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What Are PCB Annular Rings and How Do They Work?

pcb annular ring

During the PCB manufacturing process, you are required to drill holes in your board for installation, through-hole technique, or as vias for multilayer circuits. For through-hole technology and vias that link traces, the goal of the drilling exercise is to percolate a hole through the bullseye of a copper pad. Since these pads are circular-shaped, the pad area left after boring is known as an annular ring.

When drilling holes during PCB fabrication, precision is an essential factor even in the international PCB manufacturing standards. Besides, it is necessary for successful PCB production and functioning. This article takes a closer look at the important factors to consider when designing annular rings and how these rings work.

1. What is a PCB Annular Ring?

A PCB annular ring is the area of the copper pad surrounding a drilled hole, usually a through-hole via. The region surrounding this via should contain adequate copper to form solid connections between the via and the copper traces.

annular ring size

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Therefore, the primary function of an annular ring is to create solid connections, particularly in multilayer circuits. In a layman language, annular rings act as PCB anchors. The minimum annular ring diameter is determined by the PCB manufacturer’s assembling capacities, which differ based on the nature of holes and whether they are plated or not.

2. The Structure of a PCB Annular Ring

In math, an annulus is a ring-shaped object. It is an area surrounded by two concentric circles. In PCB, an annular ring is a difference that lies between two concentric circles. This ring is mostly circular.

PCB Annular Ring Structure

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It is essential to note that the higher the withering capacity of an annular ring, the better the drill hole’s copper connection. To minimize breakouts, you should raise the size of your annular rings.

3. The Role of PCB Annular Rings

Annular rings are integral elements of a PCB as they create room for solid electrical connections on a board. Without these rings, you will be unable to create reliable connections on your PCB. Issues related to annular rings are problematic and significantly affect the trace continuity and the functionality of the entire board.

Unfortunately, some designers ignore the importance of annular rings by creating distorted shapes. When designing annular rings, you should be accurate to avoid drilling off the mark. By doing so, you will eliminate the annular ring tangency.

4. Annular Rings in High-Speed Circuits

The annular rings in a via create a parallel plate capacitor, which affects the via impedance. This implies that more rings along the via barrel, either functional or non-functional, raise the parasitic capacitance of the via. Furthermore, this will decrease the via’s impedance and move it closer to the trace impedance. Therefore, you should carry out impedance matching on primary traces and components.

Any non-functioning stub available at the via end contains a set of resonance frequencies as high-speed digital signals and high-frequency AC signals spread through the via. Since more non-functional pads along the via barrel raise the PCB capacitance, they reduce the quarter wavelength resonance frequency of the via stub. This further intensifies the via insertion loss and lowers the signal quality.

The best strategy of preventing via stub resonance is back-drilling the stub to achieve the smallest size. You can then set the lowest order resonance frequency of the via stub higher than the signal working frequency. This will subdue the via stub resonance. Non-functional pads along the barrel minimize the lowest order resonance frequency and eliminate the standard method used to prevent insertion loss issues.

5. How to Calculate the PCB Annular Ring

While manufacturers are responsible for producing accurate annular designs, designers also have their role in creating the correct size designs. Incorrect ring sizes will negatively affect the functionality of the whole PCB. Therefore, you should state the appropriate annular ring sizes to obtain a quality and functional product.

Calculating the PCB annular ring is not a simple task. The correct annular ring width is the difference between the finished aperture’s diameter and the copper pad divided by two. Assuming the pad diameter is 40 mils, and the hole diameter is 10 mils, then the annular ring is calculated as shown below:

(40 -10)/2 = 15 mils.

5.1 Determine the Size According to Your Application

You can use PCBs in various applications, like home appliances, military, and medical equipment – based on their thermal capacities and insulating properties. Therefore, when calculating the annular ring of your PCB, you should consider its application.

If your PCB is for special applications, like the military, it may require high-quality PCB annular rings. PCBs are classified into three classes: levels 1, 2, and 3. Level 3 boards are used in heavy applications; hence, Class 3 annular ring requirements differ from Class 1 and 2. Their pad size is a bit large to house the vias’ diameter.

5.2 Design Criteria of PCB Annular Ring

An incorrect annular ring design criteria result in a PCB with low functionality. Therefore, it is vital to have everything right from the start. Basically, the right annular ring is made by drilling the center of the via pad accurately. In the real world, the drilling accuracy is determined by the manufacturing equipment.

Most manufactures have set their tolerances for designing annular rings, while designers like specifying annular rings at the dead-center of the board. This ensures good connectivity between the layers and the vias.

Therefore, it is important to establish the right setting of your annular ring during the design stage. For compact designs, try as much as possible to specify smaller annular ring sizes for your pad to take up small spaces.

