Electronics > PCB/EDA/CAD

Vias as belt and braces?

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paolo:
I've been laying out my first board. It's fairly simple, a driver chip and a bit of decoupling. Because it's based around the TI TLC5947, I've been taking some inspiration from the TI demo board.

One thing that's puzzling me... there's lots of vias all over the place, not just the minimum needed to bridge between layers.

Is there some kind of design principle of adding them beyond just making basic connectivity? e.g. minimising signal paths between areas of pours, or adding robustness by making the connections diverse? I'm guessing at the moment.

The board I've been using as inspiration:

http://e2e.ti.com/cfs-file.ashx/__key/CommunityServer-Discussions-Components-Files/192/3872.TLC5947_5F00_Layout_5F00_PCB.pdf

poptones:
Think of every connection on the board as having two wires - the wire from the driver to the load, and the wire returning the ground.

Now, imagine the ground wire runs all over the place, in this spaghetti path, back to the driver. What happens to the inductance of that connection? What does the ground look like? Remember, electricity follows the path of least resistance. So is the ground return path at high frequencies going to be distributed evenly across all the copper and vias?

The vias minimize inductance of the return path.

paolo:
OK - that makes sense, and will I'm sure help me place extra vias sensibly. Thanks. :)

But here's another question, specifically about the board design I linked to.

See the top edge of the board? There's a line of vias up there, and I can't figure out how those would help shorten the return path. To my eyes they're bridging the top and bottom and ground pours, and yet the nearest ground components seem to be miles away. Maybe there's something else I'm not understanding.

SeanB:
They join the 2 planes so that they do not act as antennas.

paolo:

--- Quote from: SeanB on November 28, 2012, 04:39:31 am ---They join the 2 planes so that they do not act as antennas.

--- End quote ---

More enlightenment. :) Good practice to scatter some round the edges of the planes then, fixup the outer reaches?

(You may be realising my analogue is massively beginner level.)

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