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I recently came across this video on YouTube, in which the guy is soldering a .5 mm QFP component.

Hand soldering a .5 mm QFP Chip

As I haven't done a lot of soldering in my life (and never used flux), I have a few questions.

  1. The guy in the video uses only flux without solder to do all of the soldering. How is this possible? What kind of flux is he using? As far as I know, flux is only used as a cleaning chemical during the soldering process. Correct me if I'm wrong.

  2. He puts flux over the PCB traces and the space between them. If the flux is conductive, how is it possible that there are no short circuits created?

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    \$\begingroup\$ Please re-watch the video, paying special attention to what is said at about 1:00. "The grooves already have solder in them." \$\endgroup\$ – WhatRoughBeast Jan 30 '16 at 17:29
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    \$\begingroup\$ I will suppose that you don't have the product advertised in the video. Either way, you really must use flux. It will be extremely difficult otherwise since the solder will not wet the pads and leads very well. Some people recommend flux pens, but I haven't had much luck with them. A good liquid flux and a glass fibre pen will serve you much better. Buy good quality, fine solder, and have a temperature-controlled soldering iron with a fine tip. It is even better if you can buy solder paste and use a hot air soldering tool. \$\endgroup\$ – Oleksandr R. Jan 30 '16 at 18:20
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    \$\begingroup\$ The key to soldering qfp's is to let surface tension do the work. You don't need the advertised product, but you do need flux as well as solder. \$\endgroup\$ – Chris Stratton Jan 30 '16 at 20:29
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First a warning to readers: That video is an advertisement for a product.

The board you are looking at comes pre-tinned. Applying solder flux removes dirt and oxidation, and makes it easy to use the tin already on the PCB. The flux itself is not conductive.

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