First thing first, I am going to suggest Acetone, that is nail polish remover. If you can obtain near 100% acetone from a hardware store please do.
Words of warning:
Acetone is flammable. It burns like gasoline.
Acetone eats plastics - It is the foundation of many plastics contact with plastics is not advised as it will attack them.
I recommend working with a fan over your work area to reduce the risk of inhalation and it will help disperse evaporating acetone in order mediate fire risk.
Sure lots of warnings and sounds like overkill... better to be safe than on fire.
- Test if the acetone attacks the type of tape residue you want to
remove on a test item. If it does so then you can proceed.
- Important, take a q-tip covered in acetone and on a small patch on the back of the ipad rub with q-tip. Don't press too hard, and wait until it dries which should take a few seconds.
- Confirm visually and by touch that no reaction has occurred. Discoloration and texture may change if the material on your ipad back is reactive with acetone.
If good to go, then have it, acetone will attack most glues and plastics used in tapes. Acetone evaporates readily so just work at it until you have removed the residue traces.
Alcohols that are very strong of ethyl and methyl types are an alternative but mostly same warnings apply, except you shouldn't have reactivity with any plastics.
They can be helpful to remove anything that wasn't destroyed by the acetone and on their own.