The most commonly used circuit board cleaning fluid is probably isopropyl alcohol. De-ionized/demineralized water might be the least expensive. You could also check out Kleanium
http://www.chemtools.com.au/product/electrical-electronics/electronics-cleaning-chemicals/pcb-flux-remover/and see if it's worth the cost. An ultrasonic tank cleaner can make the process faster and more thorough, but NASA no uses it due to concerns about possible component damage.
You might want to seriously consider how feasible it is to disassemble all your miners every month to clean the circuit boards, then reassemble them. Seems that would add a lot of overhead and potential for damage. Is it possible to get cleaner incoming air using some type of filtering?
