Printed Circuit Boards (PCBs) are the backbone that allows all electronic components to communicate seamlessly. Almost every system relies on flawless board-level signal transmission to function correctly. However, PCB design can be daunting, considering many specifications and parameters. From component placement and routing to ensuring environmental resilience and manufacturability - getting it right the first time is crucial.

At the heart of any PCB design is the choice of IPC class, which determines clearances and spacing standards. Choosing a suitable class that balances functionality, reliability, and costs can make or break a project.

Read this ultimate guide that cuts through the clutter, highlighting the key differences between IPC classes.

What is an IPC Class?

So what does the IPC class mean? IPC class refers to the standardized classification system developed by the IPC (Institute for Printed Circuits) to categorize printed circuit boards (PCBs) based on their quality levels and manufacturing capabilities. The IPC developed this classification system in the 1970s to establish common industry standards and language regarding PCB quality and complexity. 

There are three IPC class designations from Class 1 to Class 3. Class 1 PCBs have the simplest design with the fewest layers and strictest tolerance levels. As the class number increases, the board design increases in complexity with more layers, tighter tolerances, smaller circuit features, and increased density of components. Class 3 boards require the most advanced manufacturing technologies and processes, with micrometer-level tolerances and the ability to place microscopic components at high densities.

Adopting IPC class standards is vital for both PCB manufacturers and buyers. It provides a common and consistent way to specify a board design's quality level and complexity. This ensures the manufacturer has the necessary capabilities and processes to produce the board that meets design requirements accurately.

For buyers, it establishes consistent expectations of quality. The classification also allows rough cost estimates as higher classes typically correspond to more complex designs and higher manufacturing costs.

IPC Class 1: General Electronics Products

The IPC class 1 electronics classification, also called "general electronics," consists of boards designed for low-cost, short-lifespan products. These circuits have the most lenient quality control standards compared to other classes.

Think of the electronic components found in single-use gadgets, like a discounted brand electric toothbrush or novelty greeting cards that play short music clips. While entertaining for their intended brief usage period, nobody expects these disposable devices to retain full functionality over extended timeframes. Their cheap components are only engineered to last as long as the perceived value of the product.

This classification represents the low-end of the electronics market. Circuitry designs and manufacturing processes tend to cut corners to maximize profit margins. It leads to deprioritization of features like tight component tolerances, rigorous testing standards, and robust materials. As a result, general electronic goods often showcase reduced reliability and durability compared to higher classes.

Some major electronics contract manufacturers have opted out of class one production altogether. Most large firms focus solely on classes two and three due to their clientele's demands for long-term quality and durability in mission-critical applications. Specializing in meticulous board design and assembly geared for resilience fulfills customer needs better than low-budget workmanship.

IPC Class 2: Dedicated Service Electronics Products

IPC Class 2 standards encompass a wide range of electronic devices and systems intended for applications where people expect continuous operation but can tolerate temporary outages. Reliability over the product's lifespan is important, though the consequences of an unexpected failure do not pose severe risks to personnel or critical infrastructure.

Some common IPC Class 2 product types include:

● Industrial control systems
●  Automation controllers
●  Commercial HVAC equipment
● Monitoring sensors
● Test and measurement instruments
● Heavy machinery displays and HMI interfaces
● Communication radios for non-essential data transmission.

While downtime for Class 2 electronics would lead to economic inefficiencies or work stoppages, safety is not compromised.

Manufacturers of Class 2 electronics employ stringent design and production methods to maximize service lifetimes under typical operating conditions. Component selection involves screening for tolerance to heat, vibration, power fluctuations, and corrosion/chemical exposure. Circuit boards use heavy copper traces bonded securely with high-quality solder.

Conformal coating seals boards against moisture intrusion. Robust mechanical structures feature metal chassis, vibration damping, and gaskets/seals to protect internal electronics from dust, debris, and temperature extremes. Connectors withstand high mating cycles under heavy loads. Rigorous testing verifies all assembly processes meet IPC quality standards.

Electronic and software safeguards allow Class 2 systems to operate reliably even if environmental insults cause component faults over time. Redundant processors, error-checking memory, watchdog timers, and configurable firmware/parameter settings facilitate remote recovery from unforeseen issues.

Considering the ruggedness level during initial design avoids costly product reengineering later. Class 2 specifications influence enclosure strength/size, power supply ratings, interface robustness, installed component specifications, and failure logging/historian capabilities. Strategic component selection balances durability with cost versus higher IPC classes.

Adhering to IPC-A-610 acceptability standards throughout the assembly process helps attain the long service life required of Class 2 products operating for a decade or more in industrial environments. With meticulous manufacturing according to these rigorous specifications, Class 2 electronics deliver years of dependable functionality.

IPC Class 3: High-reliability Electronics Products

IPC Class 3 pertains to electronic products and assemblies that require a high degree of reliability based on their intended use. Class 3 products are expected to function without failure over long periods, often many years, in applications where failure could result in hazardous or safety-critical situations. Due to the critical nature of Class 3 products, their manufacture is held to the strictest quality control and process standards.

