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PCB Annular rings

What is Annular Ring?

Annular ring refers to the circular metallic pad on the PCB resembling a doughnut, with an inner hole used for inserting wires or component pins. After soldering, the annular ring establishes an electrical connection with the component pins inserted into its inner hole, thereby enabling electrical connections between various components on the PCB. The design and manufacturing quality of annular rings are crucial to ensuring stable and reliable electrical connections on PCBs.

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How to Calculate the Size of Annular Ring?

Mathematically speaking, the size of an annular ring is equal to half of the difference between the diameter of the pad and the diameter of the hole. For instance, if the diameter of the hole is 10 mils and the diameter of the pad is 24 mils, then the width of the annular ring is [(24-10)/2] = 7 mils.

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Why Does the Size of Annular Ring Matter?

To achieve reliable electrical connections, the width of the ring should be sufficiently wide. During design, the minimum width of the ring should be greater than or equal to the specified design width. If it is smaller than the designated size, it may have adverse effects on component connections. Additionally, it may lead to pad breakout (zero annular ring), which is related to the area where the circuit enters the pad. This can reduce the circuit's current-carrying capacity and affect normal circuit operation.

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Typically, the minimum width of the annular ring is 0.15mm (6mil), and the drill size (CNC) ranges from 0.2mm to 6.3mm. If it goes beyond this range, additional charges may apply. It is recommended to understand more about manufacturing tolerances before placing an order and to leave enough space when designing PCB vias, which can help ensure that the width of the annular ring is within an acceptable range and that the electrical connection is not affected.

Common Issues of Annular Ring

In the manufacturing process, the drill bits used by PCB manufacturers are often affected by certain variables, resulting in the annular ring may not be centered but drilled within a certain tolerance range. There are several situations may occur:

Good via: A via can be considered a good one when it is located at the center of a pad or within a specified restricted area. Such a via will form a complete annular ring and provide a good electrical connection.

Tangency: When the drill bit is not aligned with the marker, the completed via will deviate from the center and be closer to one side of the solder pad. When a portion of the via contacts the edge of the solder pad, tangency will appear.

Breakout: Breakout refers to the situation where the via deviates significantly and may end up outside the boundary of the pad.

During the PCB design or manufacturing process, both Tangency and Breakout should be avoided.

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What is Teardrop?

The additional copper added to the PCB trace and annular ring usually takes the form of a slender water droplet-shaped structure. The purpose of this structure, known as a teardrop, is to reinforce the mechanical strength of the PTH and prevent it from breaking or being damaged due to stress during installation or use.

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During the drilling process, the drill bit may deviate, causing a partial loss of copper at the connection between the through-hole pad and wire, ultimately leading to weak connections or even broken wires. In this case, teardrops can improve the drilling tolerance and further reduce the scrap rate caused by drilling issues in PCB production.

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