Use a cleanser designed for, or at least safe on, electronic circuits.

At work I had frequently to clean delicate electrical / electronic equipment, and used a substance made for the purpose, that looked and smelt a bit citrusy, and I think was in fact a citrus compound. Other than warning that it could de-fat the skin it is quite a safe solvent. Yet I think it would remove dried resin-flux – along with adhesive-tape glue and the goo that accumulates on computer keys.

I am pretty sure it was Mykal Industries 'De-Solv- It' but looking on the RS site now, it is described as in a spray-can, not the 5l bottles I recall.

The RS catalogue carries this in an impressive list of patent electronics cleaners, including simple Isopropyl Alcohol

Usual caveat with any of these – test a tiny, discrete area first for compatibility.

Clive

I have sent you a PM.

Stuart

Freon was always the flux-cleaner of choice until it was outlawed. Plain old isopropyl alcohol works for me now – if a bit more slowly – and is pretty benign with other parts on the.board.

Besides, it's multi-purpose and I have lots of it.