What Is an Annular Ring in Printed Circuit Boards?

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Dec 14, 2021|Technological Advancements and Materials

Annular rings are a vital part of any printed circuit board (PCB), but they pose quite a challenge for PCB designers. With such precise work, it’s easy to make mistakes with drilling the via, which can lead to malfunctioning on the board itself. 

PCB designers have to be careful when calculating size and placement. In this article, we’ll explore everything you need to know about annular rings, the unique challenges they present, and how to get them right.

What Is an Annular Ring in Printed Circuit Boards?

An annular ring is the part of a printed circuit board (PCB) surrounding a finished hole or via. In circuit boards, vias are holes that work as nodes connecting the different layers. The annular ring spans the area between the edge of the via and the copper pad around the hole.

What is the function of annular rings? Annular rings help stabilize the electrical connection between the via and the copper pad. They also give designers a little wiggle room if the hole isn’t perfectly centered – ensuring conductivity.

The Structure of a PCB Annular Ring 

An annular ring is a ring or circle shape. The name comes from the mathematical term “annulus,” which refers to the space between two concentric circles. The annular ring surrounds a hole in the copper plate on a PCB where a via comes through.

On a multi-layer circuit board, you need to ensure that the layers connect seamlessly with the copper conductors. The copper-plated via conducts through the layers. The annular ring is the copper circle around the via at each layer.

Annular rings can be different sizes, and generally, a larger annular ring is a bit safer in protecting from breakouts. 

Before we get to issues like breakouts, let’s talk about annular ring PCB sizes.

How Do You Calculate the Annular Size?

When constructing an annular ring, the goal is to drill right through the center. The more accurate that hole is, the more seamlessly electricity conducts through. 

To get the correct width for an annular ring, use a simple equation:

• Width of Annular Ring = (Copper Pad Diameter – Diameter of Finished Hole/Via) / 2

So you take the diameter of the copper pad, which is the outer circle, and subtract the diameter of the via, which is the inner circle. Take that number and divide it in half to get the perfect width of your annular ring.

Why Does the Annular Ring Size Matter?

The size of the annular ring matters because it determines the success of your conductivity. While you don’t want it to be too wide since you’re working with a tiny space, you do need it to be wide enough to keep the connection strong. 

How to Get the Annular Ring Size Right

Each PCB manufacturer will stipulate the tolerance for the annular ring through the PCB manufacturing process. However, to be on the safe side, you should always add a little more space. 

Most designers recommend increasing the size by 1.0mm from the manufacturer’s specifications. The slight extra width will help prevent any issues later on. 

The size of your annular rings will also depend on what you’re designing the board for. The Institute of Printed Circuit Boards specifies three categories for PCBs: Class 1, Class 2, and Class 3. Heavy-duty PCBs need larger annular rings because the via will be wider in diameter. Class 3 PCBs require more heavy-duty annular rings.

What Is the Minimum Annular Ring in PCB?

While some PCBs support heavy-duty equipment and spaces, others must be extremely compact. Think of PCBs in smartphones or small technological equipment. 

In this case, the minimum annular ring has to be correspondingly tiny. But just how small can we go?

The design standard across the industry sets a different limit for inner and outer annular rings. 

• For annular rings on the inner layer of a PCB, the minimum width is 0.1mm.
• For annular rings on the outer layer of a PCB, the minimum width is 0.05mm.

You can learn more about design limits on Candor. 

What Is the Difference Between a PCB Annular Ring and a Through-Hole?

Whenever you hear the term PCB, you probably think of the through-hole method. It is a technique used to mount electronic components. The electronic components insert into holes drilled in a PCB.

Through-holes are a type of via, but not all vias are through-holes. Through-holes use leads inserted into the hole and then soldered onto the copper pad to create conductivity.

On the other hand, Annular rings create conductivity by directly connecting the copper-plated via to the copper pad.

What Is the Teardrop Annular Ring?

A teardrop annular ring is teardrop-shaped. Instead of designing a perfect circle, you add extra copper to connect the ring’s outer edges. The additional copper gives the ring some added strength.

The main reason to use 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. 

Teardrop annular rings can:

• Reduce the risk of cracking on thin copper
• Reduce mechanical or thermal stress
• Protect small misalignments in the drill hole

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.

Undesired Annular Ring 

On smaller PCBs, an annular ring that is too large may inhibit the board’s function. Particularly if the copper pad is small, an annular ring may cause issues. 

The simplest solution to this problem is to ensure you work with prototypes when designing and creating a PCB.

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.

Breakouts happen most often when the layers of the board don’t match well, so the hole doesn’t align through every layer. 

Again, the best way to prevent breakouts is to ensure that the annular ring is wide enough to accommodate the hole.

Rupture

Annular rings – like 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. 

Final Thoughts

Annular rings are a crucial part of any printed circuit board, but they can be tricky to get right. Use our guide above to learn the ins and outs of annular rings and how to design them.