As a novice PCB designer, you may be curious as to what an annular ring is. In addition to this, you may be interested in creating one during the PCB manufacturing process. That is important, particularly if you are creating or modifying your own PCB, and you plan to place any through-hole components in it.
In this guide, we will discuss what an annular ring is and how to calculate its size. Furthermore, we will explore why its size matters and how it relates to other PCB holes. Without further ado…
What is an Annular Ring?
When you drill a hole into a copper PCB pad, the resulting copper surrounding the hole is what we know as annular rings. It essentially acts as the wall of the drilled via. Your annular ring size will depend on the size of the via and the size of the copper pad’s size.
But what is the purpose of the annular rings? As you may or may not know, PCBs are multi-layered. Your circuit board traces will need to connect to various layers of your PCB. They usually achieve this by reaching a pad which routes them through to another layer by a via.
The annular ring surrounding the via facilitates the connection between the trace and the via. What this means is the thicker the annular ring, the stronger the connection. But how thick should the annular rings be, and how do you calculate it?
How do you Measure and Calculate the Annular Ring?
To find an annular ring’s width, you need to use this formula: (pad diameter – via diameter) ÷ 2.
If the diameter of the pad is 25 mils (0.50mm) and the hole/via is 15 mils (0.38mm) then your equation will look like this:
(25 - 15) ÷ 2 = 5 mils
(0.50 – 0.38) ÷ 2 = 0.06mm.
Let us look at further real-world examples. That will include the annular ring's width for standard vias, micro via, and component holes.
Generally, a standard via will feature a hole with a diameter of 0.20mm (7.87 mils). On a 0.40mm (15.74 mils) pad, your final equations will look like this:
(0.40mm – 0.20mm) ÷ 2 = 0.10mm
(15.74 mils – 7.87 mils) ÷ 2 = 3.93 mils
Micro vias are typically smaller than your standard holes. They will have a hole diameter of 0.10mm (3.93 mils). On a pad with a diameter of 0.30mm(11.82mm), your equation will look like this:
(0.30mm – 0.10mm) ÷ 2 = 0.10mm
(11.82 mils – 3.93 mils) ÷ 2 = 3.93 mils
As we discussed earlier, component holes tend to be larger than average vias. On a pad with a 1.20mm (47.24 mils) diameter and a 0.80mm (31.50 mils) hole, your equation will look like this:
(1.20mm – 0.80mm) ÷ 2 = 0.20mm
(47.24 mils – 31.50 mils) ÷ 2 = 7,87 mils
Why Does the Width of The Annular Ring Matter?
As discussed earlier in the guide, the annular ring connects to the traces and the various layers of the PCB. Thus, it should be sizeable enough to support a reliant connection. The thicker the annular ring, the stronger the connection around the via will be. If the annular ring is not thick enough, it may cause connection issues or breaks.
You may have heard the term “teardrop annular ring” before. Teardrop pads feature additional copper near the junction of the annular ring that connects to the PCB traces. They assist in creating a more secure connection that may be more resistant to stress.
The idea here is simple. The more copper around the via, the thicker the annular ring is, and the stronger the signal it can carry to and from the traces.
Via Positioning in an Annular Ring
Not only do your annular rings have to be ample in size, but you need to position the hole (via) correctly. If you have positioned the hole too close to the pad’s boundaries, it could cause a tangency. Additionally, a tangency may also occur if the annular rings are too thin.
Nevertheless, when you drill your hole, you want to drill through the center of the copper pad. That will give you the most structurally efficient annular rings. Drilling too close to the pad’s edges will give you a compromised or bad via, which will effectively end in a tangency.
Drill too far from the middle of the pad could end up in a breakout. This is when the drill offsets the hole and breaks the pad’s boundaries’ continuity, resulting in a structurally insufficient annular ring.
The annular rings structure is largely dependent on where you drill your hole and place it via. It would help if you avoided both a breakout and tangency when you assemble your PCB. Nonetheless, this is how vias and annular rings are related.
Recommended PCB Annular Ring Size
Typically, your annular ring needs to be as large as your heaviest parts or those that you will expose to heavy force. For instance, your switch. Generally, the sweet spot for an annular ring’s diameter is 0.25mm. If you are unsure how big to make your via, you can always fall back on this size.
However, the PCB house will usually specify the minimum size for a via or annular ring. Your annular ring should at least be this size or larger. An annular ring smaller than the stipulated size may result in insufficient space or connectivity for your components.
Depending on whether you will tent, plug, and/or fill you're via, you must consider how much board space you have. Additionally, you need to consider your board’s current capacity. These are all the things you need to assess when you consider your drill size.
It will not hurt to measure the copper pad and then try to make the hole’s diameter at least half the size of the copper pad’s area.
If you have reached this section of the guide, it means you should be able to understand the fundamentals of an annular ring. In the above guide, we discussed what an annular ring is and how to calculate its size. In addition to this, we also explored the relationships that vias have with annular rings and why the size of each matter. As always, we hope you have found this guide to be helpful. Thank you for reading.