Reflow soldering is a process in which solder paste is applied to a printed circuit board (PCB), and then components are placed onto the solder paste coated pads. The entire assembly is then subjected to a controlled temperature profile that melts the solder paste, past and solders the components to the pads.

Limitations 

• Only 1 side of the board may be reflowed
  • You should therefore only have fine pitched surface mount devices that can't be hand soldered easily on one side of the board
• Large connectors may be melted
  • Dura-Cliks have so far been susceptible to this problem. They should not be reflowed with the board and instead should be either hand soldered or reworked with the hot air station. 

Required Items

• PCB
• Stencil made for the PCB 
• Solder paste
  • Solder paste should be taken out of the fridge in small quantities prior to being used. Be sure to follow the manufacturer's instructions
• Spare PCBs used for securing the board
• Microscope for inspection

Setup

1. Follow the instructions provided by the solder paste data sheet. They have a short shelf life at room temperature, so you may need to take it out of the fridge and allow it to settle to room temperature a few hours beforehand. 
2. Put a few PCBs around the board that you wish to reflow, this is to provide a smooth, level surface that you can apply solder paste to. Securely tape the entire assembly to a flat surface

...

1. Wipe down the PCB and stencil with some isopropyl alcohol to remove any residue from manufacturing or handling
2. Align the stencil such that it matches all the pads, and tape one side to your surfacethe surface. The stencil should now be able to be lifted without messing the alignment up.  
3. Apply a generous amount of solder paste to one side of the stencil
4. Hold a straight object such as a paint scrapper or a knife blade at a 30-45 degree angle, and make one smooth path while applying even pressure over all the padsLift the stencil up and . No more than one pass should be made to avoid excess solder from bridging connectors
5. Carefully lift the stencil up and inspect the pads. If the solder paste is at a uniform thickness and doesn't miss any obvious spots, it should be inspected under a microscope to check for fine pitched components. Some shifting is allowed as the solder paste will flow onto the pads when heated, but significant shifting may cause issues. 
6. If the application was not successful, return to Step 3 to clean the PCB and reapply solder paste. 

Note: Excess solder paste on the stencil can be reused within a few days, but any longer will dry it out. It is recommended that you only take a small amount every time you apply solder paste from the large jar.