Traces

Traces, also referred to as tracks, are the conductive pathways on a Printed Circuit Board (PCB) that facilitate the transmission of electrical signals and power between components. These pathways are typically fabricated from copper due to its excellent electrical conductivity and relatively low cost. This documentation provides a comprehensive analysis of traces, their design considerations, and their role in PCB functionality. The content is tailored for advanced users, emphasizing technical precision and practical insights.

Fundamental Characteristics of Traces

Composition and Function

Traces are composed of copper and are etched onto the PCB substrate during the manufacturing process. Their primary functions include:

1. Signal Transmission:
   Traces carry low-power electrical signals between components, such as digital or analog signals in integrated circuits (ICs).

2. Power Distribution:
   Wider traces are used to deliver higher currents to power-hungry components, ensuring minimal voltage drop and heat generation.

Visual Representation

In PCB design software and manufacturing outputs, traces are visually represented as thin lines connecting component pads. The color of these lines often depends on the solder mask applied during fabrication. For example, traces may appear purple under a specific solder mask chemical, as illustrated in the provided images.

Design Considerations for Traces

Trace Width

The width of a trace is a critical parameter that influences its current-carrying capacity, resistance, and thermal performance. Key considerations include:

1. Current Capacity:
   The width of a trace must be sufficient to handle the expected current without excessive temperature rise. The relationship between trace width, current, and temperature rise can be calculated using the IPC-2221 standard or online trace width calculators.

   $$I = k \cdot \Delta T^{0.44} \cdot A^{0.725}$$

   Where:

   • $I$ = Current (Amperes)
   • $k$ = Constant (0.024 for outer layers, 0.048 for inner layers)
   • $\Delta T$ = Temperature rise (°C)
   • $A$ = Cross-sectional area of the trace (mils²)

2. Signal Integrity:
   For low-power signal traces, narrower widths (e.g., 0.3 mm or less) are often used to minimize space and reduce parasitic capacitance and inductance.

3. Manufacturing Constraints:
   The minimum trace width is dictated by the PCB manufacturer's capabilities. Advanced fabrication processes can achieve trace widths as low as 0.1 mm or less.

Trace Thickness

The thickness of a trace, typically measured in ounces of copper per square foot (oz/ft²), also affects its electrical and thermal properties. Common thicknesses include:

• 1 oz/ft²: Standard thickness for most signal traces.
• 2 oz/ft²: Used for power traces to handle higher currents.

Trace Routing

The routing of traces involves determining their path on the PCB. Key routing considerations include:

1. Angle of Direction Change:
   Traces should avoid sharp angles (e.g., 90-degree bends) to prevent signal reflections and manufacturing issues. Instead, 45-degree angles or curved traces are recommended.

2. Spacing and Clearance:
   Adequate spacing between traces is necessary to prevent crosstalk and ensure manufacturability. The minimum spacing is determined by the PCB manufacturer's design rules.

3. Layer Stacking:
   In multilayer PCBs, traces can be routed on different layers to optimize space and reduce interference. Vias are used to connect traces between layers.

Practical Examples

Example 1: Signal Traces

In the provided image, thin purple lines represent signal traces connecting the golden pads of components. These traces are designed to carry low-current signals (typically less than 20 mA) and are kept narrow (e.g., 0.3 mm) to minimize space and parasitic effects.

Example 2: Power Traces

The bottom image illustrates wider traces designed to carry higher currents. These traces connect the terminals of a 240-volt relay and are significantly wider than standard signal traces to accommodate the higher current and reduce resistance and heat generation.

Advanced Considerations

Impedance Control

For high-frequency signals, trace impedance must be controlled to prevent signal degradation. Impedance is influenced by trace width, thickness, and the dielectric properties of the PCB substrate. Impedance-controlled routing is essential for applications such as RF and high-speed digital circuits.

Thermal Management

Wider and thicker traces are used for power distribution to minimize heat generation. In high-power applications, thermal vias or additional copper pours may be employed to dissipate heat effectively.

Manufacturing Tolerances

Designers must account for manufacturing tolerances when specifying trace widths and spacing. Advanced fabrication techniques, such as laser direct imaging (LDI), enable tighter tolerances and finer trace geometries.

Conclusion

Traces are the backbone of PCB functionality, enabling the transmission of signals and power across the board. By carefully designing trace width, thickness, and routing, engineers can optimize the electrical and thermal performance of a PCB. Advanced considerations, such as impedance control and thermal management, further enhance the reliability and efficiency of modern electronic designs. Mastery of trace design is essential for creating high-performance PCBs that meet the demands of increasingly complex applications.