If you have experience in fully disassembling the iPhone, you can get 92%+ alcohol, a small hobby/paint brush, jewelers loupe (20x) and an anti-static cloth. Power off the phone, remove the battery, then use the brush to gently brush on the alcohol, use the cloth to blot away any debris, and use your loupe to check where you cleaned. Don't scrub, just smoothly wipe the brush in a small section and blot, then check. It will take some time, but, with patience (and ensuring their is no current flowing) it can be cleaned and restored to working order, in most cases.