The first PCBs I made as a young engineer were pretty simple, with wires used to connect components on the top side of the board. Once things got more complicated, board space became a real consideration, and components were packed too close for comfort. I eventually had a tangled mess of wires on the top and bottom sides of the board connecting multiple components.
Later on, I learned more about using design software for multilayer design. Using vias to route connections between components via the inner board layers saves a huge amount of board space. If you’re unsure how to size or fabricate your vias, you’ll need to consider a number of aspects, ranging from thermal demands to the overall manufacturing process.
Via Sizing in Your Next Multilayer PCB
Any multilayer PCB requires vias in order to route connections between the outer and inner layers. The via size you’ll require depends on a number of factors, particularly the width of your traces. You’ll have to balance trace density with trace size, especially as component and pin count density on your components increase. Thicker traces tend to be more reliable and can handle more current, but they take up more space on the board.
Moving to smaller traces is necessary when you need to accommodate more components on your board. This means you’ll want to use smaller vias to route between layers as trace and component density become extremely high. When you are trying to reach the inner layers of a PCB with thin vias, you’ll need to consider the via aspect ratio during design. Vias with larger aspect ratio can be less reliable under thermal cycling and can be more difficult to fabricate.
There are some calculators that can be useful for estimating the thermal and electrical demands in a via. These calculators are typically one-way; you enter your desired via size, and the calculator outputs the resulting thermal electrical characteristics. Normally, choosing a via works in the other direction; the desired thermal and electrical demands are specified by the design requirements, and the minimum allowable via size needs to be determined.
Thankfully, these calculators allow you to play around with your input values, and you can quickly get a sense for the appropriate via size if you don’t have access to advanced design tools.
Microvias: Too Small to Drill
As modern electronic devices tend to require higher component and trace densities, vias are becoming progressively smaller with higher aspect ratios. Microvias are more common in multilayer PCB with smaller traces. Most PCB manufacturers can place via holes down to 100 microns in diameter, but the tooling costs actually increase with mechanical drilling. This is because the drills used are so thin that they snap very easily, especially if your microvias have high aspect ratio.
Instead of running through multiple drill bits, laser drilling is the preferred fabrication method for microvias. A laser beam can be focused down to a very small size that is comparable to or smaller than the thinnest mechanical drill bits. The vias you form tend to be just as clean as those produced with mechanical drilling, ensuring you can deposit a uniform plating on the interior of the via.
Laser via drilling has a lower drill speed compared to mechanical drilling. It can also be used to easily drill dense via holes in circuitry on a multilayer board. Over time, the tooling costs from mechanical drilling outweigh the benefits provided by laser drilling. Lasers with wavelengths ranging from deep infrared to ultraviolet can be used for laser drilling.
One problem with using a laser is that a focused laser beam is limited in the depth that can be reliably and reproducibly drilled. If a single lens is used to focus the beam, the laser is limited in the depth it can drill due to the limited depth of focus. Folks that are familiar with optics know that using a collimated laser beam would allow you to laser drill down to a greater depth. The problem with this is that the size of the via is so small that the beam experiences Fresnel diffraction, which can affect the intensity of the beam in very deep via holes.
PCB design with laser lines
Any-Layer HDI Design
Given the problems inherent in drilling deep holes with small diameter, some creative via designs are used to route between inner and outer layers in a multilayer PCB. Filled blind/buried vias can be easily stacked on top of each other, forming a longer, solid via that can reach any layer in a multilayer PCB. This design strategy is called every-layer interconnect (ELIC) or any-layer HDI design.
Stacking blind/buried vias in a multilayer board requires using a layer-by-layer process. Each layer is drilled either mechanically or with a laser. Layers are then stacked and plated to form staack blind/buried vias that allow access to any layer. This requires very tight mechanical constraints, as a slight lateral displacement in successive layers can ruin the alignment between neighboring vias.
This strategy is quite useful when working with microvias that you intend to drill mechanically. The drill bit is less likely to snap when drilling shallow holes in each layer compared to drilling all the way through a full PCB stackup with a thin drill bit. The key difference here is the aspect ratio of the holes you are drilling. If you have a large number of holes to drill, it may be faster (and cheaper) to use laser drilling.
Multiple vias on a green PCB
The number of possible via configurations and designs can boggle the mind. Top-notch manufacturers can accommodate any via design you can imagine, but always make sure to check with your manufacturer regarding their capabilities. You’ll want to make sure that your manufacturer can actually work with your intended stackup and via specifications, helping you avoid a costly redesign.
The top-notch CAD and via design tools in Altium Designer 18.1 can help ensure your next multi-layer PCB meets appropriate design standards and will meet your manufacturer’s requirements. You can define and design vias for your board, as well as place vias with extreme accuracy. Download your free trial and find out if Altium Designer is right for you. If you want to learn more, talk to an Altium expert today.
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