What is the safest liquid to clean Arduino? I use 90% IPA but it can't clean the remaining tin?
If regular IPA doesn't quite cut it, IPA + acetone in 80/20 or 75/25 proportions.
Exactly what are you trying to remove?
It's not clear if you want to clean or remove...
You need strong acid or base to dissolve tin. Try HCl, it's relatively safe.
I want to clean the sticky tin residue around the installed components so that it doesn't leave any marks
Don't use HCl for that purpose 
Did you try the formula on post#2?
That residue is flux, and you would clean it off with alcohol (IPA) and a toothbrush for example.
You can buy commercial products but they are expensive.
I use 91% IPA and yes it takes several times with brushing to remove the flux.
Tin is not sticky. Flux from solder is sticky.
Haven't tried it yet, already know it will ruin it
Tin, if it is still wet and not neat, will become sticky if left untidy.
Good.
It's efficient to remove tin residue, not flux from pcb...
Tin is metal, flux is different story.
Thanks for the info, I will try the 91% IPA liquid. Most in my country only sell around 90%-80% but it leaves white spots
Yes it does. 99% does not but it is more expensive.
By "tin" perhaps you mean solder?? Solder is typically lead/tin alloy, but there are dozens of variations.
If you need to remove solder, I don't think chemicals are the right idea.
Tin is only wet when liquefied, that is, when you solder it. I believe you mix up tin and flux. Flux when left, can absorbe humidity that will corrode the solder in time. That's why you wanna remove the flux.
Was. Not so much anymore:
I found this when shopping for pine resin (also available and inexpensive) and pure IPA to try making DIY flux. 99.9% IPA is also not nearly so dear as in the 20th century.
a7
I prefer a good Belgian Trappist Ale or Imperial Stout over an IPA. 
yeah maybe that's what it means because the tin must have a little flux in it
Soft bristle brush, much iso-propyl alcohol and repeated scrubbing. The higher the alcohol percent, the more volatile, the shorter the time it will be in liquid form. I use liberal amounts of flux to flow my solder. All my flux cleans well with 70%IPA. Cleaning flux with IPA will not make your solder joints shine. Shiny joints come from proper soldering.
Use what the manufacturer used. Hot deionized water in a device just like your home dishwasher. or just use your home dishwasher. When finished blow dry with compressed deionized air. Clean by hand using hot water and a tooth brush, let air dry or use compressed air. Blow water from under all devices.