What Is a Microvia

Designing a PCB can be pretty easy once you learn all the component types and their symbols. But even then, it can get confusing when deciding which component goes where on the board. And that is when microvias can be a big help.

You have probably noticed that in PCB, there are these very small circles. These are called microvias PCB, or sometimes just vias for short. They are an essential part of creating PCBs for some reasons. Microvias can ensure various types of durability and performance and make your PCBs look great. This guide will explain everything you need to know about microvias and what makes them beneficial to PCB design.

Microvias: Quick Overview

A microvia is a smaller version of a via, a hole in the PCB that allows you to run wires underneath the PCB and connect them to various circuits and connections. A microvia PCB can be used instead of an ordinary visa when there is too much copper around the desired connection point to allow drilling a normal-sized via. Microvias are most commonly used at the edge of the PCB, where it is difficult or impossible to drill the desired hole. They connect pads on the top layer to the bottom layer, or connect pads on two different layers of a stackup.

Microvias can be placed anywhere on the PCB, but they are most commonly found in areas where many signals or power lines are crossing each other to reduce crosstalk and improve signal integrity. Microvias have an incredible range of applications; they can be used in high-speed data circuit design, power and signal circuit design, and radio frequency designs where signals need to be transmitted in and out of the structure. Even as small as they are, they still fulfil the same purpose as their larger counterparts.

Types of Microvias

Blind Microvias

Blind microvias are a smaller form of blind via that can be used to transfer signals from one layer to another. These vias are fabricated by depositing a metal on top of the dielectric layer and then etching away the metal until the desired depth is reached. Blind microvias are becoming more popular because they have very little inductance or capacitance, providing better signal integrity than traditional vias. Moreover, the micro blind via costs less than traditional vias because it uses less material.

The advantage of this via is that it has no contact with the components, making it more reliable than other vias. It can also be used for grounding purposes.

Buried Microvias

Buried microvias PCB are similar to buried vias, except that the diameter is smaller and the pitch is larger. They are completely embedded in the substrate and covered with a layer of epoxy. The buried microvia can be used when there are no mechanical requirements on the PCB or when there is no need to have direct access to the top surface of the PCB.

Buried vias are usually used for signal traces or power planes. They are used heavily in RF applications, where many high-frequency signals pass through the board. They also have a lower impedance than traditional vias, making them ideal for high-speed digital circuits.

Stacked Microvias

Stacked microvias are created by stacking multiple layers of metal on top of each other to provide a path for electrical current to flow through. A single hole is created in one layer and drilled into another layer below. The top layer is soldered or welded onto the bottom layer, forming an interconnect between the two layers. This allows for quick, efficient routing without compromising on performance or reliability.

Staggered Microvias

A staggered microvia PCB  is aligned with a pad of the same layer but offset from its centre by half the pad's diameter. The distance from its centre to the board's edge is also half the pad's diameter. Staggered microvias allow you to place two vias within one pad without overlapping them or having to route traces on different layers together at this location. This type of microvia can be used to make electrical connections with the surrounding copper pads and traces.

Via-In-Pad Microvias

Via-In-Pad Microvias (VIP) are used when there is a need to route signals through a dense area of circuitry, such as in an FPGA or ASIC. VIPs have a smaller footprint than traditional microvias and allow more signal layers to be routed underneath them. The downside to VIPs is that they can cause problems if placed too close together, which can result in signal crosstalk between traces that pass through the same VIP.

The method for creating via-in-pad microvias involves cutting or drilling through the top metal layer of the PCB and then making contact with the bottom metal layer. They are used in various applications, including heat sinks and other thermal management devices. They are also used in electrical switches, connectors, and radio frequency (RF) circuits.

Designing Microvias

Microvias PCB can be made in three ways: laser drilling, punching and milling. Laser drilling is the most common method today as it allows for smaller diameter vias (0.004" or less) with higher precision than punching or milling methods.

Laser Drilling

Drilling is done with a laser, making very clean holes and leaving little or no residue. Laser drilling is also much faster than other methods, so it can be used when large quantities of boards need to be drilled quickly. Drilled microvias are typically 0.005 inches (0.127 millimetres) wide and 0.010 inches (0.254 millimetres) deep. They are made in batches of 200 or more at once so that they can be placed together on panels and soldered at once after they are made.

Punching

The other common method for making microvias is punching using a press machine. This machine uses sharp punches that cut through the copper laminate without leaving any burrs behind on the hole's edge like drills do. This allows for very high-quality microvias that can pass inspection easily and still fit tightly against their pads without having loose material hanging between them. The presses have been designed specifically for this purpose, so they can make hundreds of microvias at once and place them together on panels for soldering later.

Milling

Milling is cutting out a hole in the board using a tiny drill bit, which is moved by a computer-controlled cutting head. The toolpath is generated by software that considers the size and shape of the desired hole. Milling machines can be used for roughing or finishing cuts on metal parts. The spindle can be rotated in both directions so you can make either vertical or horizontal cuts.

The advantage of milling is that it can cut out extremely small features in boards with great accuracy, down to fractions of a micron. The disadvantage is that it is slow and expensive, so it is not suitable for making large numbers of holes.

Common Uses of Microvias

Microvias have many uses, but here are some common ones:

Thermal Relief for Soldered Components

Solder balls on standard-size through-hole components have a relatively small surface area compared to their volume, which causes them to take a long time to heat up during soldering. This can lead to cold solder joints and poor electrical performance. Microvias can be used as thermal reliefs to allow more heat transfer from the solder ball, reducing the time needed for proper solder reflow and increasing reliability.

Connect Power Planes

Power planes help distribute heat away from high-density board areas where multiple components draw current. To accomplish this, microvias are often placed directly above each power plane so that heat can be quickly distributed throughout the plane rather than concentrating at one point on the board like with standard vias.

Connect Ground Planes

 Ground planes are designed to absorb noise and prevent interference from creeping into sensitive sections of your circuit board like analogue or digital signal paths. Ground planes are often connected via microvias to share a common connection point on the board. This helps to ensure that all ground points are connected and properly grounded.

Resonant Circuits

Microvias can be used for resonant circuits to reduce the inductance of traces and decrease phase shifts between the primary and secondary resonating components. This helps reduce signal loss in high-frequency applications where signal integrity is important.

Power Integrity Testing

Microvias allow you to test your power integrity without having to remove components from the board or put them under stress during testing procedures such as burn-in tests or EMI testing procedures like radiated emissions testing or immunity testing.

Final Thoughts

It is undeniable that microvias have changed the game for the PCB industry. When they were first introduced in the 1990s, they provided an opportunity to expand BGA routing options and maximize routing densities. The way they are manufactured also opened up new possibilities for PCB profitability. Today, microvias can be used to create a much wider variety of printed circuit boards than ever before imagined. The only question is how you plan to use these attributes of microvias in your next project.

To help you get started, we recommend checking out the HDI Microvias PCB section from our archives for more information. Hemeixin is dedicated to going over the clear concept of microvia so people can learn more about them.