Understanding the Importance of Annular Rings in PCB Design

Annular rings play a crucial role in printed circuit board (PCB) design, ensuring the proper electrical connection of two pads. The signal and current flow are characterized based on the placement of annular rings between two layers. Incorrect calculation of size and improper placement can lead to transient behavior and flicker noise in electronic components. In this article, we will consider all important steps to improve our design from annular ring issues, tangency, and breakout.

An annular ring in a printed circuit board (PCB) refers to the copper ring that surrounds a hole drilled into the PCB. Due to the complexity of the design, PCB designers often have to use multilayer stack PCBs. To connect the track pads of these two layers, a small hole is drilled through the pad known as a “via.” This ring is part of the via and is essential for ensuring a good electrical connection between the copper pads of two different layers. For more clarity, we can define it as a cylindrical copper attachment inserted through the via for the connection of two layers. The size and electrical properties are determined by the PCB manufacturer's capabilities.

Structure of an Annular Ring

The structure of an annular ring in a PCB can be understood by breaking it down into its main components and how it interacts with the rest of the PCB. Here's a detailed look in 3D:

• Pad: The pad is the area of copper on the PCB where the annular ring is formed. It serves as the landing area for the component lead or via.
• Hole (Via or Through-Hole): The hole is drilled through the pad and the PCB. This can be either a via (for inter-layer connections) or a through-hole (for through-hole components). The hole is typically plated with copper to create a conductive path.
• Annular Ring: The annular ring is the ring-shaped area of copper surrounding the drilled hole. It is part of the pad and ensures a good electrical connection.

Why Annular Ring Size Is an Important Parameter

The size of the annular ring in a PCB is critical for ensuring reliable electrical connections and mechanical stability. An adequate annular ring width is essential for creating robust solder joints, preventing breakout during drilling, and reducing the risk of pad lifting. It directly impacts solder joint reliability, conductivity, and mechanical strength, particularly in through-hole vias.

Properly sized annular rings accommodate manufacturing tolerances, ensuring uniform plating coverage and avoiding issues like voids or thin copper layers. Compliance with industry standards, such as IPC guidelines, helps maintain high-quality PCB fabrication and enhances overall reliability. In summary, the size of the annular ring plays a vital role in the performance, manufacturability, and long-term reliability of printed circuit boards. Placing the ring without proper calculation may lead to undesirable circuit operation.

Sizing Parameters of Annular Rings

The sizing parameters of an annular ring include Outer Diameter (OD), Drilled Hole Diameter (D), and Annular Ring Width (W). The outer diameter is the overall diameter of the pad, including the annular ring and the hole. Drilled Hole Diameter is the diameter of the hole drilled through the PCB. This hole is later plated with copper to form a conductive path. Annular Ring Width is the radial distance between the edge of the drilled hole and the outer edge of the pad. These three parameters form the equation to calculate the size of an annular ring.

Size Calculation of Annular Rings

A general formula is used to calculate the annular ring which includes Outer Diameter (OD), Drilled Hole Diameter (D), and Annular Ring Width (W) and is given as:

Annular Ring Width (W)=(Outer Diameter (OD) - Drilled Hole Diameter (D)) / 2    

The formula is good to go with because the sizes of pads and drilled holes are already optimized according to the max current consumption of the PCB circuit. See the design example given here:

Design Considerations

• Width: The width of the annular ring is the distance between the edge of the drilled hole and the edge of the copper pad. It needs to be wide enough to ensure a good connection but also small enough to fit the PCB design constraints. The best possible solution is to follow industry standards such as IPC-2221, which specify minimum annular ring widths. Ensure design and manufacturing processes accommodate necessary tolerances.
• Tolerances: During the PCB manufacturing process, there are tolerances for hole positioning and drilling. Therefore, the annular ring must be designed with these tolerances in mind to ensure that even with minor positional errors, there is still enough copper around the hole to maintain a connection. The design considerations are listed in the capabilities section of a manufacturer, which gives an intuition about the maximum and minimum ratio of tolerances.

What Is the Teardrop Annular Ring?

