Micro Via in HDI PCB(High Density Interconnect)

A microvia is obviously a smaller 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 are drilled by lasers, a process that is constantly being improved. 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, HDI PCB manufacturers can make through microvias by drilling them separately in multiple layers and stacking them.

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. However, microvias have the same risks as normal vias when it comes to HDI PCB plating and solder reflow. Thus, it’s important that you discuss tenting or filling your microvias with your PCB manufacturer.

Now that you know what microvias are, how can they help you?

SPACE ADVANTAGES

PCB Board space is money, and microvias will help lower your PCB costs. Microvias’ small size will naturally save you a little space over normal vias. They’ll also help you better utilize via-in-pads (VIPs), are more easily buried, and can make high density interconnect (HDI) design a breeze.

Normally VIPs like to be the stars of the show, but in PCBs, they’re mostly hidden. VIPs save you space by making connections inside the pads of surface mounted technologies (SMTs). Microvias are especially well-suited for VIPs 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 PCB 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 and blind applications. They’re also simply smaller than traditional vias, so you’ll be able to shrink your PCB board even further.

Microvias are especially useful in certain applications, like pcb with breaking out BGAs. As I said earlier, microvia VIPs will fit inside the pads of even the finest pitch BGAs and make breakout a breeze. They can be even more useful, though, in breakout channels. Even if you use VIPs to save PCB surface space, you’ll still need lots of layers to get those outputs to other parts of the PCB board. Using microvias can increase escape path width, possibly letting you use fewer layers to breakout a BGA.

Have more questions about microvias? Contact PATT CIRCUITS for assistance: pcb@pcbprof.com

EMI ADVANTAGES

Microvias can reduce crosstalk in our audio circuits, but sadly can’t do anything about the music itself. Microvias are perfect for EMI prone circuits, like high-speed and high-frequency applications.

If you’re not careful then your high-speed digital circuits will radiate as much noise as a pop radio station. One of the big problems in high-speed circuits is signal radiation and reflection in vias. Vias make great radiating antennas for these noisy signals. A good way to reduce an antennas power is to reduce antenna size. Using microvias instead of traditional vias will reduce signal radiation through simple geometry. Vias stubs are a major problem for high-speed circuits as well. Stubs reflect signals back into your conductors, attenuating, or even cancelling, the original signal. 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 traditional vias introduce too much EMI to be useful in the higher frequency striplines. Microvias are perfect, because they won’t radiate much EMI, and won’t degrade signal integrity with too much reflection.

High frequency circuits are becoming more common, 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 PCB boards.

These tiny, laser drilled, holes are changing the landscape of PCB design. They’ll help you reduce EMI in your PCB boards, and reduce PCB surface area as well.