Water Itself is not damaging to the board. water in it's purest form is non-conductive. the actual problem is the minerals in the water. (Iron, Calcium, Sodium, Lime) These are conductive and cause damage. unfortunately, they have a much higher boiling temperature than water, so when the water evaporates, (Sped up by the rice) these minerals are left on the motherboard. Rice doesnt actually help the damage, but rather speeds up the drying porcess. If the phone functions WITH the rice treatment, it would have worked without the rice treatment.
as a water damage repair, it still is only 50/50 as to whether or not you can get the device up and running. follow the next steps, andyou may have a functional device.
1) Isopropyl: Take a 99% rubbing alcohal, also known as Isopropyl, and a toothbrush. (New, as it can not be previously contaminated by conductive materials such as saliva, or tap water) soak the board in the isopropyl, and scrub diligently. make sure not to be terribly rough on the board, but still use a heavy handr
2) DeOxit (Electronics cleaner): I have used many brands, but DeOxit is the the one i have had the most success for, so i suggest this brand. others may work, and it is only a personal preference. spray the cleaner onto the board, and seal in a tupperware container. let sit for an hour, then re-soak with isopropyl to dilute any lifted dirts, and scrub with the tooth brush. Essentially, after the DeOxit has sat, Repeat step one.
3) Sonic Wash: This is implying you have access to the tool required. usually need a repair shop environment to have one The sonic wash is a tool, usually cosing between 300$-500$. fill it with distilled water and a liquid electronics cleaning solution. this pulses the cleaner underneath components where a brush can not reach. though this is the most successful procedure, it is also the most expensive of the three.
i hope this helps, and if it does, please accept the answer.
4) After all of these procedures, if any funtions aren't working, try replacing the modular component that the function is related to. (Antenna's for signal, Charge port for charging, Camera for camera function, ETC.)
Anything further than this will be a board level failure.