Here is a generalization of my method for soldering in a new flex cable.
I always disconnect the + terminal on the battery. Think about it, if you get in their scraping away epoxy, or doing other things, and the logic board is hot, you can cause arching and burn the board up.
1. At the battery, put Kapton tape on any nearby components.
2. Use a dab of flux to remove the epoxy covering the battery contacts. Also use a brush with 92%+ alcohol to remove the flux, and a static-free cloth to dampen up any excess liquid and debris.
3. Cover the neutral and ground with Kapton tape.
4. I set my Weller to about 475, with a chisel tip and clean/tin it.
5. Put a very small drop of flux on the + connector. Use a copper braid and your chisel tip to gently push (toward the battery). You should only be touching the braid for about 2-3 seconds at most, any more and you can lift pads, etc.
6. Once free, stick a piece of Kapton tape under the + battery connector and the + pad on the board.
- Now if you work on the Flex cable and the iron slips (or one of many other things that could cause arching) you don't risk shorting out the logic board.
FLEX CABLE REMOVAL
1. With the bad flex cable still attached. Use flux, alchohol, anti-static wipe to remove any epoxy covering the contacts.
2. Add a very small dab of flux to the contacts, use your chisel and braid pushing away from the board to "suck up" the solder. 2-3 seconds at the most then let it cool. Don't push too hard. You should be able to gently, and I mean gently, lift the cable and let it free itself. If you pull up on the cable too hard, especially with too much heat, you'll defiantly lift a pad and in for a serious headache.
3. Once you have the old cable off, use your braid, flux, alcohol and anti-static wipe to clean the solder pads
SOLDERING THE NEW FLEX CABLE
1. I heat my weller to about 425. And pop on a very fine solder tip, clean, and tin it. I also use 60/40 resin core solder I think it is like .03mm
2. Get your cable lined up and use some Kapton tape (or have someone help) and get your flex cable lined up on top of the solder pads. I use a jewelers loupe to check the alignment.
3. Place the tip of your solder on one side of the flex cables contact then using a very fine solder tip, touch the tip to the other side of metal contact for about 1 second. The heat will suck the solder into the holes. As it does, again, it takes only a second, pull away your excess solder then the iron. The thing to focus on is you want to get "in and out" as fast as you can when you heat the solder. Done right, you will have a perfect shiny bead.
4. Repeat for the last two flex cable contacts.
RE-SOLDER/ATTACH THE BATTERY
At this point you should have no problem removing the kapton tape between the battery + connector and the logic board and soldering it back on. I switch back to the chisel tip for this.
Once everything is complete, I check the iPod. If all is well you can remove all the excess Kapton tape then cut two small strips placing one top of the battery connections and one over the flex cable connections where you have soldered. This will prevent accidental shorts if someone else (or you) repair the device later and accidental let something hit the contacts.
Everyone has their own method, but after killing a few logic boards when I started repairing i*devices I found what works best for me. Your mileage may vary.