Switching from full-rosin/RMA flux to no-clean flux

[Michael Craft]

I got an inquiry from one of our electronics labs with regards to their soldering processes. They use eutectic (63% Sn, 37% Pb) solder. For liquid flux they are currently using Kester 186, which is an old-school, full rosin (RMA and ROL1) flux that contains 36% solids. For solder wire they are using Kester 282, which contains RMA/ROL1 flux paste in the core.

They are now thinking of switching to a "no clean" flux for both liquid and solder wire. What are the pros and cons to switching to a "no clean" flux? Will the solder junctions be just as good? Will they have to change their soldering processes? Will they have to change their cleaning processes?

Thank you

[tooki]

A key thing to remember is that no-clean fluxes are only no-clean if heated FULLY. This always happens to the flux core in solder wire, and in solder paste or paste flux when run through a reflow oven. But unheated or partially heated no-clean flux can be left in an active, corrosive state! So when you use external no-clean flux (liquid or paste/gel) for hand soldering, you must clean it, because any flux that spreads around (which it loves to do) beyond the joint is unlikely to reach the neutralization temperature.

Additionally, no-clean flux residues tend to be harder to remove than classic rosin fluxes, because they’re not really intended to be removed, so ease of cleaning isn’t a design priority.

No-clean is basically designed for mass production, where controlled processes make it absolutely safe to use without cleaning. They’re designed to be visually appealing without cleaning. (Many people unnecessarily worry about cleaning rosin flux, which isn’t normally needed except in very, very sensitive circuits.) And part of the no-clean systems is to simply reduce the amount of flux solids to begin with, so even a rosin-based no-clean liquid flux might only be 3% solids. This is fine for new, clean components, but often insufficient for older, tarnished parts.

Is there a reason your lab wants to make this change? If they think it will reduce their cleaning requirements, it almost certainly won’t, and may make cleaning harder.

I use no-clean solder paste for reflow, and no-clean gel flux for rework (where I know I’ll have to clean it), but stick with old-school rosin for hand soldering leaded solder.

Most lead-free solder wires are no-clean, because the old rosin flux formulations don’t hold up to the higher temperatures needed, and by the time lead-free became relevant, no-clean had already become dominant in mass production, so newer flux formulations tended to be no-clean. There do exist some lead-free rosin solders, they’re just harder to find.



(Also, Kester 282? Fancy! That’s a very expensive solder! Note that solder wire does not contain flux paste; it’s a crystalline solid, or compressed powder.)