Yup, flux is great, under appreciated stuff. First you apply the flux to the area you'll be soldering, then you put just enough solder onto your iron and drag it along the pins. The evaporating flux will pull the solder onto the pads. The hard part is in judging the right amount of solder to use.
There's another technique I've used in the past for fine pitch TQFP's. I call it "impact soldering".
It goes something like this:
- Mask everything around the component to be soldered with masking tape
- Flux as usual
- Place component down and align it
- Tack on the corners in order to ensure alignment
- Don't worry about solder bridging
- Now, apply solder to the pins
- Use a healthy dose
- You actually want an entire row to be brideged
- You should see a solid strip of solder across all pins
- Get all four sides done the same way
- Now, take the iron and heat up one of the beads of solder to the melting point
- Without any delay, hit the board edge-wise on the table
- The molten solder will come flying out
- A small amount of solder (just enough) will remain on the pins
- With practice you can get perfect factory-looking joints with no bridging whatsoever
It takes a little practice, but I have found that if you need to do a lot of TQFP's this technique, once mastered, works far better than trying to apply solder precisely. It's messy at first and you might even ruin some parts. Once the technique is perfected it works amazingly well and it is very fast.
Obviously, if you have a lot of parts on a single board you are entering territory where reflow soldering in an oven is a far better idea.