5.3 PCB Annular Ring Calculation Formula

The PCB annular ring calculation formula is not complicated as some people think. Unfortunately, few designers are aware of using it. To make vias, the drilled holes need copper surrounding them. The copper ring is the via size/the outer diameter. According to IPC-2221A standards, this is how you should calculate the minimum PCB annular ring size:

Minimum Annular Ring = (Min Ringer Border) * 2 + (Tolerance) + (Hole Size)

The annular PCB ring is (the pad diameter – the hole)/2. Assuming that your pad diameter is 30 mils and hole diameter is 20 mils, your annular ring width will be 30 – 20 = 10 mils.

5.4 Minimum Annular Ring

The internal and external PCB layers have their own minimum annular rings. Generally, 0.1 mm is the standard minimum annular ring for inner layers, while 0.05 mm is the standard minimum annular ring for external layers.

However, you can create small annular rings based on the surrounding elements’ tight spacing. Remember, small rings increase the likelihood of drilled vias being close to the edges of the rings. In some cases, the via may break out of the annular ring, causing a broken trace connection. Besides, it can lead to a fragile relationship and hinder the PCB’s functionality. Generally, the smallest annular ring is -1 mil. However, it is not recommended as it increases the chances of a breakthrough.

6. The Difference Between an Annular Ring and a Though-Hole

Many people confuse the PCB annular ring with a through-hole via. However, the two differ a lot. The region between the copper pad and the edge of the finished via is known as an annular ring. As mentioned earlier, an annular ring with a large width offers solid connections for the vias. In many cases, the annular rings of most PCBs have the exact width measurement.

On the other hand, a through-hole is a mounting method applicable for electronic parts. It entails the application of leads on materials put into drilled PCB holes. This method is suitable for PCB hobbyists as it is easy to use and has minimal accidental faults.

7. What is a Teardrop Annular Ring?

The teardrop annular rings were named so because of their shape – a dropping tear. The shape maintains structural integrity against mechanical stress. You can use a teardrop pad to add more copper at the junction of annular rings.

Teardrop Annular Ring

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During via drilling, you may misalign the drill bit, which extracts a lot of copper from the junctions of your via pad and PCB trace leading to broken traces or thin connections. In this case, a teardrop shape can raise the drill tolerance. This will further minimize the rejection count in the PCB production process.

The need for teardrops increases as the traces become small. It is important to note that teardrop rings are not needed in conductors bigger than 20 mils. Teardrops are suitable for flex circuit boards because of their structural reliability against shear and vibrations. There are numerous benefits of using teardrops:

  • They minimize mechanical and thermal stress where traces connect with pads, leading to fewer hairline cracks in paths.
  • They increase the manufacturer tolerance of the drill to pad, making PCB manufacturing easy and minimizing rejected circuits during production.
  • They minimize the cracking risk.
  • They enhance resistance to heat shock, mainly hindering rework and wave soldering during production.

Teardrops also improve resistance to impact shearing.

8. Common Problems of PCB Annular Rings

There are various PCB annular ring issues you should be aware of as you will face them in your PCB designing projects. These issues are common because manufacturers use different materials, personnel, and even design approaches. There are several annular ring problems, but we will address only three of them in this article.

8.1 Undesired Annular Ring

An unwanted annular ring is a standard PCB problem that affects many designers. If the pad size is small, the PCB may not function because the drill hole will occupy more space on the pads. To avoid this problem, you should order prototypes before embarking on an entire production run. Prototypes prevent the occurrence of unwanted annular rings before the mass production of boards.

8.2 Tangency

Tangency is another main PCB annular ring problem that most designers encounter. Tangency arises when a drill misses the desired mark, and the spot touches the edge of the place. If the drilling diverges further, it will lead to breakouts. To prevent the formation of tangencies, you must drill the holes accurately.

8.3 Breakout

Annular ring breakouts occur when the drilled via gets a wrong encompassing pad. Most breakouts take place when the lamination process displaces some circuit components from their right locations. If you want to avoid this, ensure you assign a large width to your annular rings.

Conclusion

An annular ring is a copper pad surrounding a via. Basically, it is a copper pad remnant when you bore a hole through it. This ring significantly influences PCB functionality. It facilitates solid connections between the via and the traces. Without it, you would simply have a via that cannot conduct signals.

Are you in need of high-quality printed circuit boards with the correct annular rings? If so, you are at the right place! MKTPCB manufactures some of the finest boards with the recommended annular ring width. Our products are rarely affected by tangency and breakouts issues. Contact us today to place your order or learn more about our products and services.

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