Ratio for Blind and Buried Via Holes

Printed circuit boards (PCBs) require via holes to connect inner layers. These are copper plated holes that connect to different parts of the internal layer and allow more space for wiring. The via hole also ensures electrical conductivity of the PCB. Vias are commonly used in high-density multilayer PCBs, where they are critical to interlayer connectivity.

However, it’s important to know that not all via holes are created equal. Vias come in several forms, including blind and buried via hole. While both can be beneficial for your circuit board design, you must understand the differences between these two types. This will help you choose the best via type for your circuit board layout.

A blind and buried via is a type of plated through hole connection that establishes connections between the outer and inner layers of a multilayer PCB. It is used in high-density circuit boards to enable more functionality in less board space. It is also known as a hidden copper via or a microvia.

The basic structure of a via is composed of a barrel, pad, and antipad. The barrel is the conductive pole tube that fills the pre-drilled/percolated hole. The pad is a small, circular metal structure that links the end of the barrel to a trace or component on the inner layer. The antipad is a circular clearance hole that separates the barrel from non-connective layers.

Maximum Aspect Ratio for Blind and Buried Via Holes

When designing a PCB with blind and buried vias, it is important to consider the maximum aspect ratio for the holes. This is because larger diameters are harder to plate through the inner layer pads. The reason is that the chemical solution used to plate the copper ions cannot reach the center of the hole. This is especially true for longer barrels.

In general, the aspect ratio is defined by the drilled hole size and the distance between the pad and the edge of the buried via hole. Usually, the aspect ratio of a buried via hole is around 1:8. However, this can vary depending on the manufacturer’s requirements.

While this is a good ratio for most applications, you may want to use a smaller aspect ratio for some designs. For instance, a 10mil via is much easier to plate than a 12mil one. In addition, the 10mil via is more stable against thermal cycling, which is crucial for some applications.

While the benefits of blind and buried vias are obvious, you should keep in mind that they aren’t as cost effective as plating through-holes. You should use them only if they are necessary for the circuit board’s operation. Otherwise, you should consider using other PCB layout techniques to save money and time.

Blind and buried vias are essential components in multilayer PCBs, enabling the connection of different layers of traces without traversing the entire board thickness. Blind vias connect an outer layer to one or more inner layers, while buried vias connect two or more inner layers without penetrating the outer layers. These vias optimize space utilization, enhance signal integrity, and reduce signal delay, making them indispensable in modern electronics.