Phone dead after Tristar replacement

Hi all, I'm hoping for some assistance from somebody out there. Working on an iPhone 6; phone was working perfectly in all respects except for one thing - while it would charge a partially charged battery it wouldn't charge a flat battery at all. After trying different batteries and using my own charge port and doing some measurements with the multi-meter I finally decided to replace Tristar. I did this carefully and once finished I put the board back in the housing temporarily to test it. I first used a partially charged battery; the phone booted no problem and began to charge the battery. I then tried a known good battery which I'd allowed to go flat; the board began charging the battery and booted a few minutes later, bingo! I let it charge for a while until there was about 15% in the battery then disconnected it and switched it off in order to properly reassemble it in the housing. Did that, connected the screen and turned it on....nothing! Phone is now completely dead; it shows 0A on my USB ammeter but on the DCPS it is pulling a small and constantly fluctuating current of approx. 30mA but it's up and down all over the place (but always less than 100mA). When I try to measure voltages they are also fluctuating wildly and I can't get decent readings anywhere on the main lines. There are no shorts (I've physically checked every large and medium-sized cap on the board in continuity mode) and strangely in diode mode everything seems to be reading correctly when compared to my known-good IP6 board (which definitely still works fine). When reassembling it I didn't use heat to replace the Tristar/Tigris large shield I just used my iron with a tiny bit of solder & flux. I've since replaced Tigris but no change. I'm starting to think it may be PMIC but still have a niggling feeling that it's something dead simple which I'm missing. I've been over board under the microscope at x20 but can't see anything unusual. Help!!