[sdiy] Vias...

[Steve Lenham]

On 20/07/2012 14:46, Justin Owen wrote:
> PCB cart quote their minimum drill/line width and spacing as being
> 0.2mm.
>
> I'm currently using vias with a 0.3mm hole and a 0.8mm diameter.
>
> I haven't done this before...

Those are quite small, especially for a first time!

> ...but is there any reason why I shouldn't be putting vias under SOIC
> chips (I've always tried to avoid that previously - I thought the
> slight curve of the via ring under the chip might cause problems for
> the pick'n'place machine)?

AFAIK that is completely fine (I do it all the time). The top of the via 
ring is level with the pads that the IC attaches to, so shouldn't raise 
it any further.

> ...and is there any reason why I couldn't put a via on top of an SMT
> pad that was bigger than the via (not sure if Eagle will even allow
> that - I've always put vias off to the side of pads)?

You CAN do that, but it isn't good practice. When the solder paste is 
reflowed during manufacture, it tends to disappear down the via hole 
leading to bad joints. The furthest I tend to go is to overlap the via 
and pad such that the via hole is placed just on the edge of the pad.

Hope this helps,

Steve L.

Benden Sound Technology
www.bendensound.co.uk