SMD291 is the worst tacky flux I tried ever. Wetting not that good, when heated bubbles as hell, leaves ton of awful sticky residues, somewhat conductive. Worse than most of Chinese counterfeits.
Are you sure--? That sounds like a counterfeit product to me. Or maybe it was really old and absorbed moisture or something. What I have, hardly bubbles on heating (say, if I put some under a QFN to solder in place, it doesn't wobble around as it heats up), it does contain volatiles which sizzle off quickly under the soldering iron, and, I don't have anything to measure really low leakages but it at least doesn't show up on my ohmmeter.
Can't say about older deposits (absorbs moisture and then becomes conductive, maybe?), also it's winter (dry) here so I wouldn't expect a fair test even if I had, but, I usually clean things off anyway, as the softer consistency washes away easily with IPA or flux remover.
Is that SMD921 active enough to solder so-called NOS components? I've got a bunch of old components with oxidized legs. Especially silver plated ones can be difficult to solder.
Heh, much of my inventory by now could be considered "NOS"... I've got chip resistors from 2010ish, they definitely don't stick as well, but some flux (any kind, really) perks things right up again.
Can't think if I have any silver plated parts or leads around right now, that are also tarnished, and that I would want solder on... maybe some switches? Oh let me check...
Just testing right now, with a surely "old stock" carbon comp resistor, 82k 10%, with very obviously grayed leads: a drop of said flux on the lead, then heating with 370°C soldering iron touched further up the lead, shows the wetted spot turning silvery in about five seconds. Mind, this isn't all that challenging of a task; the same is true of some straight pine resin I have here (which I do actually use a little of here and there, when I'm tinning things that are large enough not to be worth using something good on, and that aren't super dirty to need mechanical cleaning anyway, nor a more active flux).
...Aha, I have some old (Russian, ex-soviet?) fuse holders here, the sprung terminal is tinned, but the side contact is not, and is fairly badly tarnished. Let me see:
- Rosin core solder... I get a blob that seems to be in contact, but it looks rough, isn't strong, and doesn't spread out at all. I think if I use a lot, I could eventually clean it up, but it'll be a mess, and I'd rather sand it down if that were the case.
- SMD291: putting a couple drops on the terminal and heating, the solder-blob area does seem to consolidate and flow, and there is a subtle change to the tarnished surface; almost more like, the surface crud is separating from the metal, maybe, but only in patches. Looks like I can rub through the crud to tin it, but it's not so active that it'll just dip and coat in one shot.
- Pine resin: doesn't look like it does much of anything, or much more anyway. Main advantage, I have a ton of it.
After heating with this combination for a minute or two, and wiping off the excess, it does look like the tarnish has been removed, at least in relevant patches. Let me see if I can get a meaningful photo of this...


There's some texture in the background that's been added by the camera/app, just... ignore that. The backside view with the bright spot, the difference in albedo is so dramatic it looks tinned, or nearly overexposed, but it's actually bare silver with its subtle yellowish tint. On the front side, it's easier to see the color against the solder around it.
Note that the fluxes didn't flow all the way around, the resin in particular kind of staying in place. So there's only a small "etch" spot on the back side as you can see. Obviously I'd do a more thorough job if I were intending to make a proper connection here.
I don't have any strongly activated or aggressive fluxes to try and compare, but I'm sure something would cut through much faster. I don't have much old junk laying around like this so it's not something I have to deal with; consider a stronger one if that's the case.
Tim