When I selected the tiny QFNs for the LDO and the MCU, I imagined it would be hell to solder them correctly. I was sooo wrong !

Check this out:

You need three things:

• Liquid flux: I used this.
• Hot air soldering station. The cheapest kind available is perfect.
• Cleaning alcohol, or acetone if available. It does a great job removing flux residue.

Here is how I did it:

1. Turn your components upside down, to see the pins.
2. Mount a needle on the flux syringe and place a small amount of flux on the component's pads.
3. Holding the part with tweezers, place solder "cushions" on the pads with the soldering iron. It doesn't have to be a insanely small tip, the flux will not allow formation of solder bridges.
4. For the exposed pad, place a smaller amount of solder. Too muck will make the component float on it and the pins will remain unconnected.
5. Clean with alcohol.
6. Repeat steps 2 - 5 for the pads on the PCB.
7. It is important to clean flux residue on the pcb and component.
8. Place FRESH flux on the PCB pads (an amount similar to the one used for tinning the pads).
9. Place your component on the pcb. The flux stickiness will hold it in place. Make sure you centered the pins on the pads.
10. Set the hot air station at the lowest airflow and at 350 deg Celsius (mine does not actually output 350 deg., but below this setting solder won't reflow). By the way, I used Sn60Pb40 solder wire.
11. If properly aligned, the component will auto-position on the pads when the solder reflows. If not, while in liquid phase, gently move the component in place. If there is enough flux. it will "snap" into place.
12. That's it!

The photos were taken with the phone stuck to a microscope eyepiece.

Alright, 2 components down (the most challenging ones), 67 more to go. I won;t be able to finish 2 boards today, so I'll be back tomorrow with fresh photos.