In PCB design, we are also always looking for new technological improvements to simplify our work and achieve more as PCB boards become smaller and denser. One of these improvements is micropores. These laser drilled vias are smaller than regular vias and have different aspect ratios. Here's a look at microvia aspect ratios and how using microvias can help you with your PCB design.

Review PCB Through Holes

First, let's take a look at some basic information about vias and how they are used on PCB boards. Through-holes are holes drilled in a PCB, plated holes conduct electrical signals from one layer to another. Just as traces conduct signals horizontally in a PCB, vias can also conduct these signals vertically. Vias can vary in size from small to large, with larger vias for power and ground nets, and even for connecting mechanical features to the board. Standard through holes are created by mechanical drilling, which can be divided into three categories:

1. Through holes: holes drilled from the top layer to the bottom layer all the way to the PCB.

2. Blind holes: holes drilled from the outer layer of the circuit board to the inner layer without going all the way through the circuit board like through holes.

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

Microvias, on the other hand, differ from standard vias in that they are drilled with a laser, which makes them smaller than regular drilled holes. Depending on the width of the board, mechanically drilled holes are typically no smaller than 0.006 inches (0.15 mm), and microvias start at that size and get smaller. Another difference with microvias is that they typically span only two layers, as it can be difficult for PCB manufacturers to plate copper inside these vias. Microvias can be stacked together if direct connections through more than two layers are required.

Microvias starting from the surface layer do not need to be filled, but buried microvias will be filled with different materials depending on the application. The stacked vias are typically filled with electroplated copper to enable connections between stacked vias. Another way to connect vias through a layer stack is to stagger them and connect them with short traces. When the profile of a microvia is different from that of a regular via, it will result in a different aspect ratio.

What is microvia aspect ratio and why is it important for PCB design?

The aspect ratio of a through hole is the ratio between the depth of the hole and the diameter of the hole (hole depth to hole diameter). For example, a standard circuit board at 0.062" thick (with 0.020" vias in it) will have an aspect ratio of 3 to 1. This ratio is used as a guide to ensure that the PCB manufacturer does not exceed the manufacturer's capabilities. their equipment while drilling. For standard drilling, the aspect ratio should generally not exceed 10:1, which will allow a 0.062" board to drill a 0.006" (0.15mm) hole through it.

When using microwells, the aspect ratio varies greatly due to their size and depth. Plating smaller holes can be difficult, and plating a small hole on 10 layers of a board can cause a lot of problems for the PCB manufacturer. However, if the hole spans only two of these layers, plating becomes much easier. IPC used to define micropores based on their size, which was equal to or less than 0.006 inches (0.15 mm). Over time, this size became common and the IPC decided to change its definition to avoid constantly updating its specifications as technology changed. The IPC now defines a microhole as a hole with a 1:1 aspect ratio, as long as the hole does not exceed 0.010 inches or 0.25 mm in depth.

How Microvias Help with Trace Routing on Circuit Boards?

As the density of PCB technology increases in PCB design, everyone wants to get more lines in a smaller area, which leads to the use of blind and buried vias, and the method of embedding vias in surface mount pads . However, blind and buried vias are more difficult to manufacture due to the extra drilling step involved, and drilling can leave residues in the holes that can lead to manufacturing defects. Conventional vias are also often too large to fit into the smaller surface mount pads found in today's high-density devices. However, micropores can help with all of these issues:

1. Microvias make it easier to create small blind and buried vias.

2. Micro vias will fit smaller surface mount pads, making them especially suitable for high pin count devices such as ball grid arrays (BGAs).

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

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

Tiny vias are an advanced method of PCB fabrication, and if your board doesn't require them, you'll obviously want to use standard vias to keep costs down. However, if your design is dense and needs extra space, see if using microvias helps. As always, before designing a PCB design with microvias, check with the contract manufacturer first to check its performance.