After cleaning the pads where the SMD components were supposed to be mounted on, I applied a bit of flux to them. Then, I put a bit of leaded solder paste (XG-50) on each pad (I think I could have used a bit less solder paste) using a toothpick (you could also use a solder paste syringe with a tip in the range of 0.26~0.43 millimetres). During this step it's important that you follow safety procedures because lead can cause cancer. Keep your room ventilated (I used a fan near a big open window) and don't touch it with your fingers because you could end up eating it (be careful when you use the toothpick or use nitrile gloves). You can also buy lead-free paste but it requires higher temperatures to melt, and that could well be as hazardous. When you are done, place your SMD components onto the solder paste using tweezers. After using the solder paste, you should ideally conserve it at a low temperature, within 2~10 °C, or otherwise it will dry out in a few weeks. However, you need to get a small refrigerator for that (never use the one you use to store food because the solder paste can slowly release toxic fumes even at a low temperature).
The next step consists of reflowing the solder paste by heating it up according to what it's called a thermal profile. You can use many different tools for heating the solder paste: hot air, a modified toaster oven, or a hot plate for example. In my case, I used a Yamazen HG-1200 hot plate for several reasons: first it's cheap (~3000 yen in my case), second it has a glass lid very convenient for checking the state of the process, and what's more important it can heat over 230 Celsius degrees (essential for reflow soldering). Besides that, this Japanese fellow explains a few tricks on his website that he had to learn the hard way in order to use the Yamazen hot plate successfully for reflowing solder paste. The thermal profile I ended up using was:
- Pre-heat at 160 °C for 10 minutes.
- Place the PCB onto the hot plate.
- Wait for a minute.
- Change the temperature to 170 °C.
- Check the temperature: when it reaches 170 °C, switch the hot plate off.
- Wait 2 minutes (the hot plate's temperature keeps raising up to 180 °C for some reason).
- Change the temperature to 210 °C.
- Check the temperature: when it gets to 210 °C turn it off again.
- Check the temperature: when it gets to 220 °C, wait for 40s (the hot plate will peak at about 230 °C).
- Finally, open the glass lid and let it cool down. Once its cooled down you can clean it with isopropyl alcohol and cotton sticks (q-tips)
After reflowing, I used a multi-meter to check for possible shorts. Fortunately, there weren't so I just continued soldering the rest of components by hand with my soldering iron (a hakko FX600). If you get shorts or "bridges" try fixing them with your soldering iron.
And this is the result. Next, you need to buy the ATMEGA328P-PU microcontroller either with or without bootloader, and a USB-Serial converter like this for loading your Arduino sketches.
Conclusions: reflow soldering is fun and easy once you have the appropriate tools. It is great for soldering little SMD components which are cheap and allow you to create high density boards.