A teardrop annular ring is a very popular and specialized design feature in PCBs where the trace widens as it approaches a drilled hole or via, resembling a teardrop shape. Instead of designing a perfect circle, you add an extra copper polygon plane to connect the ring’s outer edges. The additional copper gives the ring added strength and rigidity, reduces the risk of cracking on thin copper, and provides protection against thermal stress.

The main reason for using teardrop annular rings is if the drill bit goes slightly off course during drilling. This problem can remove too much copper and jeopardize the connection. These types of shapes are used with thin copper layer traces and in flex PCBs. They are commonly employed in high-reliability applications where maintaining signal integrity and mechanical stability is critical, such as aerospace, automotive, and medical electronics. However, this approach is not very effective when working with power planes.

Types of Annular Rings Used in PCB Design

Different types of annular rings are used in PCB design to accommodate various manufacturing processes, design requirements, and functional needs. Here's an overview of the main types:

Internal Annular Rings

These are found on the inner layers of a multi-layer PCB. They are critical for connecting different layers through vias. The internal annular rings must be precisely aligned and manufactured to ensure reliable inter-layer connections.

External Annular Rings

These are located on the outer layers of the PCB and are typically visible. They are used for mounting components and connecting the components’ leads to the PCB traces. These rings must withstand soldering processes and provide robust mechanical and electrical connections.

Microvia Annular Rings

Microvias are used in high-density interconnect (HDI) PCBs. They are smaller than traditional vias and typically laser-drilled. The annular rings around microvias need to be very precise to maintain connectivity in the densely packed PCB layout.

Tenting Annular Rings

In tenting, the via hole is covered with a solder mask, leaving the annular ring exposed. This method protects the via from environmental factors and prevents accidental short circuits. Tenting is often used in vias that do not need to be soldered but still require electrical connections.

Common Problems of PCB Annular Rings

As we mentioned at the start, annular rings can be tricky to get right. PCB designers may face any of the following issues with annular rings. PCB annular rings can encounter several common problems that impact the overall reliability and functionality of the printed circuit board. Understanding these issues is crucial for designing and manufacturing robust PCBs. Here are some of the common problems associated with annular rings:

Tangency

A tangency occurs when you accidentally drill the hole off-center. If you are so off-center that the hole touches the edge of the pad, you have a tangency. Another cause of tangency is an annular ring that isn’t wide enough for the hole. A tangency disrupts connectivity and can even become dangerous.

Breakout

A breakout is like the next stage of tangency and happens when the hole pushes outside the edge of the annular ring and the copper pad. It literally breaks out from the pad. Breakouts happen most often when the layers of the board don’t match well, so the hole doesn’t align through every layer. This can lead to unreliable connections or complete loss of electrical connectivity. The best way to prevent breakouts is to ensure that the annular ring is wide enough to accommodate the hole.

Rupture

Annular rings, unlike anything else, can experience stress and deteriorate over time. When an annular ring breaks or cracks, we call it a rupture. Rupture can be caused by weakening over time, mechanical stress, or even environmental stress. The best way to deal with rupture is to use high-quality materials and replace your PCB over time as it ages. This issue totally depends upon the PCB manufacturer.

Pad Lifting

Pad lifting occurs when the pad, along with the annular ring, detaches from the PCB substrate. This can happen due to mechanical stress, thermal cycling, or poor adhesive properties. Solutions include using materials and processes that enhance adhesion, avoiding excessive heat during manufacturing and rework processes, and designing pads and annular rings to distribute stress more evenly.

By addressing these common problems, PCB designers and manufacturers can improve the reliability and performance of the annular rings in their PCBs, leading to better overall product quality.

Final Thoughts

In summary, the annular ring is a key feature in PCB design that ensures reliable electrical and mechanical connections between components and the PCB itself. Its proper design is crucial for functionality, and the design must accommodate manufacturing tolerances and adhere to industry standards to maintain PCB integrity. Understanding the specific needs of your PCB design helps in selecting the appropriate type of annular ring for optimal performance and reliability.