No service after teardown

I've had the no service/searching issue with the past three 4s phones I've "repaired", I haven't changed anything in my repair process and I'm seriously considering giving up. I've tried all the usual methods mentioned here, time and date are fine, blue coil is intact, silver clip is in the right place and so is the black pressure point, I have also tried a full teardown and back up and restore through iTunes. I am wanting to try cleaning the contact points but I'm unsure of where to clean and how. Will IPA solvent cleaner work and how do I administer it?

Thanks in advance, Stewart.