I found inspiration for this tutorial after reading this handy guide on Sparkfun's website. https://www.sparkfun.com/tutorials/59#Hot%20Plate%20Reflowing
As always, follow the instructions here at your own risk. Take all necessary precautions as you will be working with small, sharp, hot, and electrically charged parts.
Below is a quick video demonstrating the reflow soldering process (for best results try playing fullscreen in 720 or 1080p):
Step 1: What You Will Need
I used an Oster 12" skillet for this tutorial but other makes and models will most likely work just as well. Make sure the skillet you are using can get hot enough to melt the solder paste your are planning to use and look for ones with lids to ensure even heat distribution.
Solder Paste -
Again this can vary, I used Chip Quik Low Melting Lead Free because it was locally available and had a fairly low melting temperature but you can use whatever solder paste suits your needs. Keep in mind the maximum temperature of your particular skillet when considering pastes. If your board will be subjected to potentially high temperatures you might want to consider using a paste that reflows at a higher temperature to prevent it from melting in these extreme environments. Also be aware that solder paste can dry and become unusable if not stored correctly so be sure to read the manufacturer's instructions on storing your solder paste.
Toothpicks and Cotton Swabs - or
A Solder Stencil and Squeegee -
These are two methods of applying the solder paste to your circuit boards. You can get by as I did using a toothpick to carefully apply small dabs of solder paste to the pads that need to be soldered. This is time consuming and I would not recommend it for a production run of boards but works well for prototyping. The solder stencil method will be more efficient if you are planning to solder a more than a couple of boards but will cost you more as you will need to have a custom stencil made.
Used to grab and position the small parts. Tweezers with an anti-static coating are recommended to dissipate the buildup of static electricity away from static-sensitive parts.
Good lighting and magnification -
A bright desk lamp or direct sunlight will be sufficient enough for this project. You should also have access to some sort of magnification as it will make it easier when the process is finished to find solder bridges and inspect your work. A small jewelers 10x or 30x loupe can be found online for very little money and will provide ample magnification for this task.
Soldering iron -
This is used after the reflow process for fixing any solder bridges that might occur between component leads and for general touch ups. A iron tip with a large, flat surface area such as a chisel or hoof tip will work well for this.
Liquid Flux -
Used to help fix bridges by allowing the solder to flow more easily. I use a water soluble flux pen for easy application.
99% isopropyl alcohol -
Also known as rubbing alcohol, this can be found at most local drugstores. This will be used remove excess flux residue from the board after the soldering process.
De-soldering Wick/Braid - (optional)
Useful during the bridge removal process to soak up excess solder but is not necessarily needed.
1/8" Aluminum Plate - (optional)
This can be used to line the bottom of the skillet for more even heat distribution and easier cleaning.
Step 2: Getting Started
To start, you should lay out all of the parts to be soldered and equipment in an easily accessible configuration.
Be sure to have any schematics, data sheets, and bills of materials out to reference so you know which parts go where.
Paste Application -
Once you have your supplies and tools ready, its time to apply your paste. The following steps will be different depending on what approach you are using to apply solder paste.
For the toothpick/cotton swab method your goal is to dab small amounts of paste on to each pad using the toothpick. The paste does not need to cover the entire pad, there just needs to be enough of it to hold the component securely and make a nice fillet between the board and component after the soldering process is over. Do not worry too much about exactness or neatness here as solder paste is fairly forgiving. If your solder comes in a syringe as mine did you may find it easier to extrude a bit of solder onto a piece of paper and manipulate it with the toothpick. I found the needle often placed down too much solder and it ended up in unwanted regions. If you need to remove solder paste for any reason during this process you can use a cotton swab to gently lift the unwanted paste off the board.
If you are using a stencil be sure to secure your stencil to the board accurately and securely. Apply a generous amount of solder paste to the top of the stencil and spread by applying even pressure and speed with your squeegee. Scrape the excess paste back into its container for later use. Reducing the angle of your squeegee as you continue your stroke across the surface of the stencil may help ensure a more even coat of paste.
Placing Components -
After the paste has been applied you can use tweezers to place components onto your board. Be sure each part is placed flat against the board and also be cautious of the orientation of the parts. If you are unsure of polarity, resistance, capacitance, or any other factor always check the schematic and data sheets, and/or use a multimeter to reassure yourself that everything is correct. Double checking everything once over before continuing is a good idea as it is at least 100x easier to fix mistakes while the solder is still a paste.
Step 3: Skillet Skills
1. While keeping board level, carefully place the PCB in the skillet.
2. Cover skillet with the clear lid.
3. Set the skillet temperature to just slightly over solder paste melting point.
4. Observe board carefully. The solder paste will spread out a little at first and look grey and dull. When the solder becomes silver and reflective it has successfully melted. If you are careful, you can utilize the moment when the molten solder is still fluid and manipulate any parts that may have moved out of place with a toothpick or tweezers.
5. Once all solder joints have been made you can remove the lid and turn the skillet off. Do not remove the board from the skillet until you are sure that it has cooled off and that the solder has solidified.
Watch the video below in full screen to see the final soldering process in action!
Step 4: Finishing Touches
Now that your board has cooled off and your parts have all successfully soldered, you should now carefully inspect your solder joints. Using a loupe or other source of magnification, check to make sure that your solder did not bridge between component leads and into other places it wasn't intended to go. There are numerous ways of clearing solder between leads of components and you can use whichever method you prefer. One method I prefer is to generously apply flux to the troublesome area and take a clean hoof-tipped soldering iron and try to draw the excess solder onto the wide surface area of the iron's tip. Another common method is to press solder wick/braid against the excess solder with your iron to draw it into the braid and away from the solder joint.
Once you are satisfied with how all of the solder joints look you can finish the procedure by carefully cleaning the flux from the board with 99% isopropyl alcohol and letting it dry. Your board should now be ready to be plugged in and tested!