> Leon and Stefan wrote:
> [About drag soldering QFNs]
If the chips have legs like TQFPs or
even stubby metal stumps like _some_
QFNs have then you can drag a pool
of solder across the pins and the
surface tension will leave a nice
clean set of legs.
However the QFNs that you can hardly
see the pads on the side of there is
not much choice but to reflow them.
Also anything with an exposed pad
you must reflow.
I have a hot air tool and all the
gear needed to do it properly,
however when I want to do it quick
and dirty I just use a butane torch.
(only on prototypes mind you)
Using the hot air tool you need to
carefully put down paste or better
still use a stencil.
The dodgy butane torch method you
just pre-tin the pads with 60/40,
pre tin the pins on the chip, run
your flux pen across the PCB and
place the chip.
I then place the target PCB on a
scrap of FR4 and preheat from
below for a bit. I then move the
heat to the top side and very
gently continue the heat. When I
see the chip move and allign itself
I give it a bit more heat close up
and then back off and let it cool
slowly.
A small off cut of 60/40 sitting on
the scrap PCB can be a good
indicator of progress. Spend about
20% more time pointing the heat at
it and when you see it melt you know
you are close.
As far as dragging the pool of
solder across anything and having
the surface tension pull the pool to
leave a clean pin - I think the temp
is very important. Too hot and the
flux burns away, too cold and it is
not liquid enough. That is the
reason I think that a thicker tip is
better. The fine needle tips I have
(on a Hakko) just don't get the heat
down the business end fast enough.
I get much better results dragging
the solder away with a tip that is
over a mm than one that is 0.7mm.
Also touching new solder to the pool
very regularly to keep flux in the
game helps me out.