FLUXES
by CursedbyFlame
First I will start with general purpose hand and hot air gun soldering. Stick to low-activity NoClean fluxes in a gel (REL0, ROL0, ORL0) form,
as it's they are easiest to work with and if you don't clean the board well enough it most likely won't lead to unforseen consequences.
Avoid using medium activity (ORM0/1, REM0/1, ROM0/1) fluxes without specific reason, as their higher activity usually caused by componnents that corrosive and conductive,
which means that those fluxes must be cleaned completely from the PCB.
NOTE ON A FAKE KINGBO: although it mentioned in a lot of categories, all of them assume it will be cleaned off completely. This flux is corrosive and it's SIR results are not up to scratch. My general recommendation is not to use it all, unless you make sure it's cleaned off (ultrasonic)
While doing GPU repairs, I have identified following use-cases that you might want different things from your fluxes:
- Wicking
- General iron soldering
- General hot air gun soldering
- Reballing chips
- Flooding chips for all balls to fully allign
- Soldering down BGA chips
- Microsoldering
In each of this use cases you are looking for different properties from the flux you use. I will go through each use case further and describe what properties are most important.
TL;DR; If you are looking for a single flux, that can do it all (to different degrees) and won't brake your back in terms of price at the same time it'll be safe for electonics (perhaps not safe for you, so fume extractor is a must in any case) - Amaoe M53 from Ali. Why M53 and not M50? M50 is a lot more active flux, but it comes with a big downside of it burning pretty fast and harder to clean, as well as worse resistance readings (can't be used for BGA soldering), also it has a milky colour and looses activity faster.
Other good cheap all around options:
- 2UUL Daily Solder Flux (SC14). More active than M53, but more prone to get burnt, a bit harder to clean. Resistance properties should be good enough for soldering BGA.
Wicking
Don't use wicks with flux in it, get yourself a clean copper wick. More often than not this flux burns very fast gunking up both your board and your iron tip.
When wicking, your iron will be decently high temperature for a quite long period of time, so as a result of it you look for the flux that doesn't burn easily and stays active for a decent period of time.
The other important consideration for this use case is wetting properties of the flux (e.g. how well it makes solder to flow), also it needs to be decently active, to remove oxidation layer from the wick, but at the same time it should not dull out your iron tip, which will prevent proper temperature transfer from the tip to the wick. Furthermore, you need a chep-ish flux, as consumption rate is quite big while wicking. Another important property for that activity is lubrication properties, to minimize damage that you deal to solder mask while wicking. And obviously - cleaning, you'd want to get rid of all that used up flux, to place new component.
Summarizing what've been said, you're looking for following properties:
- Wetting
- Good activity
- Price
- Heat resistance
- Lubrication
- Cleaning
My personal recommendation:
- Amaoe M53. My current favorite for this task, doesn't burn for a long time, tho the flux itself is not best in activity it compensates it by staying active for long period of time, doesn't dull out the tip, cleans well as it gets dissolved by IPA, good resistance properties. Pretty cheap at 4$/10CC from Ali
- 2UUL Daily Solder Flux (SC14).
- Fake kingbo (RMA-218) from Ali, it decently active, dirt cheap, have good lubrication due to the fact that it have vaseline in it's formula, it wets fine. It's not the best in heat resistance tho, so I'm still looking for a better option. Cleaning is not it's best property also.
- WIP. Still on a lookout for better options
General iron soldering
Generally for iron soldering everything described in Wicking section remains true.
General hot air gun soldering
When soldering with hot air gun, you'd want to have a flux that have a good activity and doesn't runs away from the component that you are working with, which can be a pain in the arse.
Important properties:
- Wetting
- Activity
- Should stick around the area that you work with
Recommendations:
- Amaoe M53.
- 2UUL Daily Solder Flux (SC14).
- WIP. Also on a lookout for the better flux
Reballing chips
There are multiple ways of reballing BGA chips, but for the matter of this guide I will concentrate on the non-direct heat method with using solder balls and placing them by using jigs.
The job of the flux is to attract balls to the chip, and keep them in place for the whole time of melting balls.
Here you are looking for sticky active flux, that doesn't boil during the activation phase, that is easy to apply thin layer of.
Also, you don't want for flux to became too liquid and flow too much after getting heated, to avoid balls drifting away from their pads and joining together.
The price here is non-factor really, as consumption is minimal. Bonus point for flux that doesn't evaporates too fast and allows balls to fully melt into pads, instead of just tacking them on, that allows to avoid to have additional step of flooding the chip to allign them.
Important properties:
- Activity
- Stickinnes
- Should not boil or flow
Recommendations:
- Fake kingbo (RMA-218) from Ali. General favorite for this case in this server and is what NWR uses for this. Easy to apply, active enough, decently sticky, but it does evaporates, so it rarely melts everything and requires flooding.
- Mechanic M35 from Ali. My personal favorite, it's hard to apply as it's very sticky, active enough, it does evaporate but not as fast as kingbo, so you'll find that you won't need to flood the chip in much more cases.
- Amaoe M50. It's easy to apply, but the consistency makes it hard to have a thin layer, so there is a learning curve. But it does the job well, tho you'll most likely need to flood the chip.
- IF-8300. Very sticky, easier to apply than M35, but still tricky. Balls barely move with this flux, but it evaporates quite a lot, so it requires flooding
Flooding BGA chips
Developing the topic of reballing, while you can flood the chip with the same flux that you placed balls with, here are few important considerations for this.
First and most important is cleaning - you'd want to clean all of that used up flux afterwards from between the balls.
There should be no flux left on the core (see Soldering BGA chips section on why). Second is consumtion, if you are using expensive flux, flooding takes a lot of flux, so it may be not smartest option. Also flux should be decently active and have good wetting ability, to allign balls perfectly.
Important properties:
- Cleaning
- Price
- Activity/Wetting
Recommendation (still on a lookout for perfect fit):
- Fake kingbo (RMA-218). Alligns well, cheap, decently cleans, but not the best. Must be cleaned out completely.
- Mechanic M35. Cleans a bit better than kingbo, safer for bga if not cleaned properly, tho sometimes it fails to allign the balls completely.
- ReLife RL-422-IL. Alligns well, hard to clean with IPA.
Soldering BGA chips
This is the where flux plays biggest role out of any use cases. The reason is it's close to impossible to clean out the flux out from underneath the chip, no solution or supersonic can do that. So to avoid issues chosen flux MUST NOT conduct (over 100 megaohms per standartized SIR test IPC-TM-650), it MUST NOT be corrosive (copper mirror test), it MUST NOT grow dendrites in it (part of SIR test), ideally after activation flux needs to become hard glass-like substance to protect solder, as if flux remains sticky and liquidy over time it will attract dust and other contaminantations that can introduce problems.
Importance of all of those factors comes from the fact, that over time, chips that you have soldered on can develop issues, that can't be easily identified up to even developing short-circuits.
Those requirements, unfortunately, basically disqualify any cheap chinese fluxes from Ali as they are inconsistent in quality, don't bother with proper testing and are mostly unknown in contents. At the same time flux should be active enough to remove oxidation layer from dull pads, to avoid the need to redo the work due to grey pad.
Important properties:
- Resistance
- Corrosivness
- Activity
Recommendation:
- EFD FluxPlus 6-412-A (industry leader for BGA soldering, but very expensive and not readily available)
- ERSA 0FMKANC32 5070NC
- Interflux IF-8300
- MOB39
- Amaoe M53. According to tests that I've seen it is probably the only chinese flux from Ali that could be used for BGA and the company behind it usually maintains the quality of it's products.
Microsoldering
The last distinct use case I want to touch is microsoldering. General difference between general soldering and this is that you'd want your flux to be much more active (this is the area where I would consider using higher activity classes of fluxes e.g. REM, ROM, ORM), it should not evaporate for long period of times and it should stay active as long as possible. In addition to that it should be as clear as possible, for you to be able to see what you are doing.
Important properties:
- High activity
- Clarity
- Low evaporation
- Long period of activity
Recomendations:
- Amaoe M53. Pretty clear flux, that doesn't burn too fast and stays active, it's more active than Mechanic and cleans easier, but it does have a tendency to run away from the hot iron tip, but it does not evaporates as much.
- Mechanic M35. Decent option, but with downsides. It's very clear, but it's not active enough, so you may find it hard to tin small traces with it, also it likes to run away. Must cleaned off, as it may corrode if left uncleaned.
- WIP.
