Everything You Need to Know About Microvias in Printed Circuit Design
This fine-pitch LGA processor needs microvias to reach inner layers.
Have you ever wondered how designers pack so much functionality into such a small space? You can thank HDI design techniques and microvias in printed circuit design. These small structures let traces reach inner layers of a PCB with high interconnect density and high layer count. These structures have been around for years, but they are becoming more common in a variety of systems that require multiple functions on a single circuit board. If you've done a size study and you've determined you'll need ~6 mil traces to fit all your components in your printed circuit board, you'll inevitably be using microvias to route between layers. Here's how these structures are formed and what you need to know about PCB microvias.
As it's name suggests, a microvia is just a small version of a typical via, but exactly how small are they? Most people consider microvias to be a via with a diameter less than 150 µm. These tiny holes can be drilled mechanically or with lasers, although the latter process is constantly being improved and is preferred in PCB manufacturing thanks to its high throughput. New advances in laser drilling techniques could reduce microvias down to 15 µm. The lasers involved can only drill through one layer at a time. However, manufacturers use a stacked microvia with multiple layers by drilling them on individual layers, followed by stacking and pressing each layer.
During fabrication, laser-drilled microvias have a lower potential for manufacturing defects than normal vias. This is because laser drilling doesn’t leave any material behind in the drilled out holes. Mechanically-drilled microvias can have defects due to drill vibration as the drill wears out, and mechanical microvia drilling is only useful down to ~8 mil diameter. Microvias do have the same risks as normal vias when it comes to plating and solder reflow. Thus, it’s important that you discuss tenting or filling your microvias with your manufacturer.
Microvias are almost as futuristic as the lasers that drill them.
Types of Microvias
There are a few different types of microvias. All microvias have two common characteristics:
- Low aspect ratio: Contrary to through-hole vias in typical PCBs, microvias have small aspect ratio on the order of ~2:1. Because of this, they typically only span between a single layer.
- Susceptible to fracture at the neck: High aspect ratio through-hole vias (~10:1) tend to be most susceptible to fracture near the middle of the via barrel. Due to the plating method used in microvias, copper curves inward to the barrel region and stress tends to concentrate there. As a result, microvias are most susceptible to fracture in the neck region under repeated thermal cycling, strong vibration, or mechanical shock.
Aside from these qualities, the differences between microvias are simply their typical diameter and where they appear in a circuit board. Microvia-in-pad configuration is common with fine-pitch BGA components, where the small distance between solder balls will not allow a dog bone fanout. Microvias on the surface layer can also be placed in a pad, filled with a conductive epoxy, and plated over with copper (known as via-in-pad plated over, or VIPPO). The VIPPO copper configuration has advantages in that the pad is now solderable, making it a good choice for ~4 mil pitch BGAs.
With these points in mind, here are the different types of microvias.
Blind microvias start in the surface layer and terminate 1 or 2 layers below the surface. It is best to simply place a blind microvia that only spans a single layer. If you need to span 2 layers, it's best to use stacked microvias (see below) as these will be more reliable and require fewer fabrication steps.
Buried microvias span between two interior layers and do not reach either circuit board surface. Just like blind vias, it's best to span a single layer with buried microvias to ensure reliability and ease of fabrication.
Stacked microvias are simply stacks of buried vias, or a blind microvia stacked on top of buried microvias. This is the standard way to span between multiple layers in an HDI PCB. The internal buried microvias in the stack need to be filled with conductive paste and plated over to ensure strong contact as the next via in the stack is deposited and plated.
Each of these types of microvias provides some particular advantages in terms of space savings and EMI.
Aside from some fabrication difficulties for less-advanced manufacturers, microvias are ubiquitous in HDI PCBs and they are responsible for saving space in these boards. For fine-pitch components in dense boards, they are required to ensure components can be packed into small spaces. Circuit board space is money, and microvias will help lower your costs. Aside from allowing more dense connections between layers and with smaller traces, they also allow you to save space by placing a via directly in-pad. Microvias-in-pad also save you space by making connections inside the pads of surface mounted technologies (SMTs). Microvias are especially well-suited for this because of their small form factor. Sometimes, normal vias are too large to fit inside pads for SMTs like fine pitch ball grid arrays (BGAs). Microvias can fit inside the pad without causing any fabrication issues.
You may be a bit hesitant to try incorporating lots of buried and blind vias into your designs, but microvias will change that. Traditional blind and buried vias will save you space but can be difficult to fabricate. However, because microvias are already made layer by layer, they’re perfectly suited for buried, blind, and stacked applications.
In addition to BGA breakout, microvias-in-pad will fit inside the pads of even the finest pitch BGAs, and they can be even more useful in breakout channels. Even if you use microvias-in-pad to save surface space, you’ll still need lots of layers to get those trace elsewhere on the circuit board. Using microvias can increase escape path width, possibly letting you use fewer layers to breakout a BGA.
We can’t make better music, but we can reduce noise.
Microvias have had a major effect on signal integrity in HDI circuits. The fact that microvias carry smaller parasitic capacitance and inductance, simply due to their small size, is one factor that allows them to be packed closer together without increasing crosstalk and noise coupling strength. The smaller parasitics negatively compensate for the stronger electromagnetic field when HDI traces and microvias are closer together.
One of the big problems in high-speed circuits is signal radiation and reflection in vias. In addition, a resonance in a large via stub can couple with high field strength into a neighboring via. In essence, large vias make great radiating antennas, especially when a signal is allowed to resonate in a stub. A good way to reduce an antenna's power is to reduce its size. Microvias are basically like smaller antennas, so they will already have smaller emission and absorption cross section compared larger vias. Microvias are made layer by layer, which means there won’t be any stubs unless you purposefully put them there (don’t do that).
A great application for microvias is in high frequency transmission lines. Take microwave and millimeter wave stripline conductors for example. They’re suspended in the dielectric of boards and need vias to breakout signals. Large vias introduce too much EMI to be useful in the higher frequency striplines. In addition, plated through-hole vias in mmWave boards can introduce significant insertion loss along an interconnect compared to microvias, which is again due to the larger capacitive and inductive coupling between larger vias and nearby reference planes. High frequency circuits are becoming more ubiquitous, and we’ll be designing a lot more of them in the future. It’s time to start mastering microvias to control EMI on these kinds of boards.
Now that you’ve got microvias in printed circuit design covered, you need the right PCB design software to create your next HDI PCB. Altium Designer® can help you enhance your advanced printed board with microvias and a wide range of other tools.
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Have more questions about microvias? Call an expert at Altium Designer.