Those working in electronics hardware have likely seen PCBs densely packed with holes, but have you noticed that some holes are missing the copper ring that should be around them? These "annular ring" holes may seem insignificant, but they are a key operation in high-density PCB design. Today, we'll explain their origins clearly, even for beginners!

First, let's understand: What is a PCB "annular ring"?

Before discussing annular holes, we need to understand their "predecessor"—the traditional PTH (Plain Hole) with an annular ring. A typical PCB through-hole is actually a "three-piece set": Drill hole: the hole drilled into the PCB; Copper layer on the hole wall: the copper plated inside the hole, used to connect traces on different layers of the PCB; Pad: the copper ring around the hole. Removing the hole portion, the remaining copper ring is the "annular ring."

This small copper ring may seem insignificant, but its function is crucial: Firstly, it acts like an anchor, firmly securing the copper layer of the hole wall to the copper foil of each PCB layer, preventing it from detaching due to heat or external force. Secondly, it provides tolerance for drilling deviations, ensuring uninterrupted electrical connections even if the hole is drilled slightly off-center.

Why "cut off" the via ring? It's not just pointless experimentation!

Since the via ring is so important, why are many PCBs now designed to be "ring-free"? The core reason is simple: to "squeeze out" more routing space, especially in products like mobile phones, computers, and high-end communication equipment that demand miniaturization and high density.

1. The "Lifeline" of BGA Routing The gaps between the pins of fine-pitch BGA chips are extremely small. Traditional vias with via rings would fill the limited space, making it impossible for signal lines to emerge from between the pins. Removing the via ring allows traces to pass close to the edge of the hole, directly removing the "blockage" in routing.

2. Life-Saving Design to Avoid Short Circuits
Some electronic components have metal casings. If the via rings on the PCB are exposed, they can easily come into contact with the metal components during assembly, causing a short circuit. Removing the via rings eliminates this risk at the source.

Simply put, "ringless PTH vias" are a trade-off: sacrificing some reliability for routing space. It doesn't eliminate pads on all layers, but only removes pads on layers where connections are unnecessary, making room for signal lines.

Acyclic vias are not a "one-size-fits-all" solution: 3 common implementation methods

Don't assume that acyclic vias are simply about removing the copper ring; there are actually different technological approaches:

Direct acyclic design: The most straightforward method. Either completely eliminate the pads on non-critical layers, or make the pads the same size as the via, effectively eliminating the excess copper ring.

Pad-in-hole + plugging plating: Drill a through-hole in the center of the BGA pad, with the pad itself acting as the "ring." Then, fill the hole with resin and electroplat it smoothly. The finished product looks "acyclic" and also prevents solder from seeping into the hole during soldering.

Back-drilling: Designed for high-speed signals, drill away the unused portion of the copper layer on the via wall, eliminating the "residual stub effect" for high-speed signals and creating a acyclic state.

The Advantages and Disadvantages of Vias-Free Circuits: High Returns with High Risks

✅ Advantages: Maximizes PCB Performance

Double Routing Density: A core operation for High-Density Interconnect (HDI), allowing for more complex routing within a limited number of layers;
More Stable Signals: Reduces parasitic capacitance from vias, and back-drilling further reduces reflection and attenuation of high-speed signals, improving signal integrity;
More Flexible Inner Layer Routing: Engineers no longer need to worry about cramped traces, greatly increasing design flexibility.

❌ Disadvantages: Requires advanced manufacturing processes and is more expensive.

Reliability is compromised: Without the fixation of solder pads, the adhesion between the copper layer on the hole wall and the inner layers of the PCB weakens, making it prone to cracking and breakage after multiple reflows.

High-precision requirements for factories: Interlayer alignment and drilling accuracy must be extremely precise; even a slight misalignment can lead to disconnection or short circuit. The electroplating process must also ensure the thickness and uniformity of the copper on the hole wall to compensate for the loss of mechanical strength.

Cost and inspection are complicated: High-precision processes mean higher manufacturing costs, and traditional AOI inspection is difficult to judge the connection quality of ringless vias, making later repair and troubleshooting more difficult.

Final Summary

Ringless vias on PCBs are not just "technical flair," but an inevitable choice for the miniaturization and high-performance development of electronic devices. It's not about arbitrarily eliminating copper rings, but a precise trade-off made by engineers based on their understanding of signal flow and by factories with control over process capabilities. Today, in fields such as communications, computing, and consumer electronics, ringless vias have become standard in high-end PCB designs—the core of ringless via design is to squeeze out space while controlling risks.

If your product is also designed for high-density PCBs, you might want to assess the factory's manufacturing capabilities before deciding on the application of via-less designs. After all, the key to making these trade-offs is to ensure that the benefits far outweigh the risks.