That's not strictly true. It helps on new boards (I get a better wet) and is needed with smt work. Not unless you like chasing down solder bridges. Clean with lacquer thinner and a tooth brush. Of course you can pay more for commercial canned products.
Solder wire intended for electronics normally has the required flux in the wire, it's just that SnPbCu and SnPbAg flux is a bit more agressive. There are SnAg solder wires that require >300 deg C and a separate flux (don't know what the composition is, when I used this, the flux bottle came bundled with the solder wire and actually cost almost as much as the expensive wire!).
There are flux sprays as well as eyedropper and brush-on liquids, which can be of great help for SMD components and NOS silvered or silver wire components when the silver has tarnished. In any case, I would take any flux residue off the board. Flux is normally heat activated, but that only means it's chemically agressive action is slowed down a lot at room temperature - in essence, it WILL eventually degrade the board and/or attract moisture and partciles, it will just take a long time. Still, it's really not a big investment to clean it off and you get a pristine board which will never have problems of that kind.
Point for discussion: The US military has an exemption from using lead-free solder. I suppose if you're going to litter the landscape with depleted uranium and jacketed lead, then a bit of lead in a PCB is trivial, but you have to ask the question, "Why did they apply for the exemption?"
I've used 63/37 solder on the odd occasion with a Weller no. 6 temperature tip instead of the no. 7 normally needed with 60/40 solder. I believe it does not have a plastic state but goes directly from liquid to solid so is less likely to make a bad joint. Very *shiny* too, not dull like 60/40 which is important if the work is on show for all to see.
I have tested at least a dozen different solders in my cables including a number of lead free versions.
In order of preference from a sonic point of view here is my list in order from most liked of that group to date.
1) the latest version of Wonder solder lovely to work with, smooth
2) Siltec solder sounds a little brighter still nice to use
3) Kester sn95sb05 lead free a little fussier to use
So if you are looking for a lead free solder which will sound good from an audiophile point of view the Kester would be a fine choice. Hope that this is of some interest and future use. Best regards Moray James.
Apparently, that only affects pure tin, though. So whether you add Pb or Ag or Cu in the solder, should not make that big of a difference... unless you can explain me what Pb specifically has that helps prevent tin whiskers... it may just be a matter of proportion (60/40 Sn/Ag would probably have too high a melting temperature), but I don't have enough detailed info on the growing of those tin whiskers...
One question would be: why the heck would the industry sell lead-free solder alloys if those were known to be fundamentally unreliable? Are we implicitly being told "we sell you ****, but that's ok as long as you don't make automotive, medical or military devices"... in other words, are we considering consumers electronics as a disposable commodity?
And last but not least, one hidden reason of all the debate around tin whiskers may just be the cost factor. Any lead-free solder alloy IS more expensive than it's leaded counterpart... I don't doubt there is a real risk, but is Sn/Ag or Sn/Cu prone to that too? Do we *know*? And finally, isn't there any metal apart from Sn that we could use for soldering? And one more thing I'm thinking about is: if the PCBs are well coated with appropriate lacquers or even coated in epoxy, I'd be guessing that tin whiskers would not be able to grow or at least not cause any problem?
When my company produced embedded control systems and we stuffed all our own boards, we used 63/37 with water washable flux. When the boards were finished, they went through a hot water rinse and dry before testing. We had very few solder failures in the field out of 100's of boards.