Spacing Requirements: Things That Drive Your PCB Mad!

Spacing

Are you familiar with horse racing? Well, of course you are. It is one of the oldest and traditional sports known to mankind. Horse racing involves riders or jockeys riding horses to decide which is the fastest of them over a set course. In horse racing, the eyes of the horse are covered from the side to avoid any distractions. Now, you are wondering what is the relationship between the spacing and horse racing, apart from the two words are in rhyme. Unfortunately, designers behave the same (sometimes) during the PCB design stage.

The importance of spacing rules from the designer’s point of view should be high. Standard spacing rules also specify creepage and clearance distances. However, engineers/designers usually follow the standard spacing rules and fail to look at the bigger picture. Additionally, the improper spacing between two traces can lead to crosstalk along the two-conductor traces.

Crosstalk also occurs in stripline and microstrip interconnects. A stripline is identified as a transmission line trace, which is placed between two ground planes on the internal layers of a printed circuit board, while the microstrip is a transmission line trace on an external layer of the board.

Most of the PCB design software tools use standard rules to determine line spacing. These rules are also called as the clearance rules. Though, ‘clearance rules’ is not technically the right term to use for spacing rules. The more accurate term will be the creepage rules, as these rules are applied between conductive elements over an insulating surface. As a PCB designer, you must be aware of the difference between creepage and clearance, particularly in high voltage circuits.

What Drives Your Neighbour Mad! (Strictly in Spacing Terms)

If you didn’t get the heading, go read our previous blog on line spacing, the key to your long-term relationship with PCBs. Now, we will be talking about factors affecting spacing in PCBs, which include:

  1. Working voltage
  2. Pollution degree
  3. Insulator types for components
  4. CAF (conductive anodic filament)

Working Voltages

A working voltage is defined as the highest voltage across any particular insulation when the equipment is subjected to a rated voltage. This definition is stated in various international standards, including IEC 950 and EN 60950. The values of the creepage and clearance are calculated by determining the working voltage under a certain operating voltage. While determining working voltages, we need to evaluate both peak and root-mean-square (RMS) voltages. The peak value of the DC voltage will determine the clearance, and the RMS value of the AC voltage will determine the creepage distance.

Based on the working voltage, we calculate the minimum distance between two traces in a PCB. For instance, the working voltage of the 609V secondary circuit will withstand the peak voltage of 2700V as per IEC-60950-1, so the root-mean-square (RMS) voltage will be 2700V*√2= 3818V. As per UL 796, the 40V/mil criterion is applied to calculate the required minimum distance. So, the spacing between the two traces would be 3818/40=95.45 mils.

Working Voltage V(a) Peak-to-Peak Voltage

V(p)

Root-mean-square Voltage

V(a-rms)

500 V 1750 V 3257 V
526 V 2000 V 3566 V
551 V 2200 V 3803 V
575 V 2400 V 4034 V
609 V 2700 V 4369 V
620 V 2800 V 4478 V

The above table shows how peak-to-peak voltage values vary with working voltages as per IEC-60950-1.

Pollution Degree

The pollution degree is a classification as per the amount of dry pollution and condensation present in the surroundings. Higher the pollution degree, more the dust contamination and condensation, thus affecting the safety of the product. The creepage and clearance distances are adjusted to ensure the safety of a PCB. The pollution degree varies as per the contamination level and humidity level in the atmosphere.

For instance, laboratory areas come under pollution degree 2 as per several safety standards and certification bodies. Pollution degree 1 can be applied to the products that are sealed inside air and watertight enclosures. On the other hand, pollution degree 3 is applied to the harsher environment conditions such as industrial manufacturing areas.

According to the IEC 60664.3 standard, the pollution degree is divided into four major categories:

  • Pollution degree 1: Zero pollution or dry environment. In this type, there is nonconductive pollution, which is not harmful to electronic circuit operations. Examples can be sealed enclosers or potted products.
  • Pollution degree 2: Mostly, there is non-conductive pollution. However, there is a possibility of the occurrence of temporary conductive pollution, which is caused due to the condensation. Laboratory area is one of the examples of pollution degree 2.
  • Pollution degree 3: In this type, conductive pollution or contamination occurs due to the presence of humidity or dust in the surroundings. For example, heavy industrial environments are more exposed to dust.
  • Pollution degree 4: There is a persistent conductivity, which is caused by the excess of humidity and dust contamination. External conditions, such as rain or snowfall, can lead to pollution degree 4 and result in persistent conductivity.

