How to replace the power IC, iPhone 3GS logic board?

So I posted awhile back about a 3gs I had got a hold of that did not seem to power on no matter what I do. I've already went through the standard trouble shooting. I've replaced the dock, and the battery. More recently another 3gs I got a hold of with a good logic board that works with the same combination of new parts that I've put together...

Now that being said its definitely the logic board. I also tried putting the bad logic board in another phone and it didn't work in there either. I'm having issues finding a good walk through for a power IC replacement or another means of troubleshooting that I can perform to diagnose what the issue is with the logic board itself. If anyone could point me in the right direction it would be much appreciated!

In addition I do have a multi-meter, but I can't seem to find any diagrams as to what I should be testing on the board or what kind of read outs I'm supposed to expect from it to determine whats working and whats not.

Thanks in advance!