[scarletboa]

well, it sounds like you have the skills to solder the packs yourself. you might just need a different soldering iron/gun.
i have found that in my experience, it is easier to solder on aluminum with a gun style soldering rig. all you have to do is hold the trigger down and preheat the tip until it starts turning red, holds it a few seconds longer, and then solder the joint. aluminum soldering requires a higher temperature, which can be difficult with a iron/pen style soldering rig. i personally use a radioshack 230/150w gun. i was advised against it, but i prefer it over my weller 80w iron. with the ability to preheat the gun as much as i need, i can solder larger joints faster and with much more ease.

also, make sure you have aluminum specific solder and zinc flux. if you do not properly flux the joint and use the appropriate solder, it will not stick. instead of a light application of flux usually used for wires, etc, aluminum tabs need a good brushing with flux. my advice would be to take a toothbrush and lightly scrub the flux onto the tabs before joining them.