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PCB Design, Selectingthe Right PCB Trace Widths_

PCB Design, Selecting the Right PCB Trace Widths?

Every PCB designer has a series of decisions to make PCB Design 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

Minimum Trace Width and Spacing

The manufacturer’s 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.

For a standard Printed Circuit Board, fabrication minimum trace widths/spacing is typically 5 mil (.127mm). PNC’s High Density Interconnect (HDI) PCB trace width/spacing can be as narrow as 3 mils (.076mm)

Trace Width vs Pad Width

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.

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.

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 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 in to 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.

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 asDDR 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 manufacturer

 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.