I am researching flux. I see there are lead free, acid based, and no clean up ones.
Which is best for circuit boards?
I know every situation will be different.
Basically, fluxes fall into a few categories (in terms of active ingredient, not the delivery method):
1. Rosin
2. No clean
3. Alcohol
4. Acid (water soluble)
First off, acid/water soluble fluxes are categorically unsuited to hobby electronics. Do not even think about using them. (Any flux that says it uses ammonium chloride or zinc chloride is an acid flux and must not be used.)
Rosin fluxes are made from the refined sap of pine trees, with activators added to improve the fluxing action. You can get rosin fluxes with varying concentrations of rosin and activators, making them more or less aggressive, and more or less hazardous. (Halogen activators are more effective, but bad for the lungs.)
Highly activated fluxes, which are better for hard-to-solder things like old components, are labeled as RA, ROM1 (halogenated), or ROM0 (halogen free). The flux in my favorite solder, Kester 44, is RA.
Slightly milder rosin fluxes are classified RMA, ROL1, and ROL0.
No-clean fluxes are essentially a subtype of rosin fluxes, made from either extra-refined rosin, or even from synthetic rosin (sometimes labeled as resin instead), such that the flux residues are clear and look better. They also may contain much less of the active ingredients, especially in liquid and gel fluxes. They aren't usually as active as rosin fluxes. They may be classified RMA (rarely), ROL1, ROL0, REL1, or REL0. (Other classifications exist, I just listed what you're likely to see.)
- Both rosin and no-clean fluxes exist as rosin core solder, paste and gel, and liquid flux forms. Note that despite the name, you can clean off no-clean flux, and that despite not saying so, rosin flux residue does
not need to be cleaned for most applications. For the kind of stuff you're doing, it's purely a matter of aesthetics.
Alcohol fluxes are used in some gel fluxes and in solder paste.
Lead-free fluxes are simply fluxes formulated to tolerate the higher temperatures of lead-free soldering. (Regular fluxes may degrade at those temperatures before having a chance to work.)
In a nutshell, here's what I recommend:
1. If you can, use rosin-core leaded solder as your everyday solder. It's the easiest to work with. (For example, my favorite solder, the Kester 24-6337-00xx series, which is 63/37 leaded alloy with 44 (RA) flux. I recommend either 0.031" or
0.020" size.)
2. A tacky gel or paste flux. I use the MG Chemicals
8341 No-Clean Flux Paste. This is really good for heavily corroded surfaces, or for SMD components, where the tacky texture helps hold components in place. (MG also makes the 8342 RA flux paste, which should be even more aggressive, I just haven't had the chance to try it.)
3. A liquid flux pen. The low solids content makes these not as effective as the paste, but it's easier to clean up, and works well for when you need to flux something that isn't heavily corroded. I use this often for tinning clean wire, for component legs and pads on through-hole soldering, etc. I'd suggest an MG Chemicals 835-P,
Kester 186, or
SRA 312.