We often see this in our workshop. It looks very worrying initially. The screen is often popping out and there is often warping of the logic board.

But in our experience the logic board is fairly hardy and replacing the battery normally sorts things out. The iPhone 3GS pictured above obviously needed a new iPhone 3GS rear case. But to my surprise with a new battery and a new rear case it working like a dream.

You do have to be a little more careful when doing the battery replacement yourself when the battery is swollen. This is because:

• There is a build up of pressure. So when undoing screw and clips there is an increased possibility of screws or components flying out. I would recommend some sort of eye protection.
• Due to the logic board often being warped it this situation, it often makes it a lot more difficult to get it out and even more so getting back in. The screw holes can be difficult to line back up for the same reason.
• There is always a risk when working on electrical devices as so many things can go wrong. If you are in any doubt it probably best to get a professional to do it.

For a guide on changing the battery in an iPhone 3GS please read our post on the subject.