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So, I read one of the guides on how to repair an iPhone 6s that went through the wash, and it says I need to use 90% isopropyl alcohol to clean the circuit boards inside. Will 70% work, as that is what most people will have for medical reasons? Thanks.

If your iPhone went through a washing machine cycle I’m afraid whatever alcohol you may use it won’t make much difference; when a phone is submerged water gets literally anywhere and disposing of residues would need professional tools to remove metal shield protections and ultrasonic bath to start with. Battery and screen will have to be replaced for sure and further damages needing microsoldering skills can be easily expected. Unless you need to recover important data from your phone the advantages to attempt a repair vs buying a used 6s iphone, given current market prices, would be rather questionable.

Use the 90% as the rest is water. I prefer methanol, which is pure with no water but harder to find. Electronics Water Damage