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What Is Reflow Soldering?

Learn what reflow soldering is from expert Jeff Koskulics in this Howcast video.


Reflow soldering is the modern mechanism for assembling most printed circuit boards today. Reflow soldering enables mass production at a very rapid rate by allowing placement of the components to be separated from the actual soldering process.

In reflow soldering, the pads of the printed circuit boards are first covered with solder paste, which is a mixture of tiny beads of pure solder, little spheres, and solder flux. This mixture is called solder paste. Solder paste is applied in a pattern using a stencil so that it leaves solder paste only where it's necessary to solder the terminal of a device.

Now, solder paste is sticky so that devices can be placed onto the paste and remain held there by simply sticking, a little bit of viscosity. With all of the devices in place on the printed circuit board, the board is placed in an oven, where it is raised to a temperature above the melting point of the solder. The solder then reflows onto the terminals of the device and the board is then allowed to cool. At that point, all the devices have been attached simultaneously and the board is fully assembled.

Now this makes it, this process is somewhat different from how somebody would solder by hand. When you're hand soldering you're treating one component individually and applying heat to each component at a time so the technique has to be a little bit different. And since our soldering iron is a very different tool from the reflow oven we have to come up with an alternative technique, a special technique that I've come up with.

Now, stencils are available in kits, available to amateurs who want to do reflow soldering. If you get a specialized kit for your printed circuit board so that you can apply the solder paste in the proper pattern, and then by using tweezers, placing individual components onto the board. Now, the board can be placed into an oven, such as a toaster oven, and raised to the proper temperature to melt the solder. In doing so you can get the same kind of professional quality that you'd get with a machine manufactured board.

Since the solder fumes can be caustic and they release lead oxide, it's probably not a good idea to use this in a toaster oven that you'd use for cooking. So use a special oven dedicated for soldering.

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