Aspect Ratio and Annular Ring

Coupling buried vias with microvias can solve many manufacturing challenges.

When it comes to designs, we all make choices – material selections, feature sizes, via structures, components and more. Often, we also make tradeoffs.

Maybe the 0.8mm BGA you want allows standard vias, but the 0.5mm BGA takes up less space. Or you need impedance-controlled signals (which seems to be a requirement on almost all new designs), and line widths must be balanced with dielectric thickness and Dk values.

As packaging challenges mount, so does part thickness and layer count. For many designs, two of the biggest cost and risk drivers are aspect ratio and annular ring. These two attributes are often at odds with each other. There are ways to help them coexist.

Aspect ratio, or board thickness to drilled hole diameter, can be a limiting factor for a few different reasons. Keep in mind that there are two aspect ratios to consider. The first and most commonly thought of is the mechanically drilled through-hole. The second is the blind microvia.

The most common limitation is the ability to copper-plate the drilled hole. As the hole gets deeper, it gets harder for the copper plating bath to drive copper ions into the middle of the hole to provide adequate copper thickness in the hole. Not every manufacturer will have the same limits for aspect ratio. Depending on their product focus, they will have optimized their plating lines to support the products they build the most.

For through vias, aspect ratios can be higher because we can drive solution through the hole and out the other side, providing a continuous supply of fresh solution and copper ions. For microvias, the aspect ratio is much lower because we are trying to plate a dead-end hole. It is more challenging to get solution to enter the hole, deposit copper and get out of the hole so new solution can take over.

Generally, if you can keep the through-hole aspect ratio below 10:1 (thickness to drill diameter) and the microvia ratio below 0.66:1, most suppliers can support your project. If you can’t, however, what options do you have?

Starting with the microvias, the first suggestion is to reduce the dielectric thickness. Often the microvias will be 0.004″ (100µm) deep or less, so the dielectrics are quite thin. Sometimes there is a reason to go deeper, such as connecting to two layers down. If that is the case, the aspect ratio can rise quickly and risk manufacturability. The recommendation here is slow and steady wins the race. Rather than try to increase aspect ratio, consider a set of stacked or staggered microvias. This may mean an extra layer is needed to accomplish this. Most suppliers, however, will say they would rather make two shallow microvias than one deep one. An advantage here can be extra room to increase pad size and thus annular ring on at least two of the layers involved. An extra layer can aid with impedance or with extra plane for power distribution.

A lot of conversation is out there on microvia reliability. Some is fact, some is anecdotal. What is known for sure is that larger diameter microvias are more reliable than smaller ones, and that shallower aspect ratio is also better. Material selection and via offsets also have an impact. If you are pushing the envelope, invest the time to talk with your manufacturer to make the best choice you can.

For mechanically drilled vias, whether through-holes, blind or buried, there is a relationship in terms of drill diameter, aspect ratio and pad size. Most suppliers are comfortable with 0.008″ (200µm) diameter drills.  Some will drill smaller – there are 100 and 150µm drill bits available. A couple of limitations come into play, however. As bit diameters get smaller, so does flute length. So there are limitations on how deep one can drill before running out of drill bit. Manufacturers have tricks to extend this, but they do reach limits.

Also, smaller diameter bits have a greater tendency to splay, meaning they don’t go perfectly perpendicular through the board. This can cause annular ring issues on the bottom side of the board.

Finally, if the vias must be filled with epoxy, the smaller holes are a much bigger challenge to fill. Keep in mind a 200µm hole in a 2mm board is not 10:1 at the filling operation. After drilling, the hole is copper plated, reducing the diameter to something closer to 125-150µm, making the aspect ratio in the neighborhood of 13-16:1. This makes via filling much more difficult, as the manufacturer is trying to push a thick epoxy into a very narrow and deep hole. As a result, a supplier might permit a higher aspect ratio on one board, but not another, and the reason may be due to via filling requirements on one but not the other.

Interestingly, while layer count is certainly a factor when it comes to aspect ratio, it can often be offset by thinner dielectrics to limit aspect ratio. Manufacturers may prefer a thinner core and prepreg solution to help keep board thickness down. When impedance is involved, selection of lower Dk materials can help make this a possibility.

In addition, via structures or number of laminations are a big impact for being able to achieve annular ring. Use of buried vias can help distribute signals with smaller vias in thinner portions of the board. These smaller vias can include smaller pads, meaning much less real estate being consumed internally, and leaving more room on layers external to the buried vias for larger pads on through-holes.

Coupling buried vias with microvias can eliminate through vias altogether. On boards using a buried via coupled with just microvias, the through-hole is eliminated altogether, and most of the annular ring and aspect ratio challenges disappear.

This can be a great way to solve manufacturing challenges while achieving your design goals.

Nick Koop is director of flex technology at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee; nick.koop@ttmtech.com. He and co-“Flexpert” Mark Finstad (mark.finstad@flexiblecircuit.com) welcome your suggestions.