Some of the IPC class 3 products include:

● Medical devices like pacemakers, defibrillators, implanted devices, imaging equipment, and life-support systems
● Avionics and flight control components in aircraft and spacecraft
● Engine control modules and collision avoidance systems for vehicles
● Military and defense electronics
● Radiation monitoring devices for nuclear plants, labs, hospitals
● Fire/smoke alarm and security systems for buildings
● Banking/transactions infrastructure for ATMs, POS devices, financial switches

Among the key characteristics that define Class 3 is the intended operation in safety-critical applications such as medical devices, aviation/aerospace, military, and defense systems. These applications have zero tolerance for product failures. Class 3 products are also expected to demonstrate long operational lifetimes, often 5-10 years or more before replacement is needed. They must maintain full functionality over a wide range of environmental conditions, such as temperature extremes, humidity, vibration, dust, and chemical exposure.

The manufacturing requirements for Class 3 are highly rigorous. All materials used must be tested and qualified to ensure long-term reliability under all anticipated operating conditions. Processes such as assembly, soldering, and conformal coating must be carefully controlled and validated. Statistical process control is used to monitor all key parameters and maintain processes within tight specification limits.

Components selected for Class 3 products undergo the following extensive screening:

● High acceleration life testing
● High/low-temperature operating life
● Thermal shock
●  Humidity exposure testing

Only parts that pass all tests with a high yield rate are approved for use. Additional testing on assemblies and finished products includes highly accelerated stress testing using temperature, vibration, power input, and other variables to simulate aging over the expected lifetime.

Class 3 products have zero tolerance for defects and require 100% inspection at various stages using techniques like automated optical inspection, X-ray inspection, and scanning electron microscopy. Final testing subjects finished units to environmental stress screening as well as functional testing under limits of all anticipated operating parameters. Only products that pass all tests and inspection steps are approved for shipment to customers.

Extensive documentation of all design, manufacturing, and test data is needed to comply with the stringent quality requirements of Class 3. Traceability of all materials and components is maintained using techniques like serialization. Products are also assigned long-term monitoring and record keeping for potential full product recalls if low yield issues emerge over time. The quality assurance costs involved in meeting Class 3 standards are significantly higher than other classes but are necessary for applications involving critical safety and reliability needs.

General Differences Between Class 2 and Class 3

Class 2 and Class 3 are two different classes of PCBs defined by the IPC based on their quality and performance requirements. Here are some general differences between IPC class 2 and 3. 

Applications

IPC Class 2 is intended for general commercial electronic products like computers and consumer devices. It allows smaller component spacing and thinner traces and clearances compared to Class 3.

Class 3 is meant for longer lifetime, industrial applications such as military, aerospace, automotive, and medical equipment that must withstand more extreme environments and have longer operational life expectancies.

Surface Mount Components

Class 2 permits smaller surface mount component spacing and pitch. The minimum clearance between pads is only 4 mils compared to 6 mils for Class 3. This allows greater surface mount component density for Class 2 designs.

However, the tradeoff is that close clearances make Class 2 designs more prone to issues from temperature cycling over time in harsh environments.

Annular Ring Breakout

IPC Class 2 allows for 90% annular ring breakout, provided the minimum literal spacing is maintained. This means connections can utilize almost the entire area within the annular ring, enabling very dense packaging.

In contrast, IPC Class 3 does not permit any annular ring breakout. It requires the full ring area to remain unplated as an insurance buffer against manufacturing variations.

So, while Class 2 offers design flexibility for minimal spacings, Class 3 prioritizes reliability over a long lifespan by eliminating breakouts within the annular ring.

Minimum Copper Wrap Requirements

For buried vias and blind and buried plated-through holes, Class 2 requires 100 mils of conductive wrap around the vertical wall, and Class 3 necessitates 150 mils.

The lesser copper wrap of Class 2 again allows for reduced hole diameters and tighter spacing, while Class 3 maintains conductivity and strength even if some wrap is etched away over decades of use and environmental exposure.

Class 2 vs Class 3: Differences in PCB Manufacturing

When designing a PCB, it is important to specify the appropriate IPC class based on the application's requirements. As we have already seen, IPC classes 2 and 3 are commonly used for general-purpose rigid PCBs. Despite the similarities, there are some key differences in their manufacturing specifications.

Annular Ring and Drill Breakout

The annular ring size and drill breakout allowance acceptance criteria differ between IPC class 2 and 3 printed circuit boards. Class 3 boards must adhere to more stringent standards compared to class 2 boards to ensure greater reliability and enhanced durability over the lifespan of the product.

The table below provides a summary of the IPC annular ring specifications that each class must meet:

Feature	IPC Class 2	IPC Class 3
Minimum annular ring size (external)	0.05 mm (0.002 in)	0.076 mm (0.003 in)
Minimum annular ring size (internal)	0.05 mm (0.002 in)	0.051 mm (0.002 in)
Maximum drill breakout (external)	90° or less	None
Maximum dirll breakout (internal)	Any angle	None

Design Rules of Annular Rings

The design of annular rings depends on multiple factors related to the PCB and the drilled hole. These key considerations include the drill diameter, pad size, copper thickness, and aspect ratio.

