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What is the white powdery/smokey stuff he drops on the eMMC chip using the soldering iron at 7min27s? Some kind of flux?

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Yes, it is flux (solid rosin). He vaporizes some with the iron and the smoke resolidifies on the contacts, so there is a very thin layer of flux to help with the soldering.

Interesting, I’ve never seen this before. How does it compare to regular gooey or liquid flux? Also how does it work? Just bringing a chunk of rosin next to a soldering iron would just vaporize the flux as seen on the video?

My grandfather who learned electronics work in the USSR worked with solid rosin.

When you touch the iron with the rosin, it starts smoking like incense at a church (same stuff actually), but a small amount remains on the tip. You transfer this to the contact you’re soldering, dip into the rosin again, and then solder as usual.

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