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PCB Trace Guide: From Basics to In-Depth

When we originally scratched copper wires finer than hair onto silicon wafers, we did not just create circuits – stitched the vascular system of the electrical cosmos.

Buried in the middle of every circuit board is the key to whether electronic equipment flourishes or fails: PCB traces. Copper traces of micron-level accuracy, these have a tri-fold mandate: convey high-speed signals with micron-level precision, carry ampere-level currents at ease, and address Joule heating effects. From smartphone to satellite systems, a 0.1mm PCB trace width difference is a 5ps signal delay, and a 10% current overload is a 30°C temperature rise. Understanding the design rules of these copper channels is the key to balancing performance, reliability, and cost.

pcb trace

What is a PCB Trace and Its Function

PCB traces are electrical paths etched from copper sheets glued on a PCB substrate. They are the “wiring” that guides electrical signals between components like resistors, ICs, and connectors. Without traces, your circuit would be an loose assortment of worthless parts!

When a circuit is powered, current flows over each component’s contact points, compelling their functioning. This involves the existence of a continuous current path, and this is the main function of PCB traces. Trace design is directly related to circuit performance, and trace width, thickness, and resistance are factors that need to be carefully chosen.

PCB Trace Types

  • Single-ended trace: the most common type, used to transmit single-ended signals, such as digital signals, analog signals, etc., whose signals are transmitted over traces, and return path usually goes through the ground plane or ground wire.
  • Differential trace: consists of two traces of the same width and length and is employed to transmit differential signals, can effectively reduce common-mode interference, improve the anti-interference ability of the signal and transmission quality, and is commonly used in high-speed digital circuits and high-frequency signal transmission.
  • Power trace: employed especially for the transmission of power signals and for a stable efficient supply of power to electronic components. It typically requires a larger width and smaller impedance to reduce power supply voltage noise and loss.
  • Ground trace: used to connect the ground terminal of electronic components, provide a firm ground reference potential, and play the role of shielding and anti-interference. Its configuration and composition are critical to the stability and reliability of the circuit.

PCB Trace Manufacturing Process

In the design and arrangement of PCB, the designer selects and positions components based on the functional requirements of the circuit and then interconnects them through traces. In the manufacturing phase of the PCB manufacturing process, manufacturers apply specific materials over the surface of the PCB, and the PCB trace pattern design gets exposed on the copper layer by a photolithography process. At last, the process of chemical etching is used to remove the redundant copper material, and the traces remain with accurate dimensions. These conductive lines are held onto a substrate insulator, typically consisting of such items as fuses, wiring, and copper conductors.

PCB trace manufacturing process

PCB Trace Width

Trace width refers to the width of the conductive copper traces on the surface of a PCB. Setting these parameters is crucial for ensuring the reliable operation of electronic devices, based on an analysis of project requirements, including current-carrying capacity, signal integrity, thermal considerations, and manufacturing constraints, while strictly adhering to standards.

  • Current Analysis: Conducting current analysis is essential to determine the appropriate PCB trace width required to accommodate the maximum current without exceeding temperature rise limits and voltage drop thresholds. This ensures that the PCB can operate safely and reliably under various load conditions.

Use the IPC-2221 modified formula for precision:

Width (mm) = (I / (k * ΔT^0.44)) ^ 1.0.725

Where:

  • I = Current (Amps)

  • ΔT = Permissible temperature rise (°C)

  • k = 0.024 (external layers), 0.048 (internal)

  • Signal Integrity: Considering the frequency and impedance requirements of signals helps determine the optimal trace width for high-speed digital and analog circuits, minimizing signal attenuation, reflection, and electromagnetic interference. This ensures effective transmission of high-speed signals.
  • Thermal Management: By incorporating heat dissipation holes, heat sinks, and copper layers, excess heat generated by current flow can be effectively dissipated, preventing overheating and maintaining the optimal operating temperature of the PCB and its components. This enhances the overall reliability and lifespan of electronic devices.
  • Partner Selection: Choosing to collaborate with companies like FS Technology, which includes a design team, in the early stages of design, helps ensure that trace width design aligns with manufacturing capabilities and limitations. This ensures manufacturability and cost-effectiveness, minimizing the risks of manufacturing defects and flaws.

PCB Trace Resistance

Resistance is a physical property that opposes the movement of electric current and is inherently present in all materials. Excessive resistance in traces on a PCB can lead to signal attenuation, increased power consumption, and heat generation. Therefore, designers need to assess this property rationally.

