Establishing continuity in a single layer PCB is relatively straightforward—simply link the components in your circuit together using traces. In this context, a via is nothing more than a way to tunnel vertically down to ground and complete your circuit.
But once you get to multi-layer PCBs, vias get more complex. Managing the connections between the different layers of your board becomes a multi-dimensional puzzle that requires you to balance physical constraints with EMI/EMC considerations.
If you want to get your PCB manufactured, it helps to be able to communicate what you’re trying to do with any given via. In this post, we’ll walk you through the different types of vias that are out there, so that you’ll have the flexibility you need to route your own multilayer designs.
Pick up a PCB and face it towards a light. If you can see light shining through a via, you’re looking at a through hole. The most common type of via is a plated through hole (PTH) which involves drilling a hole through all the layers in a PCB until you exit out the other side. The walls of a PTH are plated with a thin layer of copper that forms a conductive barrel that links all the layers together. There are also non-plated through holes (NPTH) which are used primarily as mounting/tooling holes for assembly and/or operational use.
If light doesn’t shine through when you face your PCB towards a light, you’re looking at a blind via. It means the hole stops at one of the inner layers in the board. Blind vias connect an outer layer of a board to one or more inner layers without exiting out the opposite side of the board. As PCB designs get more complex, PCB real estate becomes limited—there’s no reason to tunnel through more layers than you need to in order to achieve the desired connectivity.
On the manufacturing floor, it is easier to drill a through hole than it is to create a blind via, because you must stop precisely at a certain depth within the board. This added manufacturing complexity is reflected in the relative costs between blind vias and through holes.
Sometimes you need to make connections between two inner layers in a board, but don’t need to and lack the space to drill through the entire board. Buried vias connect one or more inner layers of a board together without exiting through an outer layer. The ultimate space saver, and the most expensive option on this list. Creating a blind via requires extra manufacturing steps to stack and drill the inner layers before adding them to the rest of the board. That means you need to determine whether you really need that blind via, or if it’s feasible to save costs with a blind via or through hole.
The future of PCB design: HDI and Microvias
Now that you understand the three main types of vias that are out there, it’s time to take a look at the future of PCB design: HDI (high density interconnect) PCBs. The demand for smaller boards and ever faster signals translates to a need for even smaller vias.
According to IPC standards, a microvia is a catch-all term for any via with an aspect ratio (depth to diameter) of 1:1 and a depth not to exceed 0.25 mm. The old definition you’ll see floating around the web is any via less than 15 μm in diameter, was phased out in 2013 as the size became more commonplace. Laser drilling is expensive, so you likely wouldn’t use microvias unless you had an HDI PCB design where board real-estate is a premium.
As you might imagine, keeping track of all your signal integrity, EMI, physical, thermal and cost considerations in an HDI board can get pretty complex. Fortunately, EDA software has evolved alongside advancements in PCB technology to help give designers all the tools they need to bring their designs to life. Check out Cadence’s suite of PCB design and analysis tools today.
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