Liquid vs Gel Flux

[Ranger14]

I have been playing with smd lately and would like to know what does everyone prefer as liquid or gel flux and for what purpose ??

[ovnr]

Gel/paste flux all the time. I've got a liquid flux pen in the bottom of a drawer somewhere, and it hasn't been touched since I got proper flux.

[RobertHolcombe]

Gel for rework and hand soldering assembly - it aids in component placement/alignment and will generally remain in the soldering area even after soldering. In my experience liquid flux tends to spread over a board and flow through vias, through holes etc which requires unnecessary disassembly and cleaning. In an environment that allows or requires ultrasonic/IPA bath/oven drying of boards I'd probably still choose gel flux 

When handling extremely fine pitch packages I could see the gel being a hindrance to visibility and the slight tackiness of liquid flux would probably be adequate to hold components in place

[RoGeorge]

Always gel for SMT.
Mostly colofonium for THT.
Sometimes gel for THT and multifillar oxidized wires, too.

[TheUnnamedNewbie]

I prefer gel too. Only problem I have sometimes is that they can get sticky, and it's really quite annoying when your tweezers stick to everything they touch. I use a SMD flux pen sometimes when doing wires or such because the gel can be a bit more of a hassle cleaning, but that's likely more due to the different types I use than it being gell or liquid.

[tooki]

Always gel for SMT.
Mostly colofonium for THT.
Sometimes gel for THT and multifillar oxidized wires, too.

Meaning what? Colophonium is normally called "rosin" in English, and rosin flux can be liquid, gel, paste, as well as the solid flux core in solder wire.

[RoGeorge]

My bad, thank you for letting me know the correct English name.

Yes, rosin. The solid type, crystal-like, breakable, translucent, amber: https://en.wikipedia.org/wiki/Rosin
Since the supply I have is many decades old, I guess it was not processed in any way for better soldering (it's not activated rosin or something).

And this is the flux I like most "AMTECH NC-559-ASM":
https://hackaday.io/project/7574-the-devil-is-in-the-details/log/24733-flux-paste-dispenser

It was bought from AliExpress, so probably it's not original, but the seller advertised it as a non-smelling Flux. Indeed it's almost odorless compared with the same type of flux bought from another seller.

[tooki]

How do you apply solid rosin? I know it's sold in block form for string instrument bows, and sometimes electronics nerds dissolve it in a solvent to make liquid flux.

[RoGeorge]

For Through Holes, I'm usually using a soldering gun instead of a soldering iron. The soldering gun tip does not have a big thermal inertia, and it is powered only when the trigger is pressed. The advantage is that you can easily control the tip temperature of a soldering gun by how long you press the trigger.

To apply solid rosin, heat the tip of the soldering gun just a little, enough to melt the rosin just a little, until the consistence of honey. Deep the tip into the rosin for 2-3 mm, then release the trigger and remove the tip from the rosin. As a result, a few mm cubes of rosin will solidify on the tip of the soldering gun, without burning the rosin. The tip loaded with solidified rosin can now be heated again on the soldering junction, simultaneously heating the PCB trace, the component terminal, and the soldering alloy.

Solid rosin does not work too well with a normal soldering iron, because the rosin is burned/vaporized too fast at normal soldering temperatures.

One more thing: In my experience, solid rosin is good only with SnPb solder.
For lead free, nothing can beat dedicated lead free flux, because the lead free flux can outstand the higher soldering temperatures required by Pb free alloys.

[johnyradio]

The advantage is that you can easily control the tip temperature of a soldering gun by how long you press the trigger.

You mean, the tip heats slowly enough that the longer you hold the trigger, the hotter it gets?

To apply solid rosin...

By "solid", do you mean gel?

[mariush]

RoGeorge,  just get in 2017 and use some liquid flux. My father used  your technique 20 years ago with one of those 100w mains powered soldering irons with thick copper wire instead of tip.

I'm buying noclean flux from TME.eu , they sell flux made by a Polish company really cheap and it does a good job ... seriously, you can buy a 100ml bottle for 3$ or 1 liter for 20$  LINK

You're burning that rosin when you shove the tip into it, then you're also potentially thermally shocking your tip every tip you try to heat the cold rosin and by the time you bring rosin to the work area on your tip the heat of your tip already burns it a bit.
You can just go at a pharmacy and buy a syringe and suck 2-5 from the bottle into the syringe and use where needed.

[KL27x]

I prefer liquid flux when putting SMD parts on a board.

I don't get the gel fad. Gel is great for slathering flux onto vertical surfaces or for BGA reflow to slow the evaporation rate of the solvent and increase the working time. For SMD work, the board is flat. I spread on the liquid, flow it, and now I have option to clean it or to just leave the thin layer that is left to evaporate and harden into inert residue.

[kalel]

I prefer liquid flux when putting SMD parts on a board.

I don't get the gel fad. Gel is great for slathering flux onto vertical surfaces or for BGA reflow to slow the evaporation rate of the solvent and increase the working time. For SMD work, the board is flat. I spread on the liquid, flow it, and now I have option to clean it or to just leave the thin layer that is left to evaporate and harden into inert residue.

Electronics repair school (YouTube chan) also likes to leave liquid rosin flux on after cleaning/repairing water damaged components to help "seal" the area from air and further liquid entering.

[KL27x]

Not sure what YT electronics school is. YouTube?

I just checked my fridge. I have 3 syringes of paste flux in there that haven't been touched in over 3 years. If I need to stick some flux somewhere awkward, I just put a dab of liquid flux on a board covered with dried rosin (which is built up from the rosin dripping out of solderwire) and the solvent softens it up. Then I just scrape up a bit of rosin goo onto the tip of my 35 mil wide rework chisel.

If you wanted to make some goo from scratch, you could just put some liquid rosin flux on a board or shallow tray and hit it with your hot air until you get the consistency you want. I have done this to make paste and even rosin glue. W/e is added to turn flux into a gel, I suppose it probably includes some heavier petroleum distillates and glycerin and other nonsense. Unless the long open time is needed while waiting for a large board/chip to slowly reach temp, I don't really care to use gel. It works fine, but liquid rosin works better for most of my needs. And a liter of liquid flux is the same price as a couple syringes of gel.

[kalel]

Yes, YouTube, I edited now.

[RoGeorge]

The advantage is that you can easily control the tip temperature of a soldering gun by how long you press the trigger.

You mean, the tip heats slowly enough that the longer you hold the trigger, the hotter it gets?

To apply solid rosin...

By "solid", do you mean gel?

Yes, it takes about 1-2 seconds to heat the tip from the room temperature to the melting solder temperature. I'm using a soldering gun instead of a soldering iron any time it is possible. It looks like this one, but without the fan:


Not gel, by solid rosin I mean translucent rock like this: https://en.wikipedia.org/wiki/Rosin
https://www.aliexpress.com/item/1pcs-Carton-Rosin-Soldering-Iron-Soft-Solder-Welding-Fluxes-scaling-powder-Solid-flux-Free-shipping/32747584909.html