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Creative Design with Buried Vias in Your Next HDI PCB

Created: February 11, 2019
Updated: September 25, 2020
Creative Design with Buried Vias in Your Next HDI PCB

HDI PCB with vias on the surface layer

Once I learned how to use different types of vias on my PCBs, I found that my routing strategies greatly improved, and I was able to reduce my overall board size. Judicious use of the right vias lets you place connections only between the layers you absolutely need for your board to function, instead of making connections that aren’t necessary.

As newer devices pack more functionality (and more components) onto a single board, vias are indispensable for routing connections between layers in your board. By choosing the right type of via, you’ll be able to save space on your surface layers, allowing you to pack more functionality into your board.

Buried Vias in Your HDI PCB

So, what is a buried via anyways? A buried via is like a blind via that runs between two of the inner layers in a multilayer PCB. Both of these types of vias connect between two or more layers. In a multilayer board, blind vias only connect an outer layer to one or more of the inner layers of the board, whereas buried vias only connect between inner layers. Buried vias come in the through-hole variety, or they can be be filled and used to connect between two adjacent layers.

ELIC PCBs that require HDI design can use buried vias to connect between individual layers, rather than selectively spanning pairs of layers. In an ELIC PCB, blind and buried vias can be stacked in order to access every layer in the board. All vias in the stack must be filled with copper in order to create the required electrical contacts between layers and provide structural integrity to the via stack.

There is another reason that stacked vias in an HDI PCB should be filled. Lateral manufacturing tolerances on the position of stacked vias in an HDI PCB need to be very tight. If tolerances are not tight enough, buried vias that are stacked on adjacent layers can be offset slightly. Filling these vias with copper ensures that the vias in the stack will form a reliable contact, even in the presence of some lateral offset.

Stacked vias give designers a simple way to gain access to multiple layers when the vias diameter becomes very small and drilling becomes difficult. Smaller holes require slower drilling, especially when the vias have large aspect ratio. Drilling smaller via holes also increases costs as the increased stress on small drill bits causes them to break more often. Stacked blind and buried vias in an ELIC board are drilled and plated in the individual layers, rather than drilling and plating through an entire stack.

Using buried vias can also help you reduce the number of layers required to route all of your traces and/or reduce the overall size of your board. Since these vias only run between inner layers, you’ll have more room to route traces on the surface layers. This is useful when fanning out a BGA. If you're using through-hole vias and your escape routing is taking up too much space, you can reach the inner layers with blind and buried vias and widen the breakout channels.

Because buried vias do not take up space on the surface layers in a multilayer board, components can be placed directly over a buried via. Buried vias are really no different than conventional vias as long as designers obey the typical clearance rules in the buried layers.

Vias used for fanout and escape routing from a BGA

Vias used to fanout a BGA

How Cost and Quality Standards Affect Design with Buried Vias

It’s one matter to include vias in your multilayer design, but your manufacturer may have something important to say about your design choices. Many manufacturers do their best to ensure that their processes are compliant with industry standards, including ISO, IPC, and UL standards. Manufacturers will take steps to place limits on the number of lamination and thermal press steps used in a process, the sizes of annular rings in vias, and other design aspects.

In HDI PCBs, laser drilling may be a better option for placing access between the inner layers. However, when the number of lamination and thermal press cycles is limited due to quality control standards, particularly UL standards, laser drilling between each layer may not be possible given the prescribed limits on the number of lamination cycles.

In this case, you’ll have no choice but to drill through a layer stack and use a plated through-hole buried via to reach between the internal layers. The outer layers can then be accessed using blind and buried vias. As long as you’re creative with designing your stackup and defining drill pairs between layers, then you won’t exceed the acceptable number of lamination and thermal press cycle steps.

Blind and buried vias also have a cost trade-off compared to through-hole vias. Blind and buried vias incur higher manufacturing costs per via as the board will require additional machining steps. However, they allow denser routing on the surface layers and can be used to reduce the layer count. This might offset the overall cost of the additional machining steps, depending on how creative you can get with your routing.

Dense routing on a green PCB

Utilize all your knowledge when working with buried vias

If you want to include buried vias in your next HDI PCB, the CAD tools in Altium Designer 18.1 are just what you need. These features help you automate much of your routing process and make it easy to visualize your layout in 2D and 3D. The rules checking, simulation, and deliverable generation features help guide you throughout the design process.

To learn more about Altium Designer and how it can make your design process more efficient, talk to an Altium expert today.

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