Electronics > Beginners
Hot air soldering, what am I doing wrong?
iMo:
This gel is pretty popular, there are tons of 223 cheapo fakes, I've done with the fakes too and they worked fine as well.
You need also a bottle of IPA (>=95%) for cleaning the mess up.
https://www.tme.eu/en/details/armalf223tf_35/fluxes/amtech/rma-223-lf-tpf-35g/
viperidae:
--- Quote ---The key is to maintain a LOT of flux all the time - it always works fine when there is a LOT of flux, chip must always swim in the flux.
--- End quote ---
As Louis Rossman says, the bigger the gob, the better the job.
jhpadjustable:
So 1 rossmann = how many cc of flux? :-DD
wraper:
--- Quote from: viperidae on November 17, 2019, 02:22:22 am ---
--- Quote ---The key is to maintain a LOT of flux all the time - it always works fine when there is a LOT of flux, chip must always swim in the flux.
--- End quote ---
As Louis Rossman says, the bigger the gob, the better the job.
--- End quote ---
Louis Rossmann cannot solder properly and he admits it. He uses 50 times more flux than needed. If you have good tacky flux, only a tiny bit of it is completely enough, just enough to flow over the pads. Which it does by itself when heated.
KL27x:
--- Quote ---But how do other people do it? What am I supposed to do to prevent all the flux from burning to early?
--- End quote ---
I have used MG Chemicals liquid rosin flux to hot air reflow hundreds of QFN, using lead solder to pretin the pads. I find
1. You need enough flux that it bubbles and the chip wanders away on its own, so this is annoying
2. I would pin the chip down with tweezers while heating it, and this solves the problem. So it's not really that annoying.
3. I have not "burned" the flux? I mean, I don't know what kind of flux is in your pen, but I agree with others that a pen doesn't usually lay down much flux. I would put a bead of rosin flux down, with a prominent meniscus, using a syringe.
I think Luis Rossman (slash AvE; why am I not surprised he watches AvE) is mostly right re "the bigger the gob." I have gotten feedback before about an issue where excess undried flux caused failure in production testing. It is a problem that goes away by itself in about 24 more hours. If the PCB is made on Monday and goes into testing on Tuesday, it might still be too wet. Now, if you used too much gel flux and you don't clean the residue, maybe it takes another week to dry out on its own? You can also fix this issue faster by touching an iron to the flux for a bit to dry it out.
Whether you need gel flux or not might depend. I've heard that gel flux is supposedly designed for longer reflow/working time. I don't know if this is true in all (or even any) cases, though. It might be especially desirable to have longer working time if the board has a very high thermal mass/conductivity. This might be something to suspect is your case, but you/OP desoldered the chip in the first place, presumably without a notable problem.
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