Several steps are taken to avoid the effect of pollution degree on the creepage and clearance. These steps are taken in accordance with the several design features related to the spacing and the operating characteristics of the system. For instance, pollution degree 2 is avoided by limiting the accumulation of humidity or dust particles via the provision of ventilation. Also, heaters and fans will avoid contamination. The continuous energizing or application of heat is highly preferred for cases in which equipment is operated without interruption. Continuous energizing also avoids the excess of cooling so the condensation won’t occur.

Furthermore, pollution degree 3 is avoided with the use of the appropriate enclosures. These enclosers limit external environmental factors such as moisture.

Insulation Types for Components

Typically, a single level of insulation is preferred for the electronic circuits that are not accessible. However, we prefer the use of double level insulation for protection against hazardous voltage. Several rules must be followed to implement a double-level insulation system.

The insulation barriers are necessary for circuits that fall under safety extra-low voltage (SELV) standards. The user-touchable voltage or safety extra-low voltage (SELV) systems are termed as an electrical system in which the voltage cannot exceed a permissible value under normal conditions that are defined under IEC and EN 60335 standards. These SELV voltages must be non-hazardous. These SELV circuits operate at low power and logic levels, such as ±3.3 to ±24 VDC. Some of the examples of SELV circuits include input/output connectors and cables that are attached to peripheral devices such as printers and keyboards.

Classification of Insulation Types

Insulation types are majorly categorized into five different types, which are termed functional, basic, double, supplementary, and reinforced. Definitions for these insulation types are mentioned in multiple standards and are quite complex in nature. It is important for designers to know all these rules and apply them in the design as per the requirement.

It is highly critical to insulate hazardous voltages from safety extra-low voltage (SELV) circuits. Following insulation types are defined as per international standards:

  1. Functional insulation: This type of insulation ensures proper functionality of the product but it does not guarantee safety protection.
  2. Basic insulation: This provides a single layer of insulation to avoid any harm to the electronic component.
  3. Supplementary insulation: This type of insulation adds an extra layer of protection to the basic insulation to protect it from condensation.
  4. Double insulation: This is a combination of functional, basic and supplementary insulation.
  5. Reinforced insulation: This comes under a single system that provides the same protection as the double insulation.

These safety standards help designers to protect the electronic circuit from an abnormal (single-fault) condition. Single-fault conditions are avoided by implementing double or reinforced insulation, where a second layer remains for protection.

Conductive Anodic Filament (CAF) Failure

Conductive anodic filament (CAF) is the metal filament, which is caused due to the electromigration of copper in a printed circuit board. This further leads to device failure. The growth of CAF bridges two oppositely polarised copper conductors. CAF occurs in four different ways such as through-hole to through-hole, line-to-line, through-hole to the line, and layer-to-layer. CAF majorly takes place due to the two key factors including a test or bias voltage (voltage applied during the testing of the device) and high relative humidity. CAF failure particularly occurs in the hole to hole as showcased in below diagram.

CAF Formation

The key factors influencing CAF growth include electric field strength, temperature rise, humidity, and the type of laminate. The manufacturing defects can also lead to CAF failure. However, CAF failure primarily arises in high-density circuit boards, leading to reduced spacing.

The growth of the metal filament is typically from a copper anode to a copper cathode, which ultimately leads to the electrical failure of the electronic circuit. CAF occurs in two stages: the degradation of the resin glass interface, and an electrochemical migration of copper causing the filament growth.

The degradation of the resin glass interface is a reversible process where the material’s insulation resistance is returned after baking and drying processes. The second step of actual CAF growth is believed to be irreversible. The time required for CAF failure to occur is dependent on test voltage, relative humidity, spacing, temperature rise, and the resin system.

The degradation of the resin glass interface takes as the PCB behaves like a cell, with the occurrence of the following reaction:

At the anode:

Cu -> Cu(n+) + ne(-)

H20-> ½ O2 + 2 H+2e(+)

At the cathode:

H20+ e- -> ½ H2 +2 OH(+)

Spacing for Semiconductor Components

In some cases, where electrical safety and voltage isolation is given high priority, clearance, creepage and isolation distances matter significantly. Spacing between two components, terminals and connectors are well-defined in several international standards. However, we will be dividing spacing into two parts such as:

1) Spacing between the uninsulated live parts and other uninsulated metal parts This includes spacing between terminals and heatsink, chassis, metal boxes, cabinet, etc…

2) Spacing between the uninsulated live parts with opposite polarity. This includes spacing between terminals, adjacent components, connectors, bare wires, etc…

The use of a high-temperature silicone potting on the device terminals allows an increase in spacing between two life parts on the circuit. Automatically, this measure will improve the electrical safety increasing the “spacings” of the terminations offering a higher pollution degree level.

We covered most of the parts. Do you still have any questions (I doubt it!)? Feel free to ask, share and comment.

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