Al tabs soldering tips?
 

I have a new Hakko 936 with the big tip, and I bought the aluminum flux/solder crap from CPB, but I am having a really hard time soldering to the tabs on my NeuEnergy lipos. I have several years experience and have soldered tons of NiMH packs, a123 packs, and other lipo packs, but I'm really getting frustrated with these NeuEnergy cells.

I must be doing something wrong.

Any advice?

Thanks!