Ruby Fluid. It's a liquid flux that's VERY acidic. NOT for electronics use!! - because it's so corrosive / active. But what if you neutralize + clean it all off completely when done soldering? Why bother? I have at most in my shop - a Hakko FM-203 with FM-2027 and the usual assortment of microsoldering tips. I once had to solder a copper braid ground bonding jumper onto a steel Commodore 64 RF shield. Not enough wattage! Not enough thermal transfer from my largest tip. No matter the flux or low-temp solder used. And then I tried Ruby Fluid. INSTANT perfection!! WTF?? Even without hot-air preheat soak assist.When I have to tin the contacts on the underside of a chip, I see most contacts want to fight! They won't take tinning! I use rose's metal bismuth / tin 50/50 solder. I set temp on Hakko FM-2032 low...I wet contacts with Ruby Fluid = they tin! So I've got a clean-up job to do. That sure beats reflowing a chip that only half the pins take soldering! You gotta work with it some, learn it's behavior to heat. It's well worth it.