Re: gEDA-user: OT: soldering QFN packages with exposed bottom pad?

[Stefan Salewski <mail@xxxxxxxxxxxx>]

On Mon, 2009-04-27 at 21:55 -0700, joe tarantino wrote:
> I've done this, but not always with success.
>    First, get some fine solder paste and a flux pen.
>    When you do the layout, put a pad on the other side of the board,
>    connect with a few vias to the part's bottom pad.  Be sure to have a
>    solder resist opening on the back side pad.
>    Apply flux to the large pad only, on both sides of the board.  Then
>    paste the pad, both sides.  Then place the part.  When you heat the
>    back with the iron, the part may want to move.  It often helps to
>    tack-solder a couple of the other pads on the QFN (farthest away from
>    the large pad) to keep it from moving.  Solder the remaining small
>    pins last.
>    Good luck -
>    Joe T
> 

Fine description.

>connect with a few vias to the part's bottom pad.

What diameter do you suggest for the vias?
I think smaller than 0.3 mm will be expensive. Is 0.3 mm small enough so
that solder flows in the hole driven by capillary forces and gravity. Or
may it be better to fill the (larger ?) via hole with a copper wire
(wire from a through hole resistor) to increase thermal conductivity and
capillary forces?

Best regards

Stefan Salewski




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