due to
overheating, or the high voltage drop along the
trace can cause intermittent circuit problems as
the current and thus the voltage drop in the
trace varies over time. Designers often create
an internal copperlayer with multiple buses of
various voltages. Since that layer consists only
of power busses, the buses can be quite wide. The
designer will then connect the individual
components to the bus using vias rising to the
components power pins. A bus based design
reduces voltage drop at the far from the power
supply while reducing the width of the short
connector trace to the same size as the component
pin pad.
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In the days before the internet and sophisticated
PCB layout software, designers would use the
pages of current vs trace width tables in IPC
2152 Standard for Determining Current Carrying
Capacity in Printed Board Design Now there are
online calculators based on those tables that
take into consideration all of the factors
involved in determining the appropriate trace
width for a specific current and allowable
temperature rise of the trace due to the power
dissipation. Many full-featured Printed Circuit
Board layout applications have the calculations
embedded in their design rules.
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If a PCB is intended for high power applications
such as motor control or an LED power supply, a
copper layer thicker than the typical 1 oz can be
used but note that it is difficult to etch fine
traces and pads in thicker copper. Make sure to
check with the PCB fabricator about their
capabilities. PNC has experience with thick
copper layers and can provide advice to the
designer about what is possible.
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Controlling Trace Impedance
The last consideration in selecting trace widths
is the impedance of the trace, which becomes a
factor in high-frequency signals such as DDR
busses, video such as HDMI, and high-speed serial
communication like USB and Gigabit Ethernet. At
these high frequencies, not only the trace
resistance, but the capacitance and inductance of
the trace become significant factors. Designing
controlled impedance (CI) circuits is beyond the
scope of this post, because designing a
controlled impedance circuit requires taking into
account the dielectric constant of the PCB, the
length and routing of the trace in addition to
the width of the trace.
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However, trace width is one of the most easily
controlled elements of impedance controlled
circuits, so the trace width on individual
controlled impedance circuits may be different
from the width of other low-frequency signal
traces, and those traces may be finetuned after
the prototype PCBs are tested.
The design of controlled impedance circuits is
described in detail in IPC-2141A Design Guide
for High-Speed Controlled Impedance Circuit
Boards, and many of the formulas are available
in online calculators or as options in PCB layout
applications. When designing high speed circuits,
it also pays to work with a PCB manufacturer like
PNC that has expertise in fabricating PCBs with
precise and consistent dielectric properties.
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Schedule a Design Review with your PCBA
maufacturer
The designers at PNC have experience with both
high power and high frequency RF and microwave
PCB layout designs. Because they work closely
with the manufacturing team, they know what is
possible to achieve with the thick copper layers
used in todays compact LED and motor
controllers, and they know what it takes to
maintain consistent dielectric properties in the
substrates, needed for predictable RF
performance. Let them help you with your design.
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Contact Details
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