The iPhone 4 has been plagued by a few hardware issues. Apart from the infamous antenna-gate, we have also noticed that the home button is prone to failure. And then there's the more serious issue with the audio compiler chip.
This last problem manifests itself by interrupting the audio of one of the two parties during a phone call. Most of the time, the person at the other end of the line will not hear you. While this can be a microphone issue as well as an audio compiler chip issue, there's an easy way to test it: start Voice Memos and record a memo. If you can hear it OK afterwards, then the microphone is working OK and so the problem is because of an audio compiler fault.
This fault is caused through imperfect soldering of the audio compiler chip, which causes conductivity issues. If your iPhone is still under warranty, you can take it to the Genius Bar and they'll replace it free of charge. If the warranty has expired, the best option is to have the chip re-soldered. There are companies that have the right equipment for this job and it will cost you around £50 to have the chip reballed.
However, if you feel confident and want to try to fix the problem yourself, there is another solution. We've made a video showing you what you need to do. We suggest that you try it only if you have had some experience of taking things apart. Be warned that you will have to deal with some tiny screws and that the logic board is a very sensitive component which can be easily damaged.
WARNING: Proceed at your own risk. We are not responsible for any damage you might do to your iPhone!
If you still want to go ahead, here's a list with the tools we think you'll need: