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Author Topic: Power limit of a via  (Read 190 times)

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Offline JFK422

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Power limit of a via
« on: Today at 05:42:03 AM »
Hi

So I am designing a PCB and I need to pass about 4.5W through a via. I'm uncertain on how to set the diameter. At the moment I have a inner one (of the hole) of 0.3mm and a outer one of 0.62mm. I've calculated so far, that i need about 0.5mm traces for the ammount of current. (about 1.2A)

Thanks for any help
 

Online T3sl4co1l

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Re: Power limit of a via
« Reply #1 on: Today at 06:16:06 AM »
What 4.5W, do you mean thermal conduction?

1.2A is nothing for most any via size.  Assume the via I.D. circumference is lined with 0.5 to 1oz of copper: in other words, the equivalent trace width of the via is about 1.5 times the I.D..

Tim
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Online KL27x

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Re: Power limit of a via
« Reply #2 on: Today at 03:07:41 PM »
^Good to know, on thickness of the plating.

if the id is 1, the circumference is 2 pi R or 1*D*pi. . That's 3.17 x the ID. So why only 1.5X? Is that to leave extra margin because the plating makes the drill hole smaller, or what?

The ratings for current a trace can handle take into account heat dissipation. If the trace is very short and connects to pads (heatsinks) on either end, it is going to handle much more than that without cooking. A via is typically 64 mils long on a standard board. And it has larger pads on either end. So I bet you can go much smaller on vias than w/e Ian's formula says.

I'm not sure of the reasons why, but in some of the datasheets for parts I have used, they recommend multiple small vias in order to increase current capacity, rather than making one huge one. This is how I do it, for no other reason that copypasta one via is easier than changing the size. Maybe it has to do with laminar airflow. I wouldn't expect much air flow through a via. Making the via larger takes away area you could be using for top and bottom pad/plane.

FWIW, I just came across another thread where forum id Rerouter posted this:
Quote
Unless your using a special process via plating thickness is generally half your board copper thickness, e.g. a 1 Oz board begins as a 0.5 Oz board, they then put it through some chemical baths so the copper can plate to the via hole walls, then plate the entire thing with another 0.5 Oz of copper, making a 1 Oz board with 0.5 Oz via thickness
« Last Edit: Today at 03:24:39 PM by KL27x »
 

Online T3sl4co1l

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Re: Power limit of a via
« Reply #3 on: Today at 05:48:59 PM »
I'm not sure of the reasons why, but in some of the datasheets for parts I have used, they recommend multiple small vias in order to increase current capacity, rather than making one huge one. This is how I do it, for no other reason that copypasta one via is easier than changing the size.

Solder wicking -- it tends not to flow into small holes ( <= 12 mils, particularly in a lead-free process), and if it does, it doesn't steal much solder from the pad.

This only applies to via-in-pad situations.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 


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