Annular rings in PCB are crucial in our printed circuit boards (PCBs). With them, most PCB designers would find it easier to provide proper connections between components and layers on a board.
Yet, designing the perfect annular rings may be challenging for beginners. It’s more so if they lack high-end equipment that makes their work easier.
When constructing PCBs, we drill holes through copper pads for connection or mechanical support. Typically, the pads are circular and of specific sizes depending on the PCB’s use.
Whatever remains around the hole is what we call annular rings.
But what’s their importance in PCBs? Today, we’re highlighting what annular rings are in PCBs and their importance to printed circuit board design.
Table of Contents
- What Are Annular Rings?
- Structure of an Annular Ring
- Getting the Right Size Annular Ring
- Importance of Annular Rings in PCB Design
- Problems You’ll Encounter With Annular Rings
- Final Remarks
What Are Annular Rings?
An annular ring is a circle of conductive material surrounding a hole drilled into a via or copper pad. It provides the electrical connection between the component lead or via and the PCB trace.
Additionally, annular rings introduce some tolerance into the PCB system. In other words, you don’t have to position the via perfectly at the center of the annular ring for a proper connection.
The width of an annular ring depends on your design parameters and acceptable industry standards.
For instance, you can give a three mil hole size tolerance for plated holes and a two mil tolerance for press-fit holes.
Furthermore, a designer may make very tiny annular rings because of the proximity of the surrounding pieces. Unfortunately, this raises the likelihood that the drilled via will be extremely close to the annular ring’s edge.
Moreover, it may break out of the ring in the worst-case scenario. As a result, one damages the trace connection or leaves it too thin to function correctly.
Structure of an Annular Ring
(Adding a CPU into its socket on a motherboard)
To understand more about annular rings, let’s look at their structure.
The structure of an annular ring consists of the conductive material encircling a drilled hole. Generally, the conductive material is a thin layer of copper deposits on the surface of a PCB.
Now, after you drill a hole through a PCB, the next step shall be to coat the hole and its surrounding with a layer of copper.
Afterward, edge the copper layer to eradicate excess material leaving you with a perfect annular ring.
Getting the Right Size Annular Ring
The size and shape of an annular ring depend on the design parameters of the printed circuit board. Moreover, it depends on the industry standards for the design of PCBs.
As such, getting the right size of an annular ring is beneficial as you’ll get a top-tier PCB.
To get the right size of an annular ring, use the formula below:
Annular ring width = (diameter of the copper pad – diameter of the hole/via) / 2
In other words, subtract the diameter of the via from the diameter of the surrounding copper pad. Next, divide the result by two to get your annular ring size. Remember, each PCB manufacturer specifies the tolerances for any annular ring in manufacturing.
You need to adhere to these specifications to ensure that you don’t compromise the conductivity of your board. For example, most manufacturers specify larger annular rings for heavy-duty Class 3 boards.
What to Consider:
Moreover, here are considerations you need to factor in when deciding on the annular ring size:
(A printed circuit board)
Component spacing needs: The data sheet usually specifies the components’ annular ring size.
Some components may need annular rings that match their size and use. Also, the PCB manufacturer’s design guidelines may add more insight into the annular ring size.
Manufacturing capabilities: Ensure that the annular ring size is within the capabilities of your chosen PCB manufacturer.
Some have state-of-the-art technologies that allow them to add minute annular rings to their boards.
If you lack the best equipment for PCB manufacturing, it’s better to go with wider annular rings.
Trace width and clearance: Consider the trace width and its clearance to adjacent circuits before deciding on the size of an annular ring.
Having an annular ring that’s too wide that it touches adjacent circuits will short-circuit your PCB.
Cost: The cost determines the annular rings you’ll use in your PCBs. If you have a smaller board, stick to the minimum size that the manufacturer recommends.
You can experiment with broader annular rings with a larger board with much space. However, larger annular rings are costlier and not profitable for large-scale PCB manufacturers.
Importance of Annular Rings in PCB Design
(An illustration of annular rings and traces on a PCB)
Annular rings provide the electrical connection between connecting leads and the electronic component.
During design, an annular ring size depends on the type of hole and its location. Other factors include trace width and the thickness of the copper layer.
Secondly, annular rings determine the signal integrity of a PCB. Poor annular ring designs can lead to electromagnetic interference and the weakening of signals.
Thirdly, annular rings can affect the thermal capacity of a printed circuit board. Its size and shape can impact the overall heat dissipation and conductivity of your PCB.
Problems You’ll Encounter With Annular Rings
(Electronic component assembly on a PCB)
Getting the right size for an annular ring is crucial in PCB manufacturing.
Generally, you can get a perfect annular ring by drilling straight through the middle of a via pad. Nevertheless, the process is challenging, and most manufacturers will accord you some tolerances.
Failure to honor the manufacturer’s recommendations may lead to the following problems:
Tangency: Tangency in PCB design refers to the situation when an annular ring touches a copper trace. In other words, when you drill a hole off-center, it touches the edge of the via’s pad.
Consequently, your design will have poor electrical performance and thermal management.
Breakout: Breakout in PCB design is when you drill a hole outside an annular ring. Usually, breakouts happen when you drill a hole that doesn’t align well through all layers of the PCB.
Moreover, you can evade breakouts by ensuring that the annular ring is broad to accommodate the hole.
Rupture: Like any electrical system, annular rings suffer from rupture due to aging. Other causes include mechanical stresses that make the annular ring deteriorate.
What is the smallest annular ring for PCB?
The PCB design guidelines usually specify the size of annular rings for specific systems. Moreover, the minimum spacing of the components will determine how small you can go with designing the annular rings.
Standard PCBs have a minimum annular ring size of 4 mils or 0.1 mm.
More sophisticated PCB manufacturers can sometimes go lower with 3 mils or 0.075mm annular rings. However, the smaller annular rings are challenging to manufacture and may introduce defects to the system.
Does annular ring size matter?
Yes, annular ring size matters in PCB design and manufacture. It affects the reliability and performance of your board and related components. It may fail to provide a sufficient connection between the via and the PCB trace if it is too small.
Therefore, you’ll have connection breakdowns, or your circuit may fail.
Conversely, if it’s too wide, it may take up precious space, making adding other components to the board challenging.
Is an annular ring the same as a through hole?
No, an annular ring is different from a through hole.
The confusion comes in that both terms relate to holes in printed circuit boards. However, an annular ring is a circle of a conductive layer that encircles a hole in a PCB.
On the other hand, a through hole is a hole through the PCB from one side to the other.
Designers use through holes for through-hole PCB making and insertion of components. Further, they use the holes to provide the board with mechanical support.
In conclusion, a wider annular ring offers a more robust and dependable connection. On the other hand, you can use a smaller one when there is a lack of available room.
Nevertheless, your choice of annular ring size depends on the production tolerances. Lastly, it also depends on your capacity as a PCB manufacturer.