Microvia PCB

A Comprehensive Guide to Microvia PCB [2022]

Modern gadgets are all about high functionality within a small space. But have you ever wondered how designers could accommodate such extensive functionality within limited spaces? Microvia PCB, along with HDI design techniques, can be attributed to functionality packed with limited space.

The small structures enable traces to reach internal layers with increased layer count and interconnect density. Even though the structures have been around for years, they are becoming popular in recent times due to their wide applications. Explore the blog to gather detailed information about a microvia PCB.

Understanding Microvia PCBs

Microvia PCB

A microvia is nothing but a miniature version of a standard via. But if you visit microvia PCB factories, you will realize that the miniature vias have a different structure. Microvias flaunt a conical frustum shape with inward slopes.

With each transitioning layer, they terminate at a pad. Microvias usually have a single layer to provide the highest amount of reliability. But designers might utilize stacked microvias to build connections across several layers.

Features of Microvias

Microvia PCB suppliers sell different types, but they all have some common characteristics:

● Low Aspect Ratio

The preferable aspect ratio of microvias is 0.75: 1. Compared to through-hole vias spotted in typical PCBs, microvias have a much smaller aspect ratio. In some cases, the aspect ratio can be quite large, like 1:1 or even 2:1.

They can be fabricated to develop microvias. But with a larger aspect ratio, a microvia will have reliability issues. Microvias usually don’t have more than one layer to offer true reliability. Remember that a microvia aspect ratio of more than one does not comply with the IPC requirements.

● Susceptible to Neck Fractures

A through-hole via found in standard PCBs will be prone to fracture near the middle of the via barrel. But microvias can be identified with their ability to fracture at the neck. It occurs due to the plating method implemented in microvias.

Copper in microvias curves inward in the direction of the barrel region. As a result, all the stress gets concentrated in that particular region. Therefore, microvias become susceptible to fractures at the neck under strong vibration, mechanical shock, or repeated thermal cycling.

Types of Microvias

A microvia PCB supplier can offer you the following varieties:

● Blind Microvias

A blind microvia will begin at the surface layer and end a layer beneath the surface. However, they might terminate beneath two layers if the aspect ratio is low. In the case of two layers, stacked or staggered microvias would be more suitable.

● Buried Microvias

Buried microvias have a structure similar to blind vias. They span across two internal layers. However, they don’t reach the surface of the circuit board.

The aspect ratio of a buried microvia must be low. It guarantees ease of fabrication. They are not made to span beyond a single layer to ensure high reliability.

Buried microvias are filled with copper through a plating method with pure copper. They might also be filled with resin and epoxy to develop a strong connection across the tip of the microvia. The process used for plating helps with the development of void-free structures with high reliability.

● Stacked and Staggered Microvias

Incorporating too many blinds or buried vias in your designs doesn’t seem wise. But they become valuable in stacked applications due to the layer-by-layer method of creating microvias with low aspect ratios. Stacked microvias are nothing but buried or blind vias stacked above buried microvias.

The stacked models are a standard method to create expansion between multiple layers in an HDI layout. The microvias stacked internally must be plated and filled with conductive paste. It ensures that they can establish a strong contact with the next vias getting deposited and plated in the stack.

Staggered vias are an alternative to stacked vias when the microvias in consecutive layers are offset from each other. Determining the aspect ratio to be enforced in the design is crucial for both stacked and staggered microvias. Stacking becomes crucial in case of potential interface failure between the vias in a stack.

Are Microvias Filled or Unfilled?

You will find filled as well as unfilled microvias in the market. In the case of buried microvias, filling them is essential. The copper filling is crucial when there’s stacking involved. If there are voids inside the via plating, it will witness high-stress concentration along the via wall.

It might lead to premature fracture during operation or reflow. Blind microvias don’t always need to be filled with copper. In earlier times, blind microvias were unfilled.

But blind microvias must be filled using a standard process if they are meant for in-pads. Manufacturers either use pure copper or a combination of epoxy and resin for the plating. The process involves conformal plating before proceeding to pulsed plating.

Pulsed plating involves using solid copper to fill in the microvia’s body. It ensures that no void remains inside the body. The plating process includes certain additives in the filler to prevent void formation.

If additives are not used during the plating process, the copper might concentrate along the walls. Conformal plating without additives will lead to an uneven copper deposit along the via body. In the end, both conformal and pulsed plating need additives to avoid voiding.

Factors Affecting the Reliability of Microvia PCB

The factors that can affect the reliability of China microvia PCB are as follows:

  • Smaller aspect ratios guarantee higher reliability. An aspect ratio of 0.7 is efficient at surviving accelerated life testing.
  • Voiding is usually associated with failure. However, the impact of voiding depends on the shape, volume fraction, and aspect ratio of the void.
  • More than two vertically stacked microvias are inefficient.
  • Copper wrap plating thickness at the butt joint often leads to the great failure of the microvias.

Wrapping up

Microvias are known to positively impact signal integrity in HDI circuits. They can help save space on printed circuit boards and lower manufacturing costs. Microvias can also seamlessly fit inside pads without fabrication issues.

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