This is what I do when putting together packs.
1. Always grind/rough up both ends of the cell.
2. Apply a small amount of flux and lightly tin each end of the cell
3. Apply another small amount of flux and install the bar using a small amount of solder.
4. Use lighter fluid and a tooth brush to remove and left over flux.
On my 4200's I use the EA/SMC Silver Intelect bars for assembly and 2 Trinity silver bars for the hook up leads. When I put a pack together I use Pure Silver Solder as well. Voltage does not travel "through" the bars/solder but around the outside. So going from one cell to another, you use Silver Solder, Silver bar, and Silver Solder your voltage travels across only Silver.
Another important tip is to use a very hot iron. Best result is to assemble the pack in the least amount of time. The longer you let your iron sit on the cell the better chance you have to damage your cells.
This pic is an old school 3300's.