PCB Layout Design
                     PCB Design, 
Selecting the 
Right PCB Trace 
WPNCid Intchs
www.pnconline.com
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.  
To balance circuit performance and 
PCB size, the designer needs to 
balance four considerations:
• The manufacturer’s 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
The manufacturer’s minimum trace width 
Minimum Trace Width and 
and trace spacing will define the smallest 
trace width that can be used for all signal Spacing
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.
For a standard Printed Circuit Board, 
fabrication minimum trace widths/spacingis 
typically 5 mil (.127mm). PNC’s High Density 
Interconnect (HDI) PCB trace width/spacing 
can be as narrow as 3 mil (.076mm)
High Current Traces Trace Width vs Pad 
Width
Once a designer has placed the Another consideration when selecting 
components in the layout, they will often 
trace widths is that the trace should be 
focus next on creating the power and 
ground traces to the active components. smaller or equal to the pad width. 
 This is because the current carrying For the most part, if working with the 
traces need to be appropriately sized and minimum trace widths, this will not be 
routed.  Signal traces, which are typically 
an issue, however, care must be taken 
at the minimum trace width, can be more 
when laying out the traces and pads for 
easily routed around the larger power 
traces. high current applications.
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 component’s 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.
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.
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.
The last consideration in selecting trace Controlling Trace 
widths is the impedance of the trace, 
which becomes a factor in high- Impedance
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.
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.
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 
today’s 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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