PCB Trace: A Comprehensive Guide

Introduction 

A PCB trace is a thin line of conducting material, usually copper, placed on a non-conductive or isolated substrate that carries the signal and power to the whole circuit.

The role of a printed circuit board is to move signals and data from one point or junction to another. To implement this function, the signals need a path through which they can travel. That path or road is what is called a 'trace'. Traces connect all the different components of a circuit board.

The printed circuit board is one of the most used components in electronic devices today. Due to its integral role in electronics production, therefore, it is vital to pay close attention to trace design.

What is PCB Trace?

A PCB trace is a combined network of wiring, copper, fuses, and insulation that make up a part of a printed circuit board. It is a highly conductive track that is used to connect the various components on a printed circuit board including the IC, capacitors, and resistors. 

A PCB trace is used to conduct electricity, so it needs to be made of a highly conductive and stable material. The most popular material used to make traces is copper, but there are other viable options such as gold and aluminum.

Traces are integral to circuit boards. Electronic devices most often malfunction due to inconsistencies or errors in PCB traces. This is why meticulous detail must be applied in designing a PCB trace, as a fundamental part of PCB fabrication.

Consequently, several vital parameters must be carefully considered when designing PCB traces. These include trace width, trace thickness, trace resistance, and trace current, among others. [1]

PCB Trace Width

Trace width is one of the primary vital parameters in PCB design and layout. It is crucial for carrying currents through circuit boards while keeping trace temperatures below a specific limit, to prevent overheating.

Traces conduct signals to and from the pins of the soldered components on the circuit board and carry different quantities of current depending on the net. The signal’s traces may have special electrical needs which influence their size, therefore, trace widths must be carefully regulated for accurate and efficient board manufacturing.

Traces are metal connections usually fabricated from copper during the circuit board etching process. This etching process is aggressive, hence the need for fabricators to apply caution to make sure that the traces are not over-etched, resulting in the fabrication of traces that are thinner than their desired widths. 

Furthermore, when traces are isolated on the board, they may become more etched down than grouped and clustered traces. This is because of the concentration of etching on a particular area. To prevent this, it is advised to slightly widen isolated traces so they don't end up etching more than desired. Over-etched traces present the risk of seriously impairing the signal integrity of the circuit board, which is why it is important to ensure that traces meet their target widths.

The thickness or weight of the copper being etched is another vital consideration, as traces with a greater copper weight can not be etched down as much as other traces, because of their thickness. Copper layers used on PCBs come in fairly standard thicknesses - usually measured in ounces (oz) per square foot or Mil. Generally, the thicker the copper layer, the lesser the trace width required. For typical PCBs, the most common copper thickness is 35µm which is equivalent to 1oz/square foot.

Trace widths can also affect the soldering processes used during PCB assembly. Wide traces applied in power and ground routing can act like a heat sink resulting in irregular soldering temperatures and poor solder joints. When this occurs with large parts like high pin-count ball grid arrays (BGAs) or surface mount connectors, the defects can be difficult to locate and correct, thereby requiring inspection and rework usually expensive procedures.

Apart from the complex manufacturing issues that make accurate trace width fabrication crucial, it is also significant for the electrical performance of circuit boards. In this regard, trace widths play a critical role in both the signal and power integrity of the board.

Signal Integrity

Varying trace widths are usually used to improve signal integrity and aid control of crosstalk, electromagnetic interference (EMI), and other problems associated with signal interference, some of which include:

• Controlled impedance routing: Certain high-speed signals need to be routed at specific widths to control their impedance. Mismatches from different impedance values scattered through the length of the signal lines can cause reflections of the signal and disrupt its performance. To negate this, the trace widths of these lines must be strictly determined based on calculations of the board's dielectric material, copper weight used, and spacing to other signals.

• Analog routing: Analog signals should be kept short and direct, and wider than another routing. This extra width ensures that trace impedance is low. 

• Microstrip and stripline: Sensitive high-speed transmission lines need to be approximately paired with a reference ground plane for protecting their traces. An internal layer of routing sandwiched between two ground planes is referred to as a stripline configuration, while traces on the external layer of the board or its exterior, with only the adjoining plane beneath it is known as a microstrip configuration. The stripline traces will be slimmer due to the use of double-ground planes to match the impedance of the lines between the two configurations.

Power Integrity

Different trace widths are also required to guarantee clean power delivery to the components on the circuit board. Some layout aspects to consider are:

• Short and direct routing: As earlier stated with analog routing, it is important to keep the traces short to lessen the likelihood of them acting like antennas and generating additional noise. Moreover, it is beneficial to apply 45° or rounded corners instead of right angles.

• Wide traces: Using wider traces with power routing is helpful to reduce inductance and crosstalk.

• Current and heat considerations: Depending on which network is being routed, power traces conduct varying current levels. This results in the news using different trace widths and copper weights as required. The heat production should be factored in as well since more current means more heat. The power traces on the board's external layer can be subjected to air cooling, but such advantage is not available to internal power routing. Hence the necessity for internal power traces to be wider than their counterparts on the surface layers.

A trace width calculator can accurately determine various trace components, such as trace temperature, maximum current, resistance, voltage drop, and power dissipation. The tool achieves this by applying particular formulas for each component, conforming to industry standards such as the IPC-2221 standard for circuit board design.

Trace thickness calculators define the maximum allowable current that can flow through a circuit board without causing damage to it. [2]

PCB Trace Resistance

Resistance is the measure of opposition to current flow. All materials in the universe possess some amount of this opposition and are broadly classified as conductors with minor resistance or insulators with high resistance. Copper, which is the most commonly used material in PCB traces, comes with different compositions and properties therefore even though it is a very good electrical conductor, traces in PCB can still have some opposition to current flow, which is known as PCB trace resistance.

PCB trace resistance is one of the primary factors that should be calculated during PCB design, and it is determined by the thickness, width, and length of the trace. This factor is important because the value of the trace resistance may lead to various design amendments and implementation issues. Moreover, as the PCB circuit grows more complex, the implementation issues are bound to intensify as well. The trace resistance value must be adequately low to enable current flow through the circuitry of the PCB.

Calculating trace resistance is important to determine the power consumption of integrated circuits or PCBs and analyze the amount of power that can be dissipated by the PCB trace.

In addition, the current that flows through the PCB traces generates a substantial amount of heat. And as earlier mentioned, trace resistance is determined by the thickness, length, and width of a PCB trace. Consequently, if these dimensions are accurately calibrated, they can help minimize the generation of heat. More so, the wider the PCB trace, the lower its resistance to current flow, which would also affect the reduced heat generation.

A specific formula is used to calculate PCB trace resistance, it is expressed thus: 

𝑅 = 𝜌 × [𝐿 / (𝑊 × 𝑇 )] ×[1 + 𝛼 × (𝑇𝑎𝑚𝑏−25℃)]

Where: 

𝑅 = Trace Resistance

𝜌 = Resistivity of (PCB trace material)

𝐿 = Trace Length

𝑊 = Trace Width, (can be obtained using a PCB trace width calculator)

𝑇 = Trace Thickness (or Trace Height)

𝛼 = Temperature Coefficient of Copper at 20℃

𝑇𝑎𝑚𝑏 = Ambient Temperature [℃] of the environment in which the PCB trace is operating. [3]

PCB Trace Current

The current capacity of a PCB trace is the maximum current that can pass through the trace without causing damage to any of the electronic components on the board. The trace current of a PCB is dependent on the trace width and thickness.

Max Current calculation

First, calculate the area according to the following formula:

A = (T · W · 1.378 [mils/oz/ft2])

Then, calculate the maximum current:

IMAX = (k · TRISEb) · Ac                                       

Where:

A is the cross-section area [mils2] 

T is the trace thickness [oz/ft2] 

W is the trace width [mils] 

IMAX is the maximum current [A] 

TRISE is the maximum desired temperature rise [°C] 

k, b and c are constants. According to IPC-2221A Section 6.2, their values for inner layers are as follows: 

k = 0.048 b = 0.44 c = 0.725 [4]

Signals moving across PCB

PCB Trace and PCB Design

The function of a PCB trace is to connect any kind of electrical signal, from one junction to another. These signals can range from analog to digital, to power signals, and the connected junction can be the pin of a component, a portion of a larger plane or trace, or an empty pad or test point used for probing.

In the layout of printed circuit boards, width plays an important function, especially in the metal connections fabricated into the board called traces.

Trace widths are typically measured in mils or thousands of an inch. A standard trace width for an ordinary signal may range between 7-12 mil and be as long as a few inches, moreover, there are many considerations to be made when defining the width and length of a trace.

The application generally determines the trace widths and types in a PCB design and at a certain point, there is a balance in terms of board density, the board size, performance, and fabrication cost. If a board has precise design specification requirements such as speed optimization, noise or coupling mitigation, or high current/voltage, the trace widths and types might take priority over optimizing for the manufacturing cost of the circuit board or general board size.

The width of the traces is a critical factor in PCB design. Designers have to assign an appropriate width to traces to safeguard them from damages that may be incurred from a rise in temperature, which is determined by the current carrying capacity of the board.

The traces on a printed circuit board are designed to handle a maximum load of current before they fail. When higher amounts of current pass through a path, it starts to produce heat. After some time when the current load exceeds the maximum current carrying capacity, the trace will burn out, destroying PCB laminate and causing permanent damage. [5]

High Current PCB Layout Guidelines

PCB design and fabrication is an intricate and complex process. Hence it is necessary to take precautions to ensure there are no negative thermal incidents, especially when dealing with high-current boards like multi-layered PCBs.   

These are some guidelines to consider concerning high-power PCBs:

Shorter Traces

Longer traces make for higher resistance, so it is beneficial to keep the traces as short as possible when dealing with high currents. This will help limit the power loss from within the tracks, thereby increasing PCB reliability and durability.

Heavy Copper Use

The thickness of a standard printed circuit board is about 17.5 microns or 1 oz. per square foot. In the fabrication of high-power PCBs, heavier copper is typically used, as it functions to reduce the trace width and still allows it to carry the same amount of current. Higher copper thickness can range between 35 to 50 microns or 3-4 oz. per square foot, if operating at over 10 amps. 

Appropriate Mounting

When several high-power individual electronic components are likely to produce increased amounts of thermal energy, it is pertinent to carefully consider the position of these parts as they are mounted on the board. They should not be mounted close to the board's edge, as that causes a buildup of heat in that area, thereby raising the temperature drastically.

Placing this type of component across the board in an evenly distributed manner instead is necessary to prevent heat concentration.

Nevertheless, Placing components like microcontrollers near the center of the package, allows the heat to be dissipated across the board, resulting in a relatively low board temperature.

Thermal Isolation

Part of the electrical energy derived from the power source is converted into other forms such as heat energy which is then diffused into the surrounding environment. it is important to thermally isolate sensitive components used in electronic circuitry, such as 

voltage references, regulators, amplifiers, and converters because they are highly sensitive to fluctuations in their immediate ambient. This is to prevent an alteration of their signal, which can lead to board defects.

Thermal Vias and Landings

Via stitching is a technique where multiple layers are connected using vias, to continue the same trace maintaining the same width across the different layers. Thermal vias can also be used to direct the flow of heat energy within a PCB away from the sensitive components to conserve their reliability and performance.

The vias absorb the heat produced by the high current passing through the circuit. Once the heat is transferred to the thermal vias, it can be sent to a thermal landing which is typically a metal plate placed at the bottom of the board where there are no failure points and the significant components of the board are insulated from the negative impact of the high power.

Polygon Pours

Employing the use of polygon pours helps to increase the current carrying capacity of a board while ensuring the thermal isolation of sensitive components. This polygon pours can be placed beneath the chip and connected to the board, using vias.

 Eliminating Solder Mask

Removing the solder mask from a PCB is a sure way to enable the trace to carry higher amounts of current. This exposes the underlying copper material which can then be supported with additional soldiers to boost the copper thickness and decrease resistance in their current carrying components of the circuit board. This way, the PCB is imbibed with a higher power-carrying capacity without having to increase trace width. [6]

Key Takeaways

This article has defined PCB traces as tracks - often made of copper - used to conduct electricity between the components of a printed circuit board.

Additionally, it was discussed that properties like width and thickness help determine the amount of current that can pass through a trace, whilst maintaining thermal stability. 

Furthermore, we outlined and discussed key parameters that must be carefully considered in designing PCB traces, among which include trace width, trace thickness, trace resistance, and trace current.

In conclusion, we explained the significance of PCB traces to PCB design and fabrication, as it is a fundamental step in manufacturing reliable and durable boards, with the application of some best practices for high-power boards.

References

1. Yun Industrial. What are PCB Traces? 2021 [Cited 2022 Nov 2] Available from: https://yic-assm.com/what-are-pcb-traces/ 

2. Auto Desk. Everything You Need to Know About Trace Width. 2021 [Cited 2022 Nov 2] Available from: https://www.autodesk.com/products/fusion-360/blog/trace-width/#:~:text=For%20typical%20PCBs%2C%20the%20most,be%206mil%20or%200.152mm.

3. Do Supply Tech Support. Trace Resistance Calculator. 2022. [Cited 2022 Nov 2] Available from: https://www.dosupply.com/tech/2022/02/22/trace-resistance-calculator

4. Millennium Circuits Limited. Trace Width Vs Current Table. 2022. [Cited 2022 Nov 3] Available from: https://www.mclpcb.com/blog/pcb-trace-width-vs-current-table/ 

5. San Francisco Circuits. The Importance of PCB Trace Widths in PCB design. 2022. [Cited 2022 Nov 3] Available from: https://www.sfcircuits.com/pcb-school/pcb-trace-widths

6. Sierra Circuits. Role of PCB Trace Current Capacity in Design. 2020. [Cited 2022 Nov 3] Available from: https://www.protoexpress.com/blog/trace-current-capacity-pcb-design/