Recovering from failure to clean off solder flux

I'm currently building a prototype device at work, that involves a QFN-64 0.5mm pitch IC (a PIC microcontroller) soldered onto a SchmartBoard|ez prototyping board. The SchmartBoard instructions call for the use of flux, which is not something I normally need to use, so I grabbed a Kester flux pen off the shelf and proceeded with soldering the chip, with no apparent problems. My initial firmware development was quite successful, but at that point I was only using a few of the chip's I/O pins. I've since reached the point where I need all of the I/O pins to work - and am finding that several of them are stuck low, despite having a weak pull-up resistor enabled. I'm measuring voltages in the 2-3V range on these pins (in a circuit that shouldn't have anything other than 0V and 5V present), and measuring resistances as low as 500Ω between adjacent I/O pins. I won't bore you with the full list of possible explanations I went through, but it eventually occurred to me to take a closer look at that flux pen - it's Kester 2331-ZX, not the no-clean flux that I had used in the past, but the type that requires cleaning immediately after soldering. Oops! And this soldering had been done about a week ago... (Also, it turns out that the flux pen had been sitting on the shelf for 2 years past its expiration date. I have no idea if that makes things any worse than they would otherwise be.)

I've tried a heavy-duty flux remover spray from MG Chemicals, and a couple of other PCB cleaning sprays. I've tried 91% isopropyl alcohol. I've tried rinsing the board under hot water, followed immediately by drying with a heat gun. None of this has achieved more than a slight improvement (that 500Ω pair of adjacent pins is now up to 600Ω, for example).

Is there anything else I can try to salvage this board? I have little confidence that I'd be able to desolder the chip while leaving the board in good enough state to solder a new one on - and I've got a lot of other components and wiring on that board, that I really don't want to have to redo.

• Could try submerging it in lacquer thinner for an extended period (overnight) but this may yield little results and create lots of waste. Best to remove, clean, and re-solder (use solder paste with flux in it), or make a new board with solder paste. Time to practice hot-air skills. Sep 29, 2021 at 18:28
• Flux issues aside, the joints look rather questionable at a glance. Looks like "bridges" in some places, unless it's all flux residue. The much easier solder joints elsewhere on the board (to the upper right) are botched cold joints, so I'd question the state of the whole board. Oct 4, 2021 at 11:59

But the leakage was in the 100's of M$$\\Omega\$$ or G$$\\Omega\$$ (low enough to be a problem in very sensitive precision analog circuitry, not digital circuits), and not a mere few hundred ohms. So at least 5 orders of magnitude higher. I doubt even plumbing solder acid flux would be quite that bad (but it would be bad).