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Distances Between Traces in PCBs

When designing a printed circuit board (PCB), it is not enough to just connect the components with the right traces. There is an art and a science behind the layout of the traces, and one of the most critical considerations is electrical insulation. Ignoring this aspect can lead to malfunctions, safety hazards, and, in the worst case, catastrophic device failure.

But what exactly are we talking about when we talk about "insulation between traces"? And what are the rules of the game?

Why is Insulation Essential?

Imagine electricity as water in a pipe. If the pipe leaks, the water goes where it shouldn't. Likewise, if the traces on your PCB are too close together, electricity can "jump" from one trace to another. This phenomenon is known as dielectric breakdown (or arcing). In addition, the surface of the PCB can degrade over time, creating conductive paths, a process called tracking.

Adequate insulation prevents:

  • Short circuits: Prevents current from flowing where it shouldn't.
  • Electric Arcs: Prevents sparks and discharges that can damage components or cause fires.
  • Dielectric Failures: Ensures that the insulating material between the traces maintains its properties over time.
  • Electromagnetic Compatibility (EMC) Problems: Good insulation can reduce noise and unwanted interference.
  • Safety Risks: Essential for products that interact with people, especially if connected to the mains.

Clearance and Creepage: The Two Key Measures

In the world of PCB insulation, you will often hear two terms: Clearance and Creepage. They are both fundamental, but they refer to slightly different concepts:

1. Clearance (Distance in the Air)

Clearance is the shortest distance measured in the air between two conductors. Think of a bird flying in a straight line from one point to another. This distance is crucial to prevent dielectric breakdown, i.e., the "jumping" of current through the air.

The factors that influence clearance are mainly:

  • Operating Voltage: The greater the potential difference between the traces, the greater the distance must be.
  • Altitude: At higher altitudes, the air is less dense and has a lower dielectric strength, thus requiring greater distances.
  • Overvoltages (Transients): Sudden voltage spikes (such as those caused by lightning or switching of inductive loads) require greater clearance to be withstood.

2. Creepage (Distance on the Surface)

Creepage is the shortest distance measured along the insulating surface of the PCB between two conductors. Imagine a snail crawling along the profile of the surface. This distance is important to prevent tracking, a process in which a leakage current creates a conductive path on the surface of the insulating material, often due to moisture and contaminants (dust, dirt).

The key factors for creepage include:

  • Operating Voltage: Similar to clearance, higher voltages require more creepage.
  • Pollution Degree: This is a critical factor. A "dirty" or humid environment will require much greater creepage distances than a clean, sealed environment. The standards classify environments into various pollution degrees (PD1 for clean and sealed environments, up to PD4 for extreme environments).
  • PCB Material (CTI – Comparative Tracking Index): This is a measure of the resistance of an insulating material to the formation of conductive paths on its surface. Materials with a higher CTI can better resist tracking. The common FR-4 (the PCB material) has a specific CTI that affects the creepage requirements.
  • Presence of Protective Coatings (Solder Mask/Conformal Coating): A protective layer such as the solder mask (that green paint on the PCB) or a conformal coating can significantly increase the surface resistance, in some cases allowing for reduced creepage distances or classifying the environment in a better pollution degree.

The Standards: Your Infallible Guide

Fortunately, you don't have to reinvent the wheel. There are international standards that provide detailed tables and calculations to determine the minimum clearance and creepage distances. The most important are:

  • IPC-2221 (Generic Standard on Printed Board Design): This is the reference standard for generic PCB design. It provides tables for clearance based on voltage, conductor type (internal/external, coated/uncoated), and altitude.
  • IEC 60950-1 / IEC 62368-1 (Safety Standard): These standards are crucial for electronic and information technology equipment. They are very strict and specify in detail the clearance and creepage distances, considering the working voltage, the pollution degree, the material group of the PCB, and the type of insulation required (functional, basic, supplementary, reinforced).

Practical Example: 1000V DC in a Controlled Environment

Let's take a common example: you want to isolate two traces with a potential difference of 1000V DC peak, but your PCB is inside a sealed box and has the standard insulating coating (green solder mask). This means a Pollution Degree 1 (PD1), the cleanest environment.

According to the standards (referring to IEC 60664-1 and IPC-2221 for incremental calculation):

  • Clearance (Distance in the Air): For 1000V DC in PD1, the safety standards might require a clearance of about 0.2 mm (for basic insulation) up to 0.8 mm (for reinforced insulation, depending on the overvoltage requirements). The IPC-2221, which is more generic, would indicate about 1.9 mm for coated external traces at 1000V. As you can see, the safety requirements are often more stringent.
  • Creepage (Distance on the Surface): With PD1 and a PCB coated in FR-4 (Material Group IIIa), the distances are significantly reduced. For 1000V DC, we could be talking about 0.5 – 0.8 mm for functional/basic insulation, and about 1.0 – 1.6 mm for reinforced insulation.

Important Note: These are just examples based on common interpretations of the standards. The final design must always refer to the precise tables of the standards applicable to your product.

Practical Design Tips

  • Know Your Standards: The first step is to identify which safety and design standards apply to your product.
  • Evaluate the Environment: Be realistic about the pollution degree of your operating environment. A "sealed box" is only effective if it actually prevents the ingress of moisture and contaminants.
  • Consider the Type of Insulation: Is the insulation only for the operation of the circuit (functional) or is it also a safety barrier for the user (basic, supplementary, reinforced)? The requirements change drastically.
  • Add a Safety Margin: Manufacturing tolerances can reduce the effective distances. It is always wise to give a small extra margin compared to the standard minimums.
  • Use CAD Tools: Modern PCB design software (such as Altium Designer, KiCad, Eagle) have features to automatically set and check clearance and creepage rules.
  • Test Your Design: For high-voltage boards, production tests such as the Hi-Pot (High Potential) are essential to verify the integrity of the insulation.

The insulation of the traces on the PCB is an aspect that should never be underestimated. Understanding the concepts of clearance and creepage, and knowing where to find the information in the relevant standards, will allow you to create robust, reliable, and, above all, safe PCBs.