In PCB design, we are also always looking for new technology improvements to simplify our work, and achieve more achievements as the design becomes smaller and denser. One of these improvements is micropores. These laser-drilled vias are smaller than conventional vias and have different aspect ratios. Due to their small size, they simplify the task of routing traces, allowing us to package more wires in a narrower space. Here is more information about the aspect ratio of microvias and how using microvias can help you with your PCB design.

Review PCB through holes

First, let us look at some basic information about through holes and their usage on printed circuit boards. Through holes are holes drilled in the PCB. Plated holes can conduct electrical signals from one layer to another. Just as traces conduct signals horizontally in a PCB, vias can also conduct these signals vertically. The size of the through holes can vary from small to large. Larger through holes are used for power and grounding grids, and even mechanical features can be connected to the board. Standard through holes are created by mechanical drilling. They can be divided into three categories:

Through hole: a hole drilled from the top layer to the bottom layer all the way to the PCB.

Blind hole: A hole drilled from the outer layer to the inner layer of the circuit board, instead of going through the circuit board like a through hole.

Buried Holes: Holes that start and end only on the inner layer of the board. These holes do not extend to any outer layer.

On the other hand, micro vias are different from standard vias in that they are drilled with a laser, which makes them smaller than conventional drills. According to the width of the board, the mechanical drilling is usually not less than 0.006 inches (0.15 mm), and the micro-holes become smaller from this size. Another difference with microvias is that they usually span only two layers, because plating copper in these small holes can be difficult for manufacturers. If you need to connect directly through more than two layers, you can stack the microvias together.

The micropores starting from the surface layer do not need to be filled, but depending on the application, different materials will be used to fill the buried micropores. The stacked microvias are usually filled with electroplated copper to realize the connection between the stacked vias. Another way to connect microvias through layer stacks is to stagger them and connect them with short traces. As shown in the figure below, the profile of the microvia is different from the profile of the conventional via, resulting in a different aspect ratio.

What is the Microvia aspect ratio and why is it important to PCB design?

The aspect ratio of the through hole is the ratio between the depth of the hole and the diameter of the hole (the depth of the hole and the diameter of the hole). For example, a standard circuit board with a thickness of 0.062 inches and 0.020 inches through holes should have an aspect ratio of 3:1. This ratio can be used as a guide to ensure that the manufacturer does not exceed the manufacturer’s capabilities. They are drilling equipment. For standard drilling, the aspect ratio should generally not exceed 10:1, which will allow a 0.062 inch plank to drill a 0.006 inch (0.15 mm) hole through it.

When using micropores, the aspect ratio varies greatly due to their size and depth. It can be difficult to plate smaller holes. Trying to plate a small hole on the 10th layer of the circuit board will cause many problems for PCB manufacturers. However, if the hole spans only two of these layers, plating becomes much easier. IPC used to define pores based on their size, which is equal to or less than 0.006 inches (0.15 mm). Over time, this size became common, and IPC decided to change its definition to avoid constantly updating its specifications as technology changes. IPC now defines a micropore as a hole with an aspect ratio of 1:1, as long as the depth of the hole does not exceed 0.010 inches or 0.25 mm.

How Microvia helps to route the traces on the circuit board

The name of the game in PCB design is that as the density of PCB technology increases, more routing routes are obtained in a smaller area. This has led to the use of blind vias and buried vias, as well as methods for embedding vias in surface mount pads. However, blind holes and buried vias are more difficult to manufacture due to the additional drilling steps involved, and drilling can leave material in the holes, causing manufacturing defects. Conventional vias are usually too large to be embedded in the smaller surface mount pads in today’s high-density devices. However, micropores can help solve all these problems:

Microvia makes it easier to manufacture small blind and buried vias.

Micro vias will be suitable for smaller surface mount pads, making them particularly suitable for high pin count devices such as ball grid arrays (BGA).

Due to its smaller size, the microvia will allow more wiring around it.

Due to its size, microvias can also help reduce EMI and improve other signal integrity issues.

Microvias are an advanced method of PCB manufacturing. If your circuit board does not require them, you will obviously want to use standard vias to reduce costs. However, if your design is dense and requires extra space, see if using microvias helps. As always, before designing a PCB with microvias, it is best to contact the contract manufacturer to check its performance.

The precise use of Microvias depends on your PCB design tools

After establishing contact with the manufacturer, the next step is to configure your PCB design tool to use microvias. To effectively use the details of the microvia design, you need to do a lot of things in the tool. This will include the new through-hole shape and subsequent design rules. Microvias can be stacked, which is usually not available with regular vias, so your tool must also be able to deal with this.