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.