To reduce the resistance of PCB traces, copper is commonly used as the material. Additionally, trace resistance is intimately related to PCB trace thickness, width, and length. According to Ohm’s law, resistance (R) is associated with current (I), voltage (V), and the physical characteristics of the resistance material and can be expressed as follows:

R = (ρ * L) / A

Where:

  • R represents resistance (measured in ohms).
  • ρ (rho) signifies resistivity and reflects the material’s electrical resistance.
  • L corresponds to the length of the trace (measured in meters).
  • A indicates the cross-sectional area of the PCB trace, which is typically connected to its width (W) and thickness (T), as A = W * T.
 

From the formula above, it becomes evident that trace resistance is directly proportional to its length and inversely proportional to the cross-sectional area. Therefore, to adjust the trace resistance, changes in length, width, and thickness are made. These parameters are typically optimized during the PCB design phase to meet the circuit’s performance requirements. It’s important to note that resistivity (ρ) is an inherent property of the material and is typically a fixed value that is not easily modified.

How to Repair PCB Copper Traces

  1. First, you need to identify the damaged or broken traces. This can be done through visual inspection or using specialized equipment like thermal imaging or multimeters.
  2. Before proceeding with the repair, clean the damaged area with anhydrous isopropyl alcohol to remove any dust, oxidation, or impurities.
  3. Use a small knife to remove the damaged traces, then reconnect the traces using a micro soldering iron and the appropriate solder.
  4. If the damage to the PCB traces is severe, you can consider using wires or specialized PCB repair adhesive. Wires can connect two endpoints, while PCB repair adhesive can fill in cracks.
  5. After completing the repair, be sure to test the PCB to ensure its functionality.
  6. Upon successful confirmation, immediately coat and cover the repaired area with an insulating adhesive, tape, or transparent sealant for protection.

Common PCB Trace Design Rules

Spacing Regulations

  • Wire spacing: There must be a specific wire spacing between wires and wires, wires and pads, etc., typically more than 0.3mm, to avoid short circuits and crosstalk.
  • Special line spacing of signal: The spacing requirements are more stringent for high-speed signal lines such as clock signals, differential signals, etc., and the differential line spacing is usually compliant with 4w (w is the line width).

Routing Direction

  • Horizontal vertical or 45° angle routing: Try to route horizontally, vertically or at a 45° angle to reduce 90° angles and acute angles to reduce signal reflection and electromagnetic interference.
  • Adjacent layer routing direction: The routing directions of two adjacent layers should be orthogonal to avoid the routing of signals of two adjacent layers in the same direction and reduce unnecessary crosstalk between layers.

Signal / Power Line

  • Power line: The power line should be as wide as possible, typically more than 1.5mm, to accommodate sufficient current.
  • Signal line: Long-distance signal line trace over power lines should be avoided to prevent interference. Ground wire isolation can be used.

Via Setting

  • Number of vias: The number of vias should be minimized in order to keep the wiring space small and signal effect minimal.
  • By size: The aperture size and pad size must be reasonable, such as the commonly used 0.8mm aperture with 1.6mm pad.

Return Path

Provide the signal with a complete return path in order to reduce electromagnetic radiation.

Other Rules

Wire length: Routing should be as short as possible to avoid unwanted interference due to excess length. For critical signal routing, for example, clock signal routing, ensure its oscillator is placed very close to the device.

Wiring termination network rules: For high-speed digital circuits, when the delay time of PCB routing is greater than 1/4 of the signal rise time (or fall time), the routing can be treated as a transmission line and must be matched.

Routing closed loop check rules: Prevent signal lines from having self-loops between different layers, which will cause radiation interference.

Branch length rule of routing: Try to control the branch length, overall need tdelay<=trise/20.

Resonance rule of routing: Mainly for high-frequency signal design, i.e., the wiring length should not be an integer multiple of its wavelength to avoid resonance.

Conclusion

The PCB trace guide helps in designing and developing reliable and high-performance printed circuit boards. By carefully considering key factors that influence trace design, engineers can determine the optimal trace width and layout configuration for a given application. This ensures seamless transmission of electrical signals within the PCB, mitigating signal integrity and thermal issues. As a leading PCBA manufacturer, FS Technology is well-versed in various aspects of circuit board knowledge. If you have any questions about PCB traces, please feel free to contact us, and our experts will be happy to assist you.

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