Odd corrosion around edge of pads when reworking

[Gridstop]

I'm replacing the electrolytics on a Keithly 2000. Only one cap had a tiny bit of leakage, and the electrolytic didn't even reach/corrode the traces on the top. So I dodged a bullet but now I'm having some strange residue around the edges of the solder joints on the bottom that I've never seen before.
I'm using Kester 245 63/37 lead solder and isopropyl to clean. Mainly using acid-free brushes through kimwipes. I've never had a problem leaving spotless joints before.

Around the very edges of the pads I keep getting some white residue/corrosion immediately after cleaning. It is not soluble in isopropyl or water. None of the existing joints on the board show any signs like this. I can mechanically remove it a bit with a needle/pick, but I don't want to scratch up the board. In this case I used solder wick to pickup the existing joint and re-flowed it and still got this funny edge. I really don't want to keep trying to reheat the joints more than once more if possible, so I appreciate any ideas on what it is or how to stop it or clean it.

Something to do with the existing solder that was on the joint? I removed as much as I could with solder wick before replacing the capacitors. I'm not 100% certain if it was leaded or lead-free, it was manufactured in 2003 from what I can see. Or the flux reacting with... something about the board or the copper in the trace in some weird way creating salts? I'm using Metcal SSC-600 series tips so temp should be fixed around 675F / 350C so I'm not horribly overheating, and I'm seeing this on joints that are not ground planes so I don't think I'm failing to heat the joint fully. The solder farther up the fillet is nice and shiny.

Thanks for any advice.

[jpanhalt]

Here's a study from Kester about white residue.  Some methods of removal are discussed:
https://www.kester.com/Portals/0/Documents/FAQs/White_Residue.pdf

NB: water/alcohol has properties neither alone has.  Water alone may have failed due perhaps to lack of wetting.

[Gridstop]

Interesting, maybe I'll try a 50:50 alcohol:water like they mention in there.

The other slightly disturbing possibility they mention could be the flux attacking the solder mask depending on its formulation. That would explain why I get that tiny edge right around the pad. And either the flux they originally used was formulated to not do that, or over the past ~19 years the mask has started to change slightly allowing it to happen. Not sure what I could do about that if that's the case. I would need a serious microscope I think to see what's going on there too.

[bobbydazzler]

My solder makes residue like that if I apply heat too long or use too high a temperature.  If I roll a qtip+ipa over it for long enough it eventually comes off but it takes so long I don't usually bother.