how to solder QFN/DFN chips?

[Al Clark]

all the PCBs I work with are HASL'd, so yes. Sometimes I add a little
bit of solder to one corner pin, to tack the device in place. In
practice I dont have one of the nice drag-soldering style wide tips,
so solder pins one at a time. Its easy when you can see.....

And just like Spehro said, I use a flux pen.

I have never used a flux pen. I use a small bottle to squirt flux on the
pads. You can't use too much flux but it important to wash your boards
after soldering. If you need to finish your board at a later time
(perhaps the next day), take the time to wash the boards before quitting.

I have seen many approaches to soldering QFPs and other fine pitch parts.
We use a small chisel tip. Apply solder to the tip and let it wick into
your pad & pin. I know this is contrary to the way you solder larger thru
hole connections.

You NEED good vision. I solder using a 7x setting on a stereo microscope.
With a little practice, you will always know where the soldering iron tip
is even when you can't see it. I don't think I have burned a finger in 3-
4 years. Most stereo microscopes have a range from 7x-30x. The higher
magnification is great for inspection.

I also apply a little solder in one corner to tack it down. Then I rotate
and tack down the opposite corner. Make sure alignment is absolutely
perfect before soldering more pins. I like to either use a very small
sharp probe from my multimeter or an xacto knife to verify that all the
pins are secure after soldering.

Fresh solder is also helpful. Solder ages (or at least the flux inside
them). I like Kester water soluble .020 63/37. I don't know what we will
use when we move to lead free. I expect that a great flux will be the
most important element to good soldering.