The drill diameter refers to the size of the hole drilled through the board. The pad size is the circumference of the copper surrounding the hole. Copper thickness is measured in ounces per square foot and indicates how much copper is layered on the board. The aspect ratio compares the hole depth to its diameter.

To ensure a suitable annular ring width and prevent drilled edges from cracking, the pad must be larger than the drill hole. The minimum distance between these is called the annular ring allowance. This value varies according to IPC class standards and copper weight.

The following table shows some examples of design rules for annular rings based on IPC class 2 and class 3 standards for 1/2 oz copper:

Feature	IPC Class 2 Drill & Pad Diameter for 1/2 oz Copper	IPC Class 3 Drill & Pad Diameter for 1/2 oz Copper
Minimum drill size	0.25 mm (0.010 in)	0.25 mm (0.010 in)
Maximum pad diameter (external)	Drill size + 0.10 mm (0.004 in)	Drill size + 0.152 mm (0.006 in)
Maximum pad diameter (internal)	Drill size + 0.10 mm (0.004 in)	Drill size + 0.102 mm (0.004 in)

PCB Dielectric Requirements

The dielectric material between the conductive layers of a printed circuit board (PCB) plays a crucial role in its overall performance. It affects electrical characteristics like signal integrity, as well as physical properties such as heat dissipation and resistance to thermal stresses.

The specifications for the dielectric are separated into IPC classes 2 and 3, with class 3 boards having more stringent requirements. These classes differ on key dielectric metrics such as:

● Minimum thickness
● Permittivity
● Loss tangent
● Thermal expansion
● Glass transition temperature
● Decomposition temperature
● Moisture absorption.

For instance, class 3 boards demand a dielectric thickness of at least 50 microns versus 38 microns for class 2. Their permittivity and loss tangent must also be lower, while characteristics like glass transition and decomposition temperatures are set higher.

PCB Through-Hole Plating Requirements

The through-hole plating plays a vital role in circuit boards. It refers to the thin layer of copper deposited within the drilled holes to provide electrical connectivity between the different layers.

IPC Class 2 and 3 set requirements around thickness, coverage, and defects to ensure that through-hole plating performs well. Class 3 boards have higher through-hole plating requirements than Class 2 since they demand higher conductivity and durability.

For example, while Class 2 allows a minimum plating thickness of 20 microns, Class 3 bumps this up to 25 microns. Other factors, like coverage percentages, are also tighter for Class 3. This helps Class 3 boards sustain performance and last longer by reducing issues over time from cracks, voids, or separation in the through-hole plating.

Class 2 vs Class 3: Differences in PCB Assembly

PCB assembly is the process of attaching electronic components to a printed circuit board (PCB) using soldering techniques. Depending on the intended use and reliability of the PCB, different standards and specifications may apply. These are some differences between Class 2 and Class 3 PCBs regarding PCB assembly.

Surface Mount

Surface mount technology directly attaches components to the surface of printed circuit boards without through holes. This allows for smaller, more compact components and higher density board assembly at lower cost than through-hole. However, precise placement and soldering are needed for reliable electrical connections.

The IPC categorizes surface mount components into fine pitch (lead spacing ≤ 0.8mm but > 0.5mm), very fine pitch (≤ 0.5mm but > 0.3mm), and ultra-fine pitch (≤ 0.3mm). These requirements vary by class.

Class 2 allows more lead overhang/fillet height for fine/very fine pitch, while Class 3 specifies minimums like 75% lead height or 0.25mm fillet height.

Class 2 also permits a minimum solder joint width of 50% land width or 0.15mm for ultra-fine pitch, versus Class 3's 75% land width or 0.25mm minimum.

Amount of Barrel Fill

Barrel fill measures how much solder fills the holes of plated-through hole (PTH) components attached to a printed circuit board (PCB). PTH components have leads inserted into copper-plated holes to make electrical connections. The amount of barrel fill affects the mechanical strength and conductivity of the solder joint.

The IPC specifies barrel fill criteria depending on the class of PCB and type of PTH component. For single-sided components, Class 2 PCBs require a minimum 50% barrel fill, while Class 3 needs 75%.

Double-sided components on Class 2 PCBs require 75% minimum barrel fill, and Class 3 boards need 100% fill. Blind or buried vias, which connect internal layers but aren't visible on the surface, must have at least 50% fill for Class 2 and 75% for Class 3. Proper barrel fill is important for reliability.

Conclusion

Carefully selecting the optimum IPC class is crucial when designing PCBs. Following the guidelines outlined in this guide will allow you to choose the class requirements suitable for your specific design goals, component sizes, and manufacturing processes. Whether transitioning to a higher or lower IPC class, considering factors like trace width, via size, spacing, and drill size is vital to meet reliability and manufacturability needs.

For turnkey PCB manufacturing services that complete the process from design to assembly, contact PCBasic. As a leading PCB &PCBA manufacturer, we have the expertise to help evaluate your board's IPC class compliance and produce high-quality prototypes and production quantities. Our quality control procedures ensure all IPC standards are strictly followed. Contact us today to discuss partnering with us on your next project.