kind of late to the party but I have been repairing phones since I was 11 (I am 15 now c: )and back then, we used to take petrol and a tooth brush and actively scrub the motherboard and mostly everyone copied this technique even outside the institution and local repairs… BUT… I noticed that most circuits got corrosions with every phones I fixed (mostly Nokia 3310 which were still common within every individual and old keypad phones”, so I changed to using isopropyl that has a very high alcohol content (90% or higher) and that works for me very well.
Do keep in mind that you can invest in an ultrasonic cleaner which can be bought from ebay around 35$ or probably more and is worth every cents. why? It helps to clean thoroughly and reaches places which your typical toothbrush cannot reach.
best regards
- Ding0