heres how i make packs, never had any problems:
take cells, put individual shrink wrap on them.
take cells and lightly sand (dremel w/ sanding drum) the end of each cell (both ends) where you plan to solder too, nothing agressive or anything.
now put all ur cells in your jig.
now tin all of your battery bars.
now use a HUGE soldering iron to solder ea bar on. try to be quick.
now glue your pack together.
in the past i didnt sand the ends of cells. the reason i do now is that when i didnt sand the cells i could take pliers and pull the battery bar off, without a ton of force, and usually no solder would be stuck to the cell, it would all be on the battery bar. now that i sand its as if they are welded on, no pulling them off with pliers
also a large iron helps to heat the bar and the surface of the battery to soldering temperature quicker, and SEEEMS like it keeps the cell overall cooler. idk.
as for the soldering itself, if you pretin the bar you should have enuf solder between the bar and the cell, so you just need to add solder around the permiter usually. also you dont need solder on top of the cell, just under it, and along its edges where it contacts the cell.
idk if that helps, but thats how i does it