Guideline

Annular Ring in PCB: Key Considerations for Design and Manufacture

Annular ring PCB

What is an Annular Ring?

Generally, PCBs are found in single layer, double layer and multiple layers. In PCB there are copper traces that connect one end of any component to the other end of any component through vias. Also, in multiple-layer PCBs, the connection of any layer to other layers is made through vias. So, the vias are very crucial in printed circuit boards. If the vias are required to connect anything in PCB, vias should be strong enough in conductivity. To maintain strong conductivity in the printed circuit boards, vias have copper around the holes. This copper around the holes is called the Annular ring. An annular ring is very important in maintain the conductivity of vias. To understand the annular ring in a printed circuit board, please see the image below.

In the above image, there is a round black portion provided which is the finished hole diameter and the golden area is the pad diameter. The distance between the outer edge of the hole and the outer edge of the copper pad is called the Annular ring. Generally, annular ring PCBs start from 1 to 2 mil to 10 mil and more, so the minimum annular ring is 1 mil. It looks like the donut shape which you see from the top of the PCB. The annular ring PCB has different widths depending on the different PCB criteria. Circuit connections are drastically affected if the annular ring has any issues.

What is a Good or a Bad Annular Ring?

If you want the perfect via annular ring, you should drill the hole exactly to the center of the copper pad. But while manufacturing it is very critical to drill at exactly the middle of the copper pad. To eliminate this issue, manufacturers always make it intolerant. Below are some examples of good and bad annular ring PCB.

Good Annular ring

A good annular ring PCB was found when the hole exactly hit the middle of the copper pad. So, the vias are getting a full copper connection.

Tangency

Tangency is found when the hole hits slightly off from the middle of the copper pad and the outer edge of the hole hits the outer edge of the copper pad. This happens to be called tangency.

Breakout

Breakout occurs when the hole exceeds the outer edge of the copper pad and hits partially on the copper pad. These types of annular ring PCBs cause the failure of annular ring PCBs or short circuits.

We can better understand all three circumstances by the below image which has good vias, bad vias (tangency), and breakout.

Good and bad annular rings

IPC Standard of Annular Ring in PCB Design and Manufacture

Any electronics manufacturing is required to follow the IPC standard. IPC is the organization that started in 1957 as the Institute of the printed circuit board. Then the name was changed to the Institute for Interconnecting and Packaging Electronic Circuits due to the expansion of packaging and electronic assembly.

The IPC has made many standards for electronic manufacturing. Any electronics manufacturer should follow the instructions from the IPC. The IPC standard of annular ring PCB is defined by different classes, in fact, all the manufacturing processes are defined by different classes like class 1, class 2 and class 3. We will discuss all three classes of IPC standard in this article which is stated below.

Class 1 Annular Ring

Class 1 criteria are the easiest criteria for the manufacturer as class 1 boards do not require meticulous inspection. For example, if any customer wants to produce a circuit board with class 1 criteria then the cost of manufacturing the class 1 board is very less. And if the manufacturer made any mistake like tangency in the annular ring PCB or did a bad annular ring PCB, etched off extra copper, then there is no issue as the boards of class 1 are not high-density or complex boards.

Via annular ring

Class 2 Annular Ring

The most usable and famous class of IPC standard is Class 2 IPC standard of electronic manufacturing. Most of the electronic boards are manufactured in this class.

As we know this class sits in the middle of all three classes, and the manufacturing criteria also sit in the middle. For example, if we manufacture a board with class 1 criteria then we should maintain an annular ring PCB on PTH holes 1 to 2 mil and as we know the less annular ring PCB increases the chances of getting tangency and breakout type problems, but in class 1 boards these things are acceptable. For these reasons, the cheaper and less functional boards are manufactured with class 1 requirements.

Now if we talk about class 2 criteria for PCB manufacturing, we should maintain an annular ring PCB of 3 to 5 mil. These ranges of via annular rings are the middle range of via annular rings which decrease the chances of getting tangency and breakout. In this criteria of IPC class the customer sometimes accepts the tangency but the breakout is not acceptable by the customer as these boards are sometimes high-density interconnect or radio frequency boards.

So if we see the highlights of class 1 and class 2 then we know class 1 has 1 to 2 mil minimum Annular ring and class 2 criteria have a minimum 3 to 5 mil annular ring PCB. Now the other IPC standard of annular ring PCB is class 3 which is the most expensive and accurate class of all three classes. The details of class 3 boards are stated below.

Class 3 Annular Ring

The toughest and most complicated criteria of the IPC standard is the class 3 criteria. Whenever the boards required precision and accuracy the customer chose the class 3 criteria for manufacturing.

Mostly the class 3 boards are designed and fabricated in the field of medical, aerospace and military applications. Board designers and manufacturers must follow the standards of IPC like IPC-6012 and IPC-A-610 to produce the class 3 boards. The different IPC standards are applied to different applications, like space and aerospace use IPC-6012 ES, medtech devices use IPC-6012 EM and automotive industries use IPC-6012 EA.

Basically, class 3 stands for long-term board reliability and no issue for the long term. So to achieve long-term reliability the manufacturer should maintain some standards. For example, class 3 boards must have high TG material, sometimes heavy copper weight and the main purpose is to maintain a greater annular ring PCB which cannot have tangency or breakout. As we see in class 1 that tangency and sometimes breakout allowed in class 1. Then we also see that class 2 only allows some tangency but not breakout. Whereas the IPC class 3 annular ring doesn’t allow the tangency or breakout. IPC class 3 annular ring wants precision in maintaining the annular ring PCB. The minimum Annular ring of class 3 should be between 3 to 10 mil minimum. The manufacturer charged almost double for class 3 boards as class 3 boards required more time and precision in manufacturing.

 

As per above all the sentences we can understand the importance of annular ring PCBs. As we discussed above there are many differences between class 1, class 2 and class 3 IPC standards. Also, we see the annular ring PCB criteria of class 1, class 2 and class 3 boards.

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