What kind of liquid flux?

[jgalak]

I've never used liquid flux in electronics soldering before, relying on rosin core solder, even with hand soldering of SMD components.

Some videos I've watched recently make me think this is something I should get.  Bit what kind?  There seems to be a bewildering range.  Just one manufacturer I stumble on (Kester) makes dozens of kinds.  Activated rosin, mildly activated rosin, non-activated rosin, alcohol-based, inorganic acid, etc.  The halogen-free or not. 

So, what's the best choice for hand-soldering SMD components?

[Ian.M]

It depends - many users here prefer gel fluxes rather than liquid ones.  My personal preference would be a R or halogen-free RMA in alcohol flux.  For all except critical high impedance circuits, if used judiciously you can treat it as no-clean once the solvent has fully dried, and if you do need to clean it, IPA will do a decent job, for either spot or whole board cleaning.   Avoid more active fluxes and especially organic acid ones unless you are equipped and prepared to follow the manufacturer's recommended board cleaning procedure.

N.B. your flux should be compatible with your solder's flux core.  All Rosin based fluxes are generally compatible.

[sainter]

I tried using several types of liquid flux, but when I tried gel flux, going to stick with it. For me, liquid flux evaporates too quickly. Amtek NC-559 works the best for me.

[Ian.M]

Gel flux is a different way of working (and mandatory PITA cleanup if used in greater than trace quantities).  With Rosin in alcohol based liquid fluxes, its usually advantageous to let the alcohol evaporate off, maybe assisting it with warm air before soldering as the alcohol will drive the flux away from the joint if you apply a soldering iron or reflow temperature hot air while the flux is still liquid.

[GreyWoolfe]

I use MG Chemicals 835 liquid RMA flux.  I dispense from a small squeeze bottle with a needle tip.  It works well for me.

[apelly]

Anything is better than nothing.

You'll drive yourself insane researching flux if you aren't careful. There are hundreds of kinds. In my experience there is no real agreement around here about what is best.

Just buy a couple of types from ebay to try, and clean up after, no matter what the instructions say.

I've tried some gels and some pens and some home made rosin, and they're all OK.

[David Hess]

My favorite is Kester #44 RA (rosin activated) flux which is available at Kester 1544 however they no longer sell it in small bottles.  They also used to have an equivalent paste flux.

I prefer a fully activated rosin flux because if I need to use extra flux, then there is no reason for half measures.  These days I would probably go with MG Chemicals 8342 RA Rosin Flux Paste and 835 Rosin Flux but there are lots of options.  I do not like pens; it is easier to dip the solder into the rosin and then use it than to use an expensive flux pen but maybe they have some application with surface mount.

[WastelandTek]

R A

[tautech]

I've never used liquid flux in electronics soldering before, relying on rosin core solder, even with hand soldering of SMD components.

Some videos I've watched recently make me think this is something I should get.  Bit what kind?  There seems to be a bewildering range.  Just one manufacturer I stumble on (Kester) makes dozens of kinds.  Activated rosin, mildly activated rosin, non-activated rosin, alcohol-based, inorganic acid, etc.  The halogen-free or not. 

So, what's the best choice for hand-soldering SMD components?

Just a few months back I was in the same position as you. 
By chance I was following Dave's nixie build and when it came to the soldering together stage in this vid my eyes were opened some....not just by the use of liquid flux but by how liberally Dave used it.
https://www.eevblog.com/2017/06/02/eevblog-997-how-to-solder-surface-mount-components/

Since then I splashed out on some Aussie Chemtools CT-NC-DP no clean liquid flux. Felt tipped pen.
Very liquid stuff and if flashes off quite quickly but the results are clean and good.

Not long after I got this flux pen apellly (posted above) called us Kiwis for a Xmas beer and he turned up with heaps of flux in 4L plastic bottles, just one being enough for life.
https://www.eevblog.com/forum/chat/auckland-beer-and-flux-xmas-get-together/
It's Multicore PC26 and its datasheet is linked in reply #7.

Getting some of these little needle tip bottles for flux and other usages.
https://www.aliexpress.com/item/10ml-Empty-Plastic-PE-Dropper-Bottle-With-Screw-Metal-Needle-Cap-For-Juice-Eye-Liquid-and/32726354975.html?spm=2114.search0104.3.131.XCFscz&ws_ab_test=searchweb0_0,searchweb201602_1_10152_10065_10151_10068_10344_10342_10343_10340_10341_10307_10137_10060_10155_10154_10056_10055_10054_10059_100031_10099_10338_10339_10103_10102_440_10052_10053_10142_10107_10050_10051_10326_10084_10083_10080_10082_10081_10110_10175_10111_10112_10113_10114_143_10312_10313_10314_10078_10079_10073,searchweb201603_6,ppcSwitch_5&btsid=c6f99132-979a-4f8c-8a39-da2c3d78f308&algo_expid=5c27031d-fe69-4800-8dd4-a6a79b6216e4-18&algo_pvid=5c27031d-fe69-4800-8dd4-a6a79b6216e4

[rmacintosh]

I switch to Chip Quik Inc. SMD291 tacky flux. Comes in a syringe with 2 types of nozzles. Switched about 7 months ago and haven't looked back. Love it.