Tools of the Trade – Reflow

In our previous issues in this series on making circuit boards, we covered placing solder paste and placing components. Now it’s time to bake our cake!

There are a variety of methods for reflowing a circuit board, but they all rely on a single principle: heat up the solder paste (a mixture of flux and solder) until the flux burns off and the solder becomes liquid, and then cool it down. Accomplishing this once or twice is easy; once you’ve played with a hot plate you’ll swear off through hole. Scaling it up and doing it repeatedly with high yield is extremely challenging, though.

Hot Plate

A hot plate with aluminum on top. Some breakout boards are just starting to flow.
A hot plate with aluminum on top. Some breakout boards are just starting to flow.

Starting off with the most basic tools available we have the hot plate or griddle method. This involves an electric hot plate ($20 investment). Usually around medium heat is a good setting for most of these plates, and you know it’s ready when you can put some solder on it and it turns to liquid. Then you place the board on the hot plate and wait and watch until everything has flowed, then wait a little longer until most of the flux has burned off, then you take it off the plate. Generally you want to move the PCB around with tweezers or needle-nose pliers during this process, as there will usually be hot spots and you want everything to flow at roughly the same time. DO NOT use the fancy science hot plates with the magnetic stirrers in them. When the paste starts to flow the reduced friction will allow certain components, especially inductors, to fly across the board as they are attracted to the magnet, causing a hot mess. It’s also helpful to have a piece of wood or metal to transfer the PCB to after reflow, as it will be very hot and you don’t want to set it down on a kitchen counter. Finally, I like to have a 1/4″ aluminum plate on top of my hot plate to even out the temperature and provide a smooth surface. Another neat trick is to place a small chunk of aluminum or other metal on top of the hot plate and reflow only a certain section of the board. This is particularly useful if you need to remove a microcontroller from an already assembled board but don’t want to reflow the whole board.