I've found QFN are easy to work with, even using TT.
The fear of shorts below the part is mostly unfounded since solder is
pushed out by surface tension (the gap is too small for beads to
form). If it isn't soldered nicely just heat it with hot air or on a
hotplate and tap gently on top. That will express excess solder out
the sides where you can remove it, and also re-seat the part if it is
not aligned right.
The tip with the pads extending out is definitely a good one and very necessary.
You can also solder QFNs without any reflow process, drag soldering
works just fine. Small parts can even be changed with just a soldering
iron by putting a large tip on it with a lot of solder and "contact
heating" the top of the part. Certainly exceeds specified soldering
conditions but I have never killed a part this way.
ST
On Thu, Jul 14, 2011 at 6:55 PM, DJ Delorie <dj@...> wrote:
>
> In theory, you should be able to do 0.5mm pitch QFN with TT - 0.25mm is
> about 10 mil, which is "in range" for TT, assuming your process is
> accurate enough. You'll probably want to use 1/2 oz copper though.
>
> What I've found is that you need to extend the copper away from the qfn
> chip far enough to be able to hand solder it ∗also∗ - even with a
> hotplate, it's likely you'll need to fix the joints, and that's a lot
> easier if the pads are accessible next to the chip - lets you drag
> excess solder away from the chip, for example. To fix, use a flux pen
> along that edge, add a little solder to your iron, and TAP it along the
> edge. you don't want to drag solder along the edge, you want to drag
> extra solder away from it.
>
> If your QFN also needs a center pad, be careful to make it small enough
> to avoid solder bridges - you won't be able to see or fix those. If
> your center pad need electrical connectivity, either tie it to a pin or
> drill a big hole through the center so you can manually solder it from
> the other side.
>
>
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