I’ve been working with a liquid flux dispenser pen (EDSYN FL-88) for all my projects so far, be it through hole or SMD or both. Although I use ‘well-behaved’ SnPb solder with rosin core (I tried lead-free SAC solder, but it is CRAP and horribly expensive — maybe lead-free solder paste is different, I can’t tell so far), I often have to add a bit of extra flux (hence the flux pen) to make the joints look good and strong. When working with SMD components, especially chip packages, I have to constantly re-add liquid flux as it evaporates very quickly. If I e.g. add flux to a TQFP-32 package and solder one side, all the other flux is dry already. And it’s really not suitable for SMD rework for the same reason. From my personal experience it is essential to add extra flux when working with older or ill treated components (oxidation). Otherwise the solder just looks you in the face and screams: “BUGGER OFF! I won’t stick to no stinking copper wires.”
I already use ChipQuik products (SMD removal test kit so far) and I was (and still am) impressed by their paste flux. Unfortunately I don’t have a local source for it. I could get it on ebay, but the shipping times are somewhat prohibitive. I could get if from Digikey, but their shipping costs are prohibitive, as I usually have a hard time ordering enough to get above the ‘free shipping’ threshold.
Recently I stumbled across a review of ‘EDSYN FL-22’ noclean paste flux on dangerousprototypes.com. They liked it a lot. I searched for it locally and I was lucky. It’s not exactly cheap, costs even more than the ChipQuik stuff, but boy it is GOOD! The FL-88 liquid flux is toast compared to it. It sticks to the PCB well, stays on there long enough, doesn’t smell, it doesn’t smoke (285°C at least), it does its job nicely. It’s also quite easy to dispense with a 0.6mm nozzle and comes in a 5ml syringe of reasonable quality with a proper screw-on plug and screw-on tip.
Without it? Nevermore!