Ho can I detect damage in the iPhone's logic board?

Hi, I have a pair of 3G iPhones, they work, get signal and do basicly all things except get charge.

I already changed the batteries and charging ports on both iphones, so I don't know what else could be the problem.

I also tried charging them to the wall electricity and still, no charge. I tried with different power cables, and also with various data cables and still no charge.

I examined one motherboard and found some black "residue" surrounding some small components, but I don't know if this is some kind of glue used normally on the logic board or what else could it be, maybe a sign of a burnt logicboard.

This are the pictures that I took of one of the logicboards.

http://asimovil.com/iphonedamages.html

Thanks for your time.