I am sure you didn't mean it like interpreted below but just in case to avoid any misunderstanding and possible damage: We don't solder anything to the phone. You just have to have some mean to establish a contact to the "springy pins" of the phone body. This usually means that you put something in the other half that, when the other half is put on the phone, has contact to the mentioned pins. I, e.g., used some "pads" I soldered myself and which I then put in the proper place of the other half and fixated it with hot glue. Other people use flat copper cables etc. But, don't solder anything directly to your phone nor the springy pins.