PCB Trace Everything You Need To Know

What is a PCB Trace?

A PCB trace, also known as a PCB track or PCB wire, is a thin strip of copper or other conductive material that is etched onto the surface of a printed circuit board (PCB). PCB traces are used to electrically connect the various components on the board, such as resistors, capacitors, integrated circuits (ICs), and connectors. The traces act as the “wires” of the circuit, allowing electrical current to flow between the components and enabling the PCB to perform its intended function.

PCB traces are typically made of copper due to its excellent electrical conductivity and relatively low cost. The thickness and width of the traces can vary depending on the current carrying requirements and the size constraints of the PCB Design. Traces are usually coated with a thin layer of solder mask, which is a protective insulating material that prevents short circuits and helps to protect the traces from oxidation and mechanical damage.

Importance of PCB Traces

PCB traces are a critical aspect of PCB design as they directly impact the performance, reliability, and manufacturability of the circuit board. Some of the key reasons why PCB traces are important include:

Signal Integrity: The geometry and routing of PCB traces can significantly affect the quality of the electrical signals transmitted through them. Poorly designed traces can lead to signal reflections, crosstalk, and electromagnetic interference (EMI), which can degrade the performance of the circuit and cause malfunctions.
Power Distribution: PCB traces are responsible for distributing power from the power supply to the various components on the board. The width and thickness of the traces must be carefully designed to ensure that they can handle the required current without overheating or causing voltage drops.
Manufacturing Feasibility: The layout and spacing of PCB traces can impact the manufacturability of the board. Traces that are too close together or too thin can be difficult to etch accurately, leading to manufacturing defects and reduced yield.
Electromagnetic Compatibility (EMC): The placement and routing of PCB traces can affect the electromagnetic emissions and susceptibility of the circuit. Proper layout techniques, such as avoiding long parallel traces and using ground planes, can help to minimize EMI and ensure that the board meets regulatory EMC requirements.

PCB Trace Design Considerations

When designing PCB traces, there are several key factors that must be considered to ensure optimal performance and reliability. Some of the most important PCB trace design considerations include:

Trace Width and Thickness

The width and thickness of PCB traces are critical parameters that determine their current carrying capacity and resistance. Wider and thicker traces can carry more current and have lower resistance, but they also take up more space on the board. The trace width and thickness must be carefully chosen based on the expected current load and the available board space.

The IPC-2221 standard provides guidelines for determining the appropriate trace width and thickness based on the current requirements and the temperature rise of the board. The following table shows some common trace widths and their corresponding current carrying capacities for 1 oz copper (35 µm thickness) at various temperature rises:

Trace Width (mm)	Current Capacity (A) at 10°C Rise	Current Capacity (A) at 20°C Rise	Current Capacity (A) at 30°C Rise
0.25	0.5	0.7	0.9
0.5	1.0	1.4	1.8
1.0	2.0	2.8	3.5
2.0	4.0	5.7	7.0

Trace Spacing

The spacing between PCB traces is another important design consideration, as it affects the board’s signal integrity and manufacturability. Traces that are too close together can lead to signal crosstalk and EMI, while traces that are too far apart can waste board space and increase the overall size of the PCB.

The IPC-2221 standard provides minimum trace spacing guidelines based on the voltage between adjacent traces and the board’s manufacturing technology. The following table shows some common trace spacing recommendations for various voltage levels:

Voltage (V)	Minimum Trace Spacing (mm)
<15	0.1
15 – 30	0.2
30 – 50	0.4
50 – 100	0.6
100 – 150	1.0
>150	1.5

Trace Routing

The routing of PCB traces is a critical aspect of board design, as it directly impacts signal integrity and EMC performance. Some key trace routing guidelines include:

Minimize Trace Length: Shorter traces have lower resistance and inductance, which can help to reduce signal distortion and EMI. It is generally recommended to keep traces as short as possible while still meeting the other design constraints.
Avoid 90° Corners: Sharp 90° corners in traces can cause signal reflections and EMI. It is better to use 45° angles or curved traces to smooth out the transitions and minimize these effects.
Use Ground Planes: Placing a solid ground plane layer beneath signal traces can help to reduce EMI and improve signal integrity by providing a low-impedance return path for the signals.
Avoid Parallel Traces: Long parallel traces can lead to crosstalk and EMI, especially at high frequencies. It is generally recommended to route sensitive traces perpendicular to each other or to use guard traces to minimize crosstalk.
Match Trace Impedance: For high-speed signals, it is important to match the impedance of the traces to the impedance of the source and load to minimize reflections and signal distortion. This can be achieved by adjusting the trace width and height above the ground plane.

PCB Trace Materials

While copper is the most common material used for PCB traces, there are several other materials that can be used in specific applications. Some alternative PCB trace materials include:

Gold: Gold traces are sometimes used in high-reliability applications, such as aerospace and military equipment, due to their excellent corrosion resistance and low contact resistance. However, gold is much more expensive than copper and has lower electrical conductivity.
Aluminum: Aluminum traces are occasionally used in low-cost, high-volume consumer electronics due to their lower cost compared to copper. However, aluminum has lower electrical conductivity and is more prone to oxidation than copper.
Silver: Silver traces are rarely used due to their high cost, but they have the highest electrical conductivity of any metal. Silver traces may be used in specialized high-frequency applications where minimizing resistance is critical.
Carbon Nanotubes: Carbon nanotube traces are an emerging technology that promise to enable the creation of extremely fine pitch traces with high current carrying capacity. However, carbon nanotube traces are still in the research stage and are not yet commercially available.

PCB Trace Testing and Inspection

After a PCB is manufactured, it is important to test and inspect the traces to ensure that they meet the design specifications and are free of defects. Some common PCB trace testing and inspection methods include:

Visual Inspection: A simple visual inspection can often reveal gross defects in the traces, such as shorts, opens, or incorrect routing. This can be done with the naked eye or using a magnifying glass or microscope.
Automated Optical Inspection (AOI): AOI uses computer vision algorithms to automatically inspect the PCB for trace defects and compare it to the design files. AOI can quickly identify problems such as incorrect trace widths, spacing violations, or missing traces.
Electrical Continuity Testing: Electrical continuity testing uses a multimeter or other continuity tester to verify that the traces are electrically connected as intended. This can help to identify open circuits or high-resistance connections.
Isolation Testing: Isolation testing uses a high-voltage insulation tester to verify that adjacent traces are not shorted together and that there is sufficient electrical isolation between them.
Microsectioning: Microsectioning involves cutting a cross-section of the PCB and examining the traces under a microscope to verify their width, thickness, and shape. This destructive testing method is typically only used for failure analysis or quality control purposes.

Frequently Asked Questions (FAQ)

What is the difference between a PCB trace and a wire?
A PCB trace is a flat, thin strip of copper that is etched onto the surface of a PCB, while a wire is a cylindrical conductor that is typically insulated with a plastic or rubber coating. PCB traces are used to connect components on a PCB, while wires are used to connect PCBs to each other or to external devices.
Can PCB traces be repaired if damaged?
In some cases, damaged PCB traces can be repaired using conductive epoxy, solder, or wire jumpers. However, the feasibility of repair depends on the extent and location of the damage, as well as the accessibility of the trace. In many cases, it may be more cost-effective to simply replace the damaged PCB.
What is the minimum width of a PCB trace?
The minimum width of a PCB trace depends on the manufacturing capabilities of the PCB fabricator and the design requirements of the circuit. In general, most PCB fabricators can reliably produce traces as narrow as 0.1 mm (4 mil) using standard etching processes. However, advanced manufacturing techniques, such as laser direct imaging (LDI) or semi-additive processes (SAP), can enable trace widths as narrow as 0.025 mm (1 mil) or less.
What is the difference between a microstrip and a stripline trace?
A microstrip trace is a PCB trace that is routed on the outer layer of the board, with a ground plane layer beneath it. A stripline trace, on the other hand, is a PCB trace that is routed on an inner layer of the board, with ground plane layers above and below it. Stripline traces have better signal integrity and EMI performance than microstrip traces, but they are more difficult to route and require more board layers.
How do you calculate the resistance of a PCB trace?
The resistance of a PCB trace can be calculated using the following formula:

R = ρ * L / (W * T)

Where:
– R is the resistance of the trace in ohms (Ω)
– ρ is the resistivity of the trace material in ohm-meters (Ω·m)
– L is the length of the trace in meters (m)
– W is the width of the trace in meters (m)
– T is the thickness of the trace in meters (m)

For example, if a copper trace has a length of 0.1 m, a width of 0.5 mm, and a thickness of 35 µm, and the resistivity of copper is 1.68 x 10⁻⁸ Ω·m, then the resistance of the trace would be:

R = (1.68 x 10⁻⁸ Ω·m) * (0.1 m) / (0.0005 m * 0.000035 m) = 0.096 Ω

This calculation assumes that the trace has a rectangular cross-section and uniform width and thickness along its entire length. In practice, the actual resistance of a PCB trace may be slightly higher due to factors such as surface roughness, edge effects, and temperature variations.

Conclusion

PCB traces are the lifeblood of any electronic circuit, enabling the flow of electrical signals and power between components. Proper design, selection, and testing of PCB traces is essential to ensure the performance, reliability, and manufacturability of the circuit. By carefully considering factors such as trace width, thickness, spacing, routing, and materials, PCB designers can create traces that meet the specific requirements of their application while minimizing signal integrity issues and manufacturing defects. As PCB technology continues to evolve, the capabilities and limitations of PCB traces will continue to shape the design and performance of electronic devices across a wide range of industries.

HDI PCB

PCB Trace Everything You Need To Know

What is a PCB Trace?

Importance of PCB Traces