When I first started working with electronic circuits, I had no understanding of flux. At that time, I chose and used flux purely by imitating what others did:
- I saw some videos where people applied large amounts of flux on circuit boards, so I did the same, using flux generously regardless of cost.
- I saw some forum posts recommending certain brands, so I bought those brands as well.
- I read opinions stating that flux smoke contains toxic gases like formaldehyde, so even with ventilation in place, I wore a mask every time I soldered…
As I gradually learned more about flux, I developed my own understanding of its characteristics and usage principles. While they may not be entirely correct, I believe every decision I make is based on a rationale. So I’ve listed my thoughts here. If any part of it is incorrect, I hope someone who reads this blog can point it out. If my views are correct, I hope they can serve as a useful reference for you when choosing and using flux.
First, the most overlooked issue for beginners:
You should keep at least two types of flux with different activity levels on your workbench, and try to avoid using RA-grade flux.
Flux is classified into three activity levels: R, RMA, and RA. The order represents increasing activity level. The R level, which is the lowest, usually refers to rosin-based flux—either natural or synthetic—that is traditional and widely used. RA, the highest, is based on rosin but with added acidic ingredients to help remove oxides from metal surfaces more effectively.
RA-grade flux can be considered a “soldering miracle.” No matter what type of pad, how badly oxidized it is, or how difficult it is to tin a tip or solder wire, RA flux makes the process smooth and easy. As a result, some people swear by it—thinking that with RA flux, there’s no need for R or RMA. You could say, “One bottle of RA goes everywhere.”
However, in reality, hobbyists should avoid using RA flux. Even professional technicians who repair hardware for a living should try to steer clear of it.
The reason is that RA flux contains more aggressive additives than RMA. It has stronger oxide-reducing ability, which also means it’s more acidic. Its residues are more corrosive and conductive. After using RA flux, the circuit must be cleaned—not just wiped, but thoroughly cleaned.
In fact, it’s hard to clean a soldered circuit thoroughly with just a brush. Therefore, in amateur settings, it’s difficult to completely remove RA residues. These residues may corrode the circuit or even cause minor short circuits.
Additionally, the acidic vapor generated when heating RA flux is more irritating than RMA or R flux. Even with good ventilation, having less harmful substances is always better, right?
So, if RA is ruled out, is RMA the ideal choice?
That’s logical. RMA flux is milder than RA and more active than R. It’s often labeled “no-clean.” Still, I suggest that in addition to having RMA on hand, you also keep some R-grade flux. And during soldering, try to prioritize R-grade flux.
Moreover, solder wire already contains flux, usually RMA-grade. That means when you’re soldering or desoldering with solder wire, the RMA inside is already dealing with oxides. Once that’s done, rosin (R-grade) flux is more than enough for the remaining job.
So my current principle for flux usage is:
Refine your soldering skills. Rely primarily on solder wire and its built-in flux for soldering and desoldering. If the flux in the solder wire has evaporated after heating for a while, then add a little R-grade rosin flux to finish the job. Only when R-grade flux can’t solve the problem, will I resort to using RA flux as a last resort.
There are two main advantages to this approach:
- You avoid the slightly irritating acidic smell of RA flux smoke, replacing it with the mild, slightly sweet aroma of rosin vapor.
- You build your soldering capabilities on your own skills and techniques, not on acidic chemical assistance—which helps you improve your craftsmanship.
Second issue: Are you worried about smelling or inhaling flux smoke?
Some articles claim that flux smoke contains formaldehyde and acidic gases that harm we health. I think this concern is exaggerated. It’s true that in factories with assembly lines and long hours, occupational exposure is a real issue. But for hobbyists who only solder occasionally, it’s not nearly as dangerous.
As mentioned earlier, we already avoid RA flux and have good ventilation. Under these conditions, the flux smoke we’re exposed to comes from only two sources:
- The RMA flux inside the solder wire
- The extra R flux or occasionally used RMA flux
Most of the smoke they produce is extracted by the ventilation system. The small amount of odor we can still smell is actually rosin vapor. If you also follow my advice from another article—like lowering your soldering station’s temperature—you’ll notice that the visible smoke becomes minimal, and what you actually inhale is even less.
“But even a little smoke contains scary formaldehyde and still harms your body.” Is that really true?
In theory, yes. Any organic substance undergoing thermal decomposition releases complex chemical gases, including formaldehyde. But if any organic material can do that, then what about when you’re cooking in the kitchen, or grilling outdoors? The white smoke over barbecue meat is chemically similar to that of flux smoke. Yet have you ever worried about that smoke? In fact, that smoke often contains more harmful substances. Still, few people worry about it.
The reason flux smoke gets special attention is due to occupational exposure in factories—where the smoke is produced nonstop for eight hours per day, and five days per week. The intensity is high enough to pose serious health risks. But in personal, amateur workbenches, as long as you have a small exhaust fan, there’s no need to worry about your health being threatened.