BoardSurfers: PCB Electronics—Why Use Via Arrays?

Tyler in his post IC Packagers: Plan Your Escape with Modernized Structures has talked in detail about the enhancements in via structures in release 17.4-2019.  And, if you are already using via structures, I will recommend Tyler's blog. But if you are new to via structures, do read on.

We all know the ubiquitous via. What is it after all but a way to make electrical connections in a multi-layered PCB? But if you think about it, it must have been a clever solution when it first occurred to someone. Someone must have said, let's place a pad and then dig through it - make a hole, copper-plate the hole to make it conductive and we have a connection from the top to the bottom layer.  Then, innovative that we homo sapiens are, someone must have thought of a blind via - why do we need to connect the bottom to the top or vice versa, always? Why not dig till the power or ground plane and stop there? Then it must not have been too difficult a task to have come up with a buried via. Let's dig, but why from top to bottom or in between? Why not from, say, the fourth to the sixth layer in a 12 layer board?

Of course, teams must have encountered a tangency or a breakout here and there early on and realized the precautions to be taken with the annular ring size. So, a team member must have said, well, this hole is touching the boundary of the pad, let's call it tangency. Or, this hole is outside the pad altogether, there's a breakout mate!

However this must have happened, it must always have been to solve a problem—to address a need. One is obvious, of course, with increasing components and reducing size, multi-layer boards was a natural progression and with it a way to connect across layers. And, the truth is, that is but only the obvious use of a via. For example, if you are working on an RF design, most probably, you are using vias to reduce electromagnetic interference and cross-talk, that is, via shielding. Or you might be using vias to manage return path impedance, namely, via stitching. And, of course, you have thermal vias to dissipate heat.

For all the different purposes you use vias, it is rare that you will use a single via. Most often, you will deal with an array of vias or a via farm. One reason is obvious, it eases your task. With an array, you have a defined pattern that you can reuse. And, anyway, your fabrication house or manufacturer will have restrictions on the size of vias you can have and they will prefer vias of one size. So, it will most often be the only way out—an array of vias in place of a single large via. But remember that a via array does more than that. A via array can be used to mitigate electromagnetic interference (EMI), control crosstalk, suppress resonance, and improve power integrity.

That's why a good tool will give you options to create boundary arrays (think shielding) or matrix arrays (think heat dissipation or stitching). You might want a boundary array of vias to be along one side of an object or surrounding an object. Similarly, you might want your matrix array of vias across the board or a windowed area. A good tool must give you options for that, too. 

Of course, through all this, I haven't even touched the tip of the iceberg. There's a lot more to vias and via arrays and what all you can do with them (microvias, for example). But if you want to immediately get started with a boundary or matrix via array or any via pattern in general, the Working with Via Array in Allegro PCB Editor Application Note will be useful. I will encourage you to download the database accompanying this Application Note as well and try out a few of the examples. And, do keep an eye on this space for more on vias, via arrays, and related staff. Happy exploring via arrays till then.