If you are looking for a challenging soldering project, then the FemFemFemto necklace/keychain kit might be a good choice! It uses the BGA package version of the 555, 3 0402 resistors, 2 0402 LEDs. and a huge (by comparison) 1206 tantalum capacitor.
If you’ve never done solder paste/hot air soldering before, then this might be a bit tricky. Thankfully, the components involved are very inexpensive, so if you wreck something, ordering replacements is not a big deal. As the sellers suggests, use some Kapton (polyamide) tape to hold the board down and to cover the parts you don’t want solder on.
Use good-quality, fresh solder paste, with liquid flux if needed. As long as you get a decent glob of solder on each pad, the miracle of hot-air reflow will take over and all the pads will get reflowed, with the solder resist helping “pull” all the solder together. You’ll suddenly see all the parts “snap” into place once the solder melts. If you get any shorts under the BGA, this can sometimes be solved by adding more liquid flux, reflowing, and gently tapping/pushing the BGA. A very steady hand is needed for this. However, unlike most BGA packages, this one has all the solder balls at the edge of the chip, which will actually make it a lot easier to solder by hand, and decreases the likelihood of shorts. Remember that BGA packages already have solder balls on them — you don’t need to apply solder paste to the BGA footprint. Just put lots of liquid flux and heat it with your hot air gun.
Once you are done, you have (probably) the smallest 555 LED flasher kit ready to put on a necklace or keychain! It’s battery powered, and the SR521 batteries themselves are quite tiny. They will fit on the back in the much easier to solder surface-mount battery holder. I would recommend soldering these on using a normal soldering iron after you’ve completed the front side.
A very cool little trinket to test your reflow skills with! Wear it with hacker pride.
source https://blog.tindie.com/2020/09/the-smallest-555-kit-ever/
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