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Date: Thu, 27 Feb 2014 20:57:06 +0000
From: Ed Simmons <ed AT estechnical DOT co DOT uk>
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Subject: Re: [geda-user] Via under a pad?
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Hi Rob,
On 27/02/14 20:01, Rob Butts wrote:
> Is it bad practice to put vias under pads?  These pads are twice as 
> big as the vias.
>
> Just curious.
Can you afford the via being much smaller? We put vias 'in pad' but make 
sure they're really small so they don't suck up away the solder.

Typically, I tend to try and place vias next to the pad unless 
absolutely necessary.

On the other hand, for heat sinking with the PCB, we use an array of the 
smallest vias our fab house will drill (0.2mm hole) under the large heat 
sink pad of SMT mosfets or transistors, with a large copper fill on as 
many layers as possible to dissipate the heat.

It depends on what you're doing as to which approach is right. ;-)

Hope that helps!
Ed