There really is no need to solder or de-solder anything. The button is a black rubber one, held down to a plastic base by means of a silver-metal, kind of clamping plate. Get the smallest screw driver in the house (or a needle even), and pry one side of the plate loose. The button will pop loose.
In it, is a brass part that looks like a bug or a beetle. It's actually a two way switch: pressing lightly engages metering and focusing, pressing through makes the switch sink further and triggers the shutter.
Make sure not to bend any part of it while cleaning with clear vinegar and a cotton swab. The acid in the vinegar will clear all the corrosion on the brass, which normally causes the two way switch to trigger focus and metering.
Replace the brass part (hollow side down) and replace the button and silver plate that holds it down. Make sure it snaps into place. Reassemble, you're done.
Fifteen minute job, saves weeks of repair time and quite the buck.
