Good soldering flux?

[shapirus]

Whatever gauge of the needle I use, within reason, or a conical syringe tip, it keeps blocking. I really hate that.

There are a couple of solutions for that : ) (I use the same stuff, 8341).
Solution 1: Use a metal tube nozzle, quite wide (I'm using 1.5 mm hole), and when it's near a hot air tool (i.e. just gently warmed) it will auto-unclog : )
Solution 2: When using a fine flexible plastic nozzle, I use a wire (e.g. through-hole resistor, or any bare wire), poke it around in the end a bit and then it egresses in a fine thin line (if that's what you want, ideal for small SMDs).

Photo shows the metal nozzle I use. Also shows a couple of other fluxes which I've not used yet, but hope to try out sometime.

Yeah. There are workarounds. Unclogging the hole with a piece of wire (solder wire!) is what I do. The thing is, I want to be able to just use the damn flux without any workarounds, and that's where it fails.

[forrestc]

You did not mention it cost more than gold

Only if you buy it direct from indium.

$37.50 for 30ml (big) syringe:

https://www.testequity.com/product/881SO202-FLUXOT-84283-30ml?gad_source=1&gclid=EAIaIQobChMIqvnno-mJhwMVwh6tBh1Yhgx_EAYYASABEgJVwvD_BwE

Or $27.12 after eevblog discount at tequipment:
https://www.tequipment.net/Indium/FLUXOT-84283-25-GRAM-SYR/Flux/

I usually use a coupler to transfer it into another (smaller) syringe and then keep the big tube in the fridge with the solder paste. 

Still not dirt cheap, but not $150 either.

Plus, there are two formulations.   TACflux-020B and TACflux-020B-RC.   The RC is (for whatever reason) rated for a 6 month shelf life. and the non-rc is 12 months. 

[Postal2]

I recommend:

[tooki]

Thanks it makes sense. I will give it a try in my ultrasonic cleaner with IPA and heater on ... I need to buy a gallon of 99% IPA somewhere... (fun fact if you google "IPA gallon" only beer shows up).

Do not do this!!

It’s not safe to fill a regular ultrasonic cleaner with flammable solvent. And it’s expensive.

Instead, fill the machine with water. Then place into the water a smaller container of solvent with the dirty item. The container can be open (like a tray or beaker) or a plastic bag. Plastic bags are a great way to massively reduce your solvent consumption, since you can put in your board and then just enough solvent that when zipped closed, the board is submerged. Seal a bit of air into the bag; this will keep the open end of the bag floating at the surface.

[Smokey]

Ethanol eh?  How about $20 per gallon? 
https://www.totalwine.com/p/1825175-1


Two for the boards..... one for me...

[isabido]

Interesting debate! I have these purchase options from Spain, what do you think of the CHIPQUIK SMD 291 and CHIPQUIK NC191, it is for general SMD repair work with a soldering iron and hot air

https://es.rs-online.com/web/p/pastas-para-soldadura/2411579
https://es.rs-online.com/web/p/fundentes-para-soldadura-blanda/1466649
https://es.rs-online.com/web/p/fundentes-para-soldadura-blanda/2862009

THanks!!

[shapirus]

Sounds interesting. We have this one available locally. Sold for ~$5/10ml, so not exactly cheap, but I think I'll give it a try, thanks for the idea.

Okay, let's begin.

Corrosion test start date:

Lol I absolutely forgot about this one. So it's been well over a month, and there's no sign of any corrosion as such. The copper under the flux acquired a little bit more "saturated" copper-ish color, like patina, I would say, but that's totally fine.

So @Gediminas thanks for the hint. Two of the three success checkboxes are ticked: super low conductivity, no corrosion. Soldering performance remains to be seen, as I have had very little to solder with it so far to say anything certain, but it seemed good enough in those few cases.

I was looking for same thing, same requirements. The best I found so far is TermoPasty Flux Gel, no clean flux.
It is paste/gel flux. I could not measure any conductivity before and after heating (with Brymen BM869S in nano Siemens mode).
If left on pure copper - no  corrosion after several weeks.
After heating, flux remains soft and newer becomes hard. It is half soluble in IPA, but still quite easily can be cleaned with it from the board.
It will flow underneath the parts and you will definitely will not be able to fully remove it from there. But so far I did not notice any problems while using this flux.
Here is the link where I bought it: https://www.tme.eu/en/details/topnik-zel_14/fluxes/ag-termopasty/art-agt-088/

[wraper]

Interesting debate! I have these purchase options from Spain, what do you think of the CHIPQUIK SMD 291 and CHIPQUIK NC191, it is for general SMD repair work with a soldering iron and hot air

I bought two SMD 291 syringes from Digikey a few years ago and they sit barely used, should just throw them out. Probably the worst tacky flux I ever bought. Dunno about the latter.

[isabido]

Upsss

Well, it seems to me that I'm going to have to settle for the lumpy MG Chemicals 8341.

[tooki]

Interesting debate! I have these purchase options from Spain, what do you think of the CHIPQUIK SMD 291 and CHIPQUIK NC191, it is for general SMD repair work with a soldering iron and hot air

https://es.rs-online.com/web/p/pastas-para-soldadura/2411579
https://es.rs-online.com/web/p/fundentes-para-soldadura-blanda/1466649
https://es.rs-online.com/web/p/fundentes-para-soldadura-blanda/2862009

THanks!!

I have used all three of those.

I would not recommend the NC191. While it does have a truly lovely consistency, its fluxing action is depleted extremely quickly. It really is designed for things like reflowing in an oven, where it gets heated once only. It’s terrible for hand rework, where you may need to keep a joint hot for longer than you normally would, or where you need to go back and reflow, like in removing bridges from an SSOP.

The MG Chemicals works great. Does get lumpy with age. Typical somewhat acrid smell when heated.

SMD291 works pretty well (not sure why wraper had trouble with it; I’ve found it to work well). But I prefer SMD291NL: whereas the MG, the NC191, and SMD291 are all opaque/translucent when cold, SMD291NL is transparent amber, giving you better visibility, and it has absolutely no graininess of any kind. And it smells like hazelnuts when heated, rather than like acid or chemicals. It isn’t the most active flux in existence, but it works well and is pleasant to use.

[wraper]

SMD291 works pretty well (not sure why wraper had trouble with it; I’ve found it to work well).

Leaves sticky darkened mess that is not that easy to clean, boils when heated (especially noticeable when soldering BGA), short working time before it becomes useless.
EDIT: Also it's somewhat conductive, so should be avoided for hand soldering SMD.

[tooki]

Thanks for the reply!

I haven’t done anything with BGAs, so my experience is solely with using them for hand rework. Seems very plausible to me that those applications have very different needs. I don’t remember the SMD291 boiling when used by hand, but to be honest I didn’t look out for that when I tested it.