Ok so I had multiple failed attempts to solder the chip on. Despite pretinning the PCB, on both attempts when the solder went molten the chip did not center itself and lock into place as would be expected but instead slid out of position significantly both attempts. After the first attempt I assumed it was the outlet bus/landing pad on the left side of the board that was to blame - it being a larger solder mass than the other solder masses on the other edges of the board - causing the chip to slide its way harder and pulling it off center. However, even after cutting off that part of the board entirely and trying again I still had the chip slide out of place during reflow attempt. It's unclear to me the cause of the issue at this point. I put on AMTECH tacky flux that expired in 2022 onto the pcb and then placed the chip as close as I could to centered and then put it onto a hot plate for cooking food or boiling a coffee pot those AC electric hot cooking plates. I had the heat set to max on there. The first time I had the board on the whole heatup time period and watched it gradually smoke off flux vapors and eventually the solder go molten and then I switched off the hotplate but it didn't matter since the chip slid 5mm off center. The second time I added it after plate was much hotter to avoid so much flux burnoff during the heating up process. I also added some flux while it was molten after which time it slid off center. Perhaps that was my mistake? Not sure. In any case, after the failed second attempt I cleaned everything up really well and chose to hand solder it. This seemed to go better. For this process, since both chip and PCB were fully tinned and no shorts to my knowledge, I proceeded to line up the pins the best I could under max magnification and then while pinching it in place I held my solder iron tip onto a trace near a pin for 5-6 seconds to get the solder very hot and hopefully locally heat the whole trace up to and under the IC's pad. I did this for trace after trace. Then to clean up since it was I guess cold joints at this point I applied flux and drag soldered all around every edge. This cleaned up the appearance and got all flux properly seated on traces since it was drifting a bit in the previous step and looking quite sloppy and bad. I did not want to have flux involved during the initial alignment phase and tacking down phases because once that is on I cannot see well the alignment from the sides since the flux blocks my view. Even without the flux its hard to see the traces and their alignment with the pins of the chip. The chip does not have pins is the problem. It has pads on its underside and little pads on its sidewalls. I have to use the sidewall pads as to verify the pads on my pcb line up with those and any flux in the way prevents me from seeing this clearly even with max magnification on my visor.
So anyways yeah, this whole thing was a bit of a disaster but MIGHT have ended ok in the end. For now hand soldering instead of reflow looks like my best bet at least until I can figure out what i'm doing wrong...
Note: I'm not sure if the flux expiration matters I haven't looked into what that means for performance.
Note: I still have no way to verify all joints are good until I test the whole finished PCB I guess. And my confidence they are all good is a bit low. Sucks that I have to invest even more time to finish the PCB while doubtful it will even have reliable connections at present ugh!
Note: I had to remove the trace from the previous post that ran under the chip because during my reflow and cleanup from failed attempts while cleaning some of the solder mask came off and I just wanted to remove that potential short or source of issues as a variable entirely for now. I can always run a bodge wire to reconnect those two points of the board later on.