Re: gEDA-user: QFN soldering

[Geoff Swan <shinobi.jack@xxxxxxxxx>]

For quick breadboarding of an accelerometer I've deadbug soldered one
successfully. Just superglued it upside down and soldered directly to
the pads with very thin wire..



On Mon, Sep 13, 2010 at 12:04 PM, DJ Delorie <dj@xxxxxxxxxxx> wrote:
>
>> does anyone have experience with this package?
>
> Just did one today.
>
>> I want to know if they
>> are hard to work with.
>
> Harder than a QFP, but not impossible for home-fab.  Pen flux the
> bottom of the chip before placing it on the paste - I wish I'd
> remember this more reliably :-)
>
>> The exposed pad underneath is a problem for hand soldering - but
>> maybe could be left unsoldered for prototypes.
>
> Depends on the chip.  Some require an electrical connection, it may be
> the only ground for example.
>
>> Maybe just place some solder paste under there ?
>
> If you have to solder an exposed pad manually, you have (IMHO) exactly
> two choices:
>
> 1. Put a via right under it, with a drill big enough to get your
>   soldering iron in there to solder it from the back side.  Keep in
>   mind the big thermal sink this will be; your smallest iron tip
>   might not be up to the task.  Obviously, do this after soldering
>   the edge pads :-)
>
> 2. Use a solder paste stencil of some sort and reflow it (heatgun,
>   oven, hotplate).  I make QFN stencils out of alumimum foil and UV
>   film, I've done it with toner and thin brass too, and once with
>   brass and a dremel drill press.  But you can't just squeeze paste
>   out of a tube and expect it to work - too little won't conduct and
>   too much keeps the other pins from touching.
>
> Note that for home-etched boards, #1 requires a tiny wire, else you
> don't really have anything to solder to.  Surface tension will keep
> the obvious idea from working :-)
>
>> If the pcb pads are long enough, is it feasible to solder to the
>> edge of the chip instead of getting it underneath the device?
>
> If you use #1 above, and the pads extend contiguously up the edges of
> the chip and not just on the bottom, yes.  Use LOTS of pen flux and
> make sure the pcb's pads extend out far enough for a thermal
> connection with your iron.  I've done this before, and the flux/iron
> trick can be used to fix reflow problems too.
>
> Note: some QFNs have copper on the side which is *not* contiguous with
> the pads on the bottom.  The FT232RQ is such a chip.  You have to
> reflow these, although the flux/iron trick can still repair them once
> there's *some* solder under the chip.
